UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

PROXY STATEMENT PURSUANT TO SECTION 14(a) OF THE

SECURITIES EXCHANGE ACT OF 1934

(Amendment No.    )

Filed by the Registrant  ☒                            Filed by a Party other than the Registrant  ☐

Check the appropriate box:

Altimmune, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

Altimmune, Inc.

910 Clopper Road, Suite 201S

Gaithersburg, Maryland, 20878

July 26, 2018August             , 2019

To the Stockholders of Altimmune, Inc.:

You are invited to attend the 20182019 Annual Meeting of Stockholders (the “AnnualMeeting”) of Altimmune, Inc. (the “Company”) scheduled for Thursday, August 30, 2018September 26, 2019, at 10:00 a.m., Eastern Time, at the offices of Proskauer RoseGoodwin Procter LLP, 1001 Pennsylvania Avenue901 New York Ave NW, Washington, D.C. 20004.DC 20001. The Company’s Board of Directors and management look forward to seeing you.

Details of the business to be conducted at the Annual Meeting are given in the attached Notice of Annual Meeting and Proxy Statement, which you are urged to read carefully.

Your vote is important to us. Whether or not you plan to attend the Annual Meeting in person, your shares should be represented and voted. After reading the enclosed Proxy Statement, please cast your vote via the Internet or telephone or complete, sign, date and return the proxy card in thepre-addressed envelope that we have included for your convenience. If you hold your shares in a stock brokerage account, please check your proxy card or contact your broker or nominee to determine whether you will be able to vote via the Internet or by telephone.

On behalf of the Board of Directors, thank you for your continued support.

Sincerely,

Mitchel Sayare,Vipin K. Garg, Ph.D.

Chief Executive ChairmanOfficer

Altimmune Inc.,Inc.

910 Clopper Road, Suite 201S

Gaithersburg,Maryland20878

NOTICE OF

20182019 ANNUAL MEETING OF STOCKHOLDERS

To the Stockholders of Altimmune, Inc.:

NOTICE IS HEREBY GIVEN that the 20182019 Annual Meeting of Stockholders (the “AnnualMeeting”) of Altimmune, Inc., a Delaware corporation (the “Company”), will be held on Thursday, August 30, 2018,September 26, 2019, at 10:00 a.m., Eastern Time, at the offices of Proskauer RoseGoodwin Procter LLP, 1001 Pennsylvania Avenue901 New York Ave NW, Washington, D.C. 20004.DC 20001.

At the Annual Meeting we will:

1. vote to elect the seven nominees named in the attached Proxy Statement as members of the Company’s Board of Directors for terms expiring at the 20192020 Annual Meeting of Stockholders;

2. vote to ratify the appointment of Ernst & Young LLP as the Company’s independent registered public accounting firm for the year ending December 31, 2018;2019;

3. hold an advisory vote on the compensation of the Company’s named executive officers as disclosed in the attached Proxy Statement;

4. vote to approve an amendment to the Company’s 2017 Omnibus Incentive Plan (the “2017 Incentive Plan”);2019 Employee Stock Purchase Plan;

5. vote to approve, for purposes of complying with Nasdaq Listing Rules 5635(d)5635(a) and 5635(b), to approve the issuance of shares of our Common Stock pursuant toin connection with the termsoccurrence of certain exchange agreements between the Company and the investors named therein;

6. vote to approve and adopt an amendment to our Amended and Restated Certificate of Incorporation to increase the number of authorized shares of Common Stock from 100,000,000 to 200,000,000;

7. vote to approve and adopt an amendment to our Amended and Restated Certificate of Incorporation to effect a reverse stock split of our Common Stock at a ratioMilestone Payments that may become payable in the rangefuture to former securityholders of1-for-5 to1-for-30, such ratio to be determined Spitfire Pharma, Inc., pursuant an Agreement and Plan of Merger and Reorganization we entered into in the discretion of our Board of Directors;July 2019;

8.6. vote to approve the authorization to adjourn the Annual Meeting, if necessary or advisable, to solicit additional proxies in favor of the foregoing proposals if there are not sufficient votes to approve the foregoing proposals; and

9.7. transact such other business as may properly come before the Annual Meeting or any adjournment or postponement thereof by or at the direction of the Board of Directors.

These items are more fully described in the Company’s Proxy Statement accompanying this Notice.

The record date for the determination of the stockholders entitled to notice of, and to vote at, the Annual Meeting, or any adjournment or postponement thereof, was the close of business on July 23, 2018.August 9, 2019. You have the right to receive this Notice and vote at the Annual Meeting if you were a stockholder of record at the close of business on July 23, 2018.August 9, 2019. Please remember that your shares cannot be voted unless you cast your vote by one of the following methods: (1) vote via the Internet or call the toll-free number as indicated on the proxy card; (2) sign and return a paper proxy card; or (3) vote in person at the Annual Meeting.

Gaithersburg, Maryland

July 26, 2018August             , 2019

YOURVOTEISVERYIMPORTANT,REGARDLESSOFTHENUMBEROFSHARESYOU OWN. OWN.PLEASEREADTHEATTACHEDPROXYSTATEMENTCAREFULLYANDCOMPLETEANDSUBMITYOURPROXYCARDVIATHEINTERNETORSIGNANDDATEYOURPAPERPROXYCARDASPROMPTLYASPOSSIBLEANDRETURNITINTHEENCLOSED ENVELOPE. ENVELOPE.ALTERNATIVELY,YOUMAYBEABLETOSUBMITYOURPROXYBYTOUCH-TONEPHONEASINDICATEDONTHEPROXY CARD.CARD.

TABLE OF CONTENTS

Altimmune, Inc.

910 Clopper Road, Suite 201S

Gaithersburg, Maryland 20878

PROXY STATEMENT for theFOR THE

20182019 ANNUAL MEETING OF STOCKHOLDERS

To Be Held On Thursday, August 30, 2018TO BE HELD ON THURSDAY, SEPTEMBER 26, 2019

GENERAL INFORMATION

This Proxy Statement is furnished in connection with a solicitation of proxies by the Board of Directors (the “Board” or “BoardofDirectors”) of Altimmune, Inc., a Delaware corporation (“Altimmune,”, the “Company,”, “we” or “our”), to be used at our 20182019 Annual Meeting of Stockholders (the “AnnualMeeting”) scheduled for Thursday, August 30, 2018,September 26, 2019, at 10:00 a.m., Eastern Daylight Time, at the offices of Proskauer RoseGoodwin Procter LLP, 1001 Pennsylvania Avenue901 New York Ave NW, Washington, D.C. 20004.DC 20001.

This Proxy Statement, the accompanying Notice of Annual Meeting of Stockholders and proxy card are first being mailed to stockholders on or about July 26, 2018.August     , 2019. Whenever we refer in this Proxy Statement to the “Annual Meeting,” we are also referring to any meeting that results from any postponement or adjournment of the August 30, 2018September 26, 2019 meeting.

Holders of record of our Common Stock, par value $0.0001 per share (“CommonStock”), at the close of business on July 23, 2018August 9, 2019 (the “RecordDate”) are entitled to notice of, and to vote at, the Annual Meeting. On that date, there were 38,506,650                  shares entitled to be voted.

We encourage you to vote your shares, either by voting in person at the Annual Meeting or by granting a proxy (i.e., authorizing someone to vote your shares). If you vote via the Internet or telephone or execute the attached paper proxy card, the individuals designated will vote your shares according to your instructions. If any matter other than the Proposals listed in the Notice of Annual Meeting of Stockholders is presented at the Annual Meeting, the designated individuals will, to the extent permissible, vote all proxies in the manner they perceive to be in the best interests of the Company.

If you indicate when voting via the Internet that you wish to vote as recommended by the Board or if you execute the enclosed paper proxy card but do not give instructions, your proxy will be voted as follows: (1) FOR the election of the nominees for director named herein, (2) FOR ratification of the appointment of Ernst & Young LLP as our independent registered public accounting firm for the year ending December 31, 2018,2019, (3) FOR the approval, on an advisory basis, of the compensation of the Company’s named executive officers as disclosed in this Proxy Statement, (4) FOR the approval of an amendment to the Company’s 2017 Omnibus Incentive2019 Employee Stock Purchase Plan, (the “2017 Incentive Plan”), (5) FOR the approval of the issuance of shares of our Common Stock for purposes of complying with Nasdaq Listing Rules 5635(d)5635(a) and 5635(b), (6) FOR the approval of an amendment to the Company’s Amended and Restated Certificate of Incorporation to increase the authorized shares of Common Stock from 100,000,000 to 200,000,000, (7) FOR the approval of an amendment to the Company’s Amended and Restated Certificate of Incorporation to effect a reverse stock split at a ratio in the range of1-for-5 to1-for-30, (8) FOR the authorization to adjourn the Annual Meeting, if necessary or appropriate, to solicit additional proxies in favor of the forgoingforegoing proposals if there are not sufficient votes to approve the proposals and (9)(7) in accordance with the discretion of the persons appointed as proxies with respect to any other matters that properly come before the Annual Meeting. If your shares are held in a stock brokerage account or by a bank or other nominee, see the information under the headingVoting—Brokerauthoritytovote.

Information on how you may vote at the Annual Meeting (such as granting a proxy that directs how your shares should be voted, or attending the Annual Meeting in person), as well as how you can revoke a proxy, is contained in this Proxy Statement under the headingsSolicitationofProxies andVoting.

Our Proxy Statement and 20172018 Annual Report to Stockholders are available at

https://materials.proxyvote.com/02155H

SOLICITATION OF PROXIES

General

The attached proxy card allows you to instruct the designated individuals how to vote your shares. You may vote in favor of, against, or abstain from voting on any proposal except for Proposal 1. With respect to Proposal 1 (the election of directors), you may, if you desire, indicate on the proxy card that you are not authorizing the designated individuals to vote your shares for one or more of the nominees.

Solicitation

We will bear the entire cost of solicitation, including the preparation, assembly, printing and mailing of this Proxy Statement, the proxy card and any additional soliciting materials furnished to stockholders. Copies of solicitation materials will be furnished to brokerage houses, fiduciaries and custodians holding shares in their names that are beneficially owned by others so that they may forward the solicitation materials to such beneficial owners. In addition, we may reimburse such persons for their costs of forwarding the solicitation materials to such beneficial owners. The original solicitation of proxies by mail may be supplemented by solicitation by telephone or other means by our directors, officers, employees or agents. The solicitation of proxies is being made primarily by mail and through the internet,Internet, but directors, officers, employees, and contractors retained by the Company may also engage in the solicitation of proxies by telephone. The cost of soliciting proxies will be borne by the Company. The Company has retained the services of Innisfree M&A Incorporated (“Innisfree”Innisfree”) to assist in the solicitation of proxies, at a cost to the Company for basic services of approximately $12,500, plusout-of-pocket expenses. Depending upon the circumstances, the scope of services to be provided by Innisfree may expand, and the cost would be expected to increase correspondingly.

VOTING

Stockholders entitled to vote and shares outstanding

Each stockholder is entitled to one vote for each share of Common Stock held on each matter submitted to a vote at the Annual Meeting. As of the Record Date, 38,506,650                  shares of Common Stock were outstanding and entitled to be voted at the Annual Meeting.

How to vote

Submitting a proxy via mail, the Internet or telephone

You may vote by calling the toll-free telephone number listed on the proxy card or visiting the website address listed on the proxy card. If you choose to submit your proxy with voting instructions by telephone or through the Internet, you will be required to provide your assigned control number noted on the Notice before your proxy will be accepted. In addition to the instructions that appear on the Notice,step-by-step instructions will be provided by recorded telephone message or at the designated website on the Internet. Votes submitted by telephone or via the Internet must be received by 11:59 p.m., Eastern Time, on Wednesday, August 29, 2018September 25, 2019 in order for them to be counted at the Annual Meeting.

If you are a stockholder of record, or otherwise received a printed copy of the proxy materials, in addition to the methods described above, you may also submit your proxy with voting instructions by mail by following the instructions set forth on the proxy card included with the proxy materials. Specifically, if you are a stockholder of record on the Record Date, you may vote by mailing your proxy card, with voting instructions, to the address listed on your proxy card.

Voting your shares in person at the Annual Meeting

ForSharesDirectlyRegisteredintheNameofthe Stockholder:Stockholder: You may vote in person at the Annual Meeting; however, we encourage you to vote by proxy card, via the Internet or by telephone even if you plan to attend the meeting. If you plan to attend the Annual Meeting, you will need to bring government issued identification.

ForSharesRegisteredintheNameofaBrokerageFirmor Bank:Bank: In order to vote at the Annual Meeting, you must contact the brokerage firm or bank in whose name your shares are registered to obtain a proxy and bring it to the Annual Meeting. In order to attend the Annual Meeting you will need to bring an account statement or other acceptable evidence of ownership of Common Stock as of the close of business on July 23, 2018.August 9, 2019.

Revoking a proxy

A proxy that was submitted via the Internet, by telephone or by mail may be revoked at any time before it is exercised by (1) executing a later-dated proxy card via the Internet or by telephone; (2) subsequently sending another proxy bearing a later date; (3) attending the Annual Meeting and voting in person by ballot; or (4) giving written notice revoking the proxy to our Corporate Secretary at Altimmune, Inc., 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878.

If your shares are registered in the name of a brokerage firm or bank, you must contact your brokerage firm or bank to change your vote or obtain a proxy to vote your shares if you wish to cast your vote in person at the meeting.

Your attendance at the Annual Meeting in and of itself will not automatically revoke a proxy that was submitted via the Internet, by telephone or by mail.

Broker authority to vote

If your shares are held in a stock brokerage account or by a bank or other nominee, you are considered to be the beneficial owner of shares held in street name. These proxy materials are being forwarded to you by your broker or nominee, who is considered to be the holder of record with respect to your shares. As the beneficial owner, you have the right to direct your broker or nominee how to vote by filling out the voting instruction form provided by your broker or nominee. Telephone and Internet voting options may also be available to beneficial owners. As a beneficial owner, you are also invited to attend the Annual Meeting, but you must bring an account statement or other acceptable evidence of ownership of our Common Stock as of the close of business on July 23, 2018.August 9, 2019. You must obtain a proxy from the holder of record of your shares in order to vote in person at the Annual Meeting.

If your shares are held in street name, your broker or nominee will ask you how you want your shares to be voted. If you provide voting instructions, your shares must be voted as you direct. If you do not furnish voting instructions, one of two things can happen, depending upon whether a proposal is “routine.” Under the rules that govern brokers that have record ownership of shares beneficially owned by their clients, brokers have discretion to cast votes only on routine matters, such as the ratification of the appointment of independent registered public accounting firms, without voting instructions from their clients. Brokers are not permitted, however, to cast votes on“non-routine” matters without such voting instructions, such as the election of directors. A “brokernon-vote” occurs when a beneficial owner has not provided voting instructions and the broker holding shares for the beneficial owner does not vote on a particular proposal because the broker does not have discretionary voting power for that proposal. Proposal 2 Proposal 6, Proposal 7 and Proposal 86 are considered “routine” proposals for this purpose.

Quorum

A quorum is required for the conduct of business at the meeting. The presence at the meeting, in person or by proxy, of the holders of shares having a majority of the voting power represented by all outstanding shares entitled to vote generally for the election of directors will constitute a quorum, permitting us to conduct the business of the meeting. Proxies received but marked as abstentions, if any, and brokernon-votes (as described above) will be included in the calculation of the number of shares considered to be present at the meeting for quorum purposes. If we do not have a quorum, we may adjourn the Annual Meeting to a later date.

Votes necessary to approve each proposal

Electionof Directors.Directors. The election of directors pursuant to Proposal 1 will require the affirmative vote of the plurality of the shares of Common Stock voted at the Annual Meeting, either in person or by proxy. This means that the seven nominees receiving the most votes will be elected as directors. With respect to Proposal 1, you may vote in favor of a particular nominee or elect to withhold your vote from a particular nominee.

Other Items.Items. For the ratification of our independent registered public accounting firm (Proposal 2), the adoption of a resolution approving on anon-binding, advisory basis the compensation of the Company’s named executive officers (Proposal 3), the approval of the amendment to the 2017 Incentive Plan2019 ESPP (Proposal 4), and the approval of the issuance of our Common Stock pursuant to certain exchange agreementsfor purposes of complying with Nasdaq Listing Rules 5635(a) and 5635(b) (Proposal 5), and approval of the authorization to adjourn the Annual Meeting (Proposal 8)7), the affirmative vote of a majority of the votes cast is required to approve each of these proposals. This means that the number of shares voted “for” the proposal must exceed the number of shares voted “against” the proposal. Abstentions and brokernon-votes are not considered votes cast for the forgoing purpose, and will have no effect on Proposal 3, Proposal 4 or Proposal 5. Proposal 2 and Proposal 86 are considered “routine” for these purposes. Therefore, if your shares are held by your bank, broker or other nominee in street name and you do not vote your shares, your bank, broker or other nominee may vote your shares on Proposal 2 and Proposal 6. Accordingly, abstentions will have no effect on Proposal 2 or Proposal 8,6 and there will be no brokernon-votes on these proposals. In addition, pursuantPursuant to the Marketplace Rules of the Nasdaq Listing Rule 5635(d) and applicable guidance,Stock Market, the Investorsshares of our Common Stock issued to Spitfire Equityholders (as defineddescribed below) are not entitled to vote on Proposal 5 and are not counted in determining votes cast for purposes of Proposal 5.

Where can I find more information about the terms of the Spitfire Acquisition?

We are including in this Proxy Statement a summary of the material terms of the acquisition (the “Spitfire Acquisition”) of Spitfire Pharma, Inc., a Delaware corporation (“Spitfire”) because we believe an understanding of the Spitfire Acquisition is necessary in order to make an informed voting decision with respect to the potential issuance of our securities in connection with Milestone Payments (as defined below) that may become due in the future under the terms of the Merger Agreement (as defined below). We are not seeking stockholder approval or ratification of the Spitfire Acquisition because the transaction has been consummated and the issuance of the consideration paid at closing did not require stockholder approval. Your vote will determine whether we will have the ability to elect to pay certain Milestone Payments that are earned in the future in Common Stock (or if we will instead be required to make such Milestone Payments in cash). A summary of the terms of the Spitfire Acquisition is set forth below:

To better understand Proposal 5 and the Spitfire Acquisition, you should carefully read this entire document and the other documents to which we refer. For a more detailed discussion of the Spitfire Acquisition, please see the section entitled “Proposal 5 —APPROVAL OF THE ISSUANCE OF OUR COMMON STOCK IN CONNECTION WITH MILESTONE PAYMENTS THAT MAY BECOME PAYABLE IN THE FUTURE TO FORMER SECURITYHOLDERS OF SPITFIRE PHARMA, INC. PURSUANT TO NASDAQ LISTING RULES 5635(A) AND 5635(B).”

Why is stockholder approval necessary in order to pay Milestone Payments in stock?

Our Common Stock is listed on the Nasdaq Global Market, and we are subject to the Nasdaq listing standards set forth in its Marketplace Rules. Although we were not required to obtain stockholder approval in

connection with the issuance of the Closing Consideration because the shares of our Common Stock that have been issued pursuantat closing constituted less than 20% of our outstanding shares, we are required under Marketplace Rules 5635(a) and 5635(b) to certain Exchange Agreements (as defined below in Proposal 5) with respect to Proposal 5.

Forseek stockholder approval for the approvalissuance of an amendment to our Amended and Restated Certificate of Incorporation to increase our authorized shares of Common Stock (Proposal 6) orin connection with the Milestone Payments as further described below.

Marketplace Rule 5635(a) requires stockholder approval prior to effect the reverse stock split (Proposal 7),issuance of securities in connection with the affirmative

Postponement or adjournment of the Annual Meeting

Your proxy may be voted at the postponed or adjourned meeting. You will still be able to change your proxy until it is voted.

Results of the voting at the Annual Meeting

Preliminary voting results will be announced at the Annual Meeting. Final voting results will be published in a Current Report on Form8-K, or Form8-K, that we expect to file with the U.S. Securities and Proposal 7.Exchange Commission (“SEC”) within four business days after the Annual Meeting. If final voting results are not available to us in time to file a Form8-K within four business days after the Annual Meeting, we intend to file a Form8-K to publish preliminary results and, within four business days after the final results are known to us, file an additional Form8-K to publish the final results.

Multiple proxy cards or voting instruction forms

If you receive multiple proxy cards, it means that you have multiple accounts at the transfer agent or with brokers. Please complete and return all proxy cards or voting instruction forms to ensure that all of your shares are voted.

Certain stockholder-related matters

We do not know of any stockholder proposals that may be properly presented at the Annual Meeting. For information regarding inclusion of stockholder proposals in our 2019 Annual Meeting of Stockholders Proxy Statement, see the information in this Proxy Statement under the section headingOtherMatters—Stockholderproposalsfor2019Annual Meeting.Meeting.

About the MergerAltimmune Mergers

On May 4, 2017, we completed our merger with privately-held Altimmune, Inc. (“PrivateAltimmune”) in accordance with the terms of the Agreement and Plan of Merger and Reorganization, dated January 18, 2017 (as amended, the “AltimmuneMergerAgreement”), whereby one of our wholly owned subsidiaries merged with and into Private Altimmune, with Private Altimmune surviving as our wholly owned subsidiary (the “AltimmuneMerger”). In connection with the Altimmune Merger, we changed our name from PharmAthene, Inc. to Altimmune, Inc. In connection with the closing of the Altimmune Merger, our Common Stock began trading on The NASDAQNasdaq Global Market under the ticker symbol “ALT.”

Pursuant to the terms of the Merger Agreement, our former executive officers and several of the members of our Board resigned, and our current executive officers and new members of our Board were appointed to their applicable offices, concurrent with the closing of the Merger. Throughout this Proxy Statement we primarily discuss our current executive officers and members of our Board, but in some contexts our former executive officers and Board members prior to the Merger are also discussed.

Also on May 4, 2017, in connection with and prior to the completion of the Merger, we effecteda 1-for-10 reverse stock split of our Common Stock (the “Reverse Stock Split”). Under the terms of the Merger Agreement, we issued shares of our Common Stock to Private Altimmune’s stockholders, at an exchange rate of 0.749106 shares of Common Stock in exchange for each share of Private Altimmune Common Stock outstanding immediately prior to the Merger. We also assumed all of Private Altimmune’s outstanding stock options and warrants, with such stock options and warrants going forward representing the right to purchase a number of shares of our Common Stock equal to 0.749106 multiplied by the number of shares of Private Altimmune’s Common Stock previously represented by such stock options and warrants, as applicable.

Except where the context indicates otherwise, references to “we,” “us,” “our,” “Altimmune” or the “Company” refer, for periods prior to the completion of the Altimmune Mergers, to Private Altimmune and its subsidiaries, and for periods following the completion of the Mergers to the combined company and its subsidiaries.

AVAILABILITY OF CERTAIN DOCUMENTS

Householding of Annual Meeting materials

The Company and some banks, brokers and other nominee record holders may participate in the practice of “householding” proxy statements and their accompanying documents. This means that only one copy of our Proxy Statement is sent to multiple stockholders in your household. We will promptly deliver a separate copy of these documents to you upon written or oral request to our Investor Relations Department at Altimmune, Inc., 901 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878 or (240)654-1450. If you want to receive separate copies of our proxy statements in the future, or if you are receiving multiple copies and would like to receive only one copy per household, you should contact your bank, broker or other nominee record holder, or you may contact us at the above address and phone number.

Additional information

We are required to file annual, quarterly and current reports, proxy statements and other reports with the U.S. Securities and Exchange Commission (“SEC”).SEC. Copies of these filings are available through our Internet website atwww.altimmune.comwww.altimmune.com or the SEC’s website atwww.sec.govwww.sec.gov. We will furnish copies of our SEC filings (without exhibits), including our Annual Report for the year ended December 31, 2017,2018, without charge to any stockholder upon written or oral request to our Investor Relations Department at Altimmune, Inc., 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878 or (240)654-1450.

If you have any questions or require any assistance with voting your shares, please contact our proxy solicitor at the contact listed below:

INNISFREE M&A INCORPORATED

Stockholders call toll free: (888)750-5834

Banks and brokers call collect: (212)750-5833

PROPOSAL 1 — ELECTION OF DIRECTORS

The first proposal to be voted on at the Annual Meeting is the election of directors. Our Board currently consists of seven directors: Mitchel Sayare, Ph.D. (Executive Chairman)(Chairman), William J. Enright,Vipin K. Garg, Ph.D., David J. Drutz, M.D., John M. Gill, Philip L. Hodges, Wayne Pisano and Klaus O. Schafer, M.D., MPH and Derace L. Shaffer, M.D. Mr. EnrightMPH. Dr. Garg is also our President and CEO,Chief Executive Officer and our other six directors are not our employees. As previously disclosed, Dr. Shaffer is not standing for reelection as a member of the Board, and his term will expire at the conclusion of the Annual Meeting. The Board has nominated Wayne Pisano for election to the Board, whose Board service would commence upon his election at the Annual Meeting. The directors and director nominees will be elected for a term that begins at the Annual Meeting and ends at the 20192020 Annual Meeting of Stockholders. Each director and director nominee will hold office until such director’s successor has been elected and qualified, or until such director’s earlier resignation or removal.

Vote Required

If you sign the enclosed proxy card and return it to the Company, your proxy will be voted FOR all director nominees, for terms expiring at the 20192020 Annual Meeting of Stockholders, unless you specifically indicate on the proxy card that you are withholding a vote from one or more of the nominees. Abstentions and brokernon-votes will have no effect on the election of our director nominees.

Directors are elected by a plurality of the votes cast. Accordingly, the seven nominees receiving the highest number of affirmative votes will be elected. Shares represented by executed proxies will be voted, if authority to do so is not withheld, for the election of the seven nominees named below. If any nominee becomes unavailable for election as a result of an unexpected occurrence, your shares will be voted for the election of a substitute nominee proposed by the Board. Each person nominated for election has agreed to serve if elected. Our management has no reason to believe that any nominee will be unable to serve.

Board recommendation

THE BOARD RECOMMENDS THAT STOCKHOLDERS VOTE “FOR” THE ELECTION OF EACH OF THE FOLLOWING NOMINEES:

Mitchel Sayare, Ph.D., Executive Chairman

William J. EnrightVipin K. Garg, Ph.D.

David J. Drutz, M.D.

John M. Gill

Philip L. Hodges

Wayne Pisano

Klaus O. Schafer, M.D., MPH

Each of these nominees except for Mr. Pisano, is currently serving as a director on our Board, and each nominee has agreed to continue to serve on the Board if he or she is elected at the Annual Meeting. If any nominee is unable (or for good cause declines) to serve as a director at any time before the Annual Meeting, proxies may be voted for the election of a qualified substitute designated by the current Board, or else the size of the Board will be reduced accordingly. Biographical information about each of the nominees is included below underDirectorinformation.

Qualifications for director nominees

Candidates for Board consideration should have experience in positions with a high degree of responsibility, be leaders in the companies or institutions with which they are affiliated, and will be selected based upon contributions they can make to the Company. A director must have an exemplary reputation and record for honesty in his or her personal dealings and business or professional activity. All directors must demonstrate

be committed first and foremost to the interests of the stockholders of the Company. Persons who represent a particular special interest, ideology, narrow perspective or point of view would not, therefore, generally be considered good candidates for election to our Board. The key experience, qualifications and skills each of our directors brings to the Board that are important in light of our business are included in their individual biographies below.

Our Corporate Governance Guidelines require the Board’s Nominating and Corporate Governance Committee to review the qualifications of the directors and the composition of the Board as a whole, and the Nominating and Corporate Governance Committee seeks members from diverse professional backgrounds who combine a broad spectrum of experience and expertise with a reputation for integrity. This assessment includes not only the independence of the directors, but consideration of required minimum qualifications, skills, expertise and experience in the context of the needs of the Board and its ability to oversee the Company’s business.

Director information

At the recommendation of the Nominating and Corporate Governance Committee, the Board has nominated the persons named below to serve as directors of the Company for a term beginning at the Annual Meeting and ending at the 20192020 Annual Meeting of Stockholders. The date of commencement of service as a director refers to the Board of Directors of PharmAthene, Inc. prior to the closing of the Mergers in May 2017, and the Board of the Company from and after the closing of the Mergers.

CORPORATE GOVERNANCE AND BOARD MATTERS

Director independence

The Board of Directors has determined that each of our current directors, other than Drs. DrutzSayare and Schafer,Garg and Mr. Hodges, and our director nominee Mr. Pisano,Gill, currently meet, the independence requirements contained in the NASDAQNasdaq listing standards and applicable tax and securities rules and regulations. None of these threenon-employee directors or our director nomineenominees has a relationship with the Company or its subsidiaries that would interfere with the exercise of independent judgment in carrying out the responsibilities of a director.

In compliance with the NASDAQNasdaq listing standards, we have a Board of Directors comprised of a majority of independent directors. The NASDAQNasdaq listing standards have both objective tests and a subjective test for determining who is an “independent director.” The objective tests state, for example, that a director is not considered independent if he is an employee of the Company or is a partner in or controlling stockholder or executive officer of an entity to which the Company made, or from which the Company received, payments in the current or any of the past three fiscal years that exceed 5% of the recipient’s consolidated gross revenue for that year. The subjective test states that an independent director must be a person who lacks a relationship that, in the opinion of the Board, would interfere with the exercise of independent judgment in carrying out the responsibilities of a director.

Other than Dr. Sayare and Mr. Gill, none of thenon-employee directors nor our director nominee were disqualified from “independent” status under the objective tests. In assessing independence under the subjective test, the Board took into account the standards in the objective tests, and reviewed and discussed additional information provided by the directors and nominee with regard to each director’s business and personal activities as they may relate to Altimmune’s management. Based on all of the foregoing, as required by the NASDAQNasdaq listing standards, the Board made a substantive determination as to each of thenon-employee directors other than Dr. Sayare, that no relationship exists which, in the opinion of the Board, would interfere with the exercise of independent judgment in carrying out the responsibilities of a director.

The Board has not established categorical standards or guidelines to make these subjective determinations, but considers all relevant facts and circumstances.

In addition to Board-level standards for director independence, the directors who serve on the Audit Committee and the Compensation Committee each satisfy standards established by the SEC and the NASDAQ listing rulesNasdaq Listing Rules providing that to qualify as “independent” for purposes of membership on the Audit Committee or the Compensation Committee, members of such committees may not accept directly or indirectly any consulting, advisory or other compensatory fee from the Company other than their director compensation. Also, each of the directors who serve on the Compensation Committee has been determined to be a“non-employee director” for purposes of the applicable SEC rules and regulations and an “outside director” for purposes of the applicable tax rules.

In making its independence determinations, the Board considered transactions occurring since the beginning of 20152016 between the Company and entities associated with the independent directors or members of their immediate family. In each case, the Board determined that, because of the nature of the director’s relationship with the entity and/or the amount involved, the relationship did not impair the director’s independence.

We do not have a director tenure requirement, as we believe our efforts to regularly refresh the Board with new directors, as well as natural turnover, has achieved the appropriate balance between maintaining longer-term directors with deep institutional knowledge and new directors who bring new perspectives and diversity to our Board. Notwithstanding this belief and the fact that our corporate governance guidelines and NASDAQNasdaq Global Market rules do not deem long-tenured directors to benon-independent, our Board reviews director tenure in connection with its director independence determinations.

How nominees to our Board are selected

Candidates for election to our Board of Directors are nominated by our Nominating and Corporate Governance Committee and ratified by our full Board of Directors for nomination to the stockholders. The Nominating and Corporate Governance Committee operates under a charter, which is available on our corporate website atwww.altimmune.comwww.altimmune.com.

The Nominating and Corporate Governance Committee will give due consideration to candidates recommended by stockholders. Stockholders may recommend candidates for the Nominating and Corporate Governance Committee’s consideration by submitting such recommendations directly to the Nominating and Corporate Governance Committee as described below underCommunicatingwithourBoardmembers. In making recommendations, stockholders should be mindful of the discussion of minimum qualifications set forth above underQualificationsfordirectornominees. However, just because a recommended individual meets the minimum qualification standards does not imply that the Nominating and Corporate Governance Committee will necessarily nominate the person so recommended by a stockholder. The Nominating and Corporate Governance Committee may also engage outside search firms to assist in identifying or evaluating potential nominees.

Board leadership structure

Currently, Dr. Sayare serves as the Executive Chairman of the Board and Mr. EnrightDr. Garg is the Company’s President and CEO.Chief Executive Officer. The Board believes that having different individuals serving in the separate roles of Chairman of the Board and CEOChief Executive Officer is in the best interest of stockholders in the Company’s current circumstances because it reflects the CEO’sChief Executive Officer’s responsibility over management of the Company’s operations and the Executive Chairman’s oversight of board functions, strategic development and financial stability.

Board committees

The Audit Committee of our Board reviews, acts on and reports to our Board with respect to various auditing and accounting matters, including the recommendation of our independent registered public accounting firm, the scope of the annual audits, the fees to be paid to the independent registered public accounting firm, the performance of the independent registered public accounting firm and our accounting practices. The Audit Committee currently consists of Messrs. Hodges (Chair) and, Gill and Drs. SchaferPisano and Shaffer. Dr. Schaffer’s term on the Audit Committee will come to an end after the conclusion of the Annual Meeting.Schafer. The Board has determined that each member of the Audit Committee, other than Mr. Gill, is an independent director in accordance with NASDAQNasdaq listing standards and that each of Messrs. Hodges and Gill is an Audit Committee financial expert, as defined by SEC guidelines and as required by the applicable NASDAQNasdaq listing standards. Mr. Gill is not independent under the NASDAQNasdaq listing standards due to his service as Chief Executive Officer of PharmAthene prior to the Merger. However, the Board has determined that Mr. Gill satisfies the specific independence criteria set forth in Section 10A(m)(3)(B) of the Securities Exchange Act of 1934, as amended (the “ExchangeAct”) and the rules thereunder, and that his membership on the Audit Committee is in the best interest of the Company and its stockholders due to Mr. Gill’s substantial experience and expertise in financial accounting, internal control and audit committee processes. For information regarding the experience and qualifications of our Audit Committee financial experts, see the information in this Proxy Statement under the section headingProposal1—ElectionofDirectors—Directorinformation.

The Compensation Committee of the Board recommends, reviews and oversees the salaries, benefits and equity incentive plans for our employees, consultants, directors (other thannon-employee directors) and other individuals whom we compensate. The Compensation Committee also administers our compensation plans. The Compensation Committee currently consists of Drs. Drutz (Chair) and Schaffer,Schafer and Mr. Hodges. Dr. Schaffer’s term on the Compensation Committee will come to an end after the conclusion of the Annual Meeting. The Board has determined that each member of the Compensation Committee is an “independent director” in accordance with NASDAQNasdaq listing standards, a“non-employee director” under the applicable SEC rules and

The Nominating and Corporate Governance Committee of the Board selects nominees for director positions to be recommended by our Board for election as directors and for any vacancies in such positions, develops and recommends for our Board the Corporate Governance Guidelines of the Company and oversees the annual review of the performance of the Board, each director and each committee. The Nominating and Corporate Governance Committee currently consists of Mr. HodgesMessrs. Pisano (Chair), and Drs. DrutzGill and Schafer.Dr. Drutz. The Board has determined that each member of the Nominating and Corporate Governance Committee, other than Mr. Gill, is an independent director in accordance with NASDAQNasdaq listing standards. As described above, Mr. Gill is not independent under the Nasdaq listing standards due to his service as Chief Executive Officer of PharmAthene prior to the Merger. However, the Board has determined that Mr. Gill’s membership on the Nominating and Corporate Governance Committee satisfies the standards set out in Nasdaq Rule 5605(e)(3) fornon-independent committee members and is in the best interest of the Company and its stockholders due to Mr. Gill’s broad experience in the pharmaceutical industry.

Meetings and attendance

During the fiscal year ended December 31, 2017,2018, the Board held nine19 meetings and the Board Committees held a total of 1210 meetings. Each incumbent director attended all75% or more of the total number of meetings of the Board and the Board Committees of which he was a member during the period he served as a director in fiscal year 2017.2018. The Company has no specific policy regarding director attendance at its Annual Meeting. Generally, however, Board meetings are held immediately preceding and following the Annual Meeting, with directors attending the Annual Meeting. Our 20172018 Annual Meeting was attended by all of our directors.

Non-Management Director Meetings

In addition to the meetings of the committees of the Board of Directors described above, in connection with the Board of Directors’ meetings, thenon-management directors met 20 times in executive session during the fiscal year ended December 31, 2018. The Chairman of the Board of Directors presides at these executive sessions.

Board involvement in risk oversight

The Company’s management is responsible for defining the various risks facing the Company, formulating risk management policies and procedures, and managing the Company’s risk exposures on aday-to-day basis. The Board’s responsibility is to monitor the Company’s risk management processes by informing itself of the Company’s material risks and evaluating whether management has reasonable controls in place to address the material risks. The Board is not responsible, however, for defining or managing the Company’s various risks.

The Board of Directors monitors management’s responsibility for risk oversight through regular reports from management to the Audit Committee and the full Board. Furthermore, the Audit Committee reports on the matters discussed at the committee level to the full Board. The Audit Committee and the full Board focus on the material risks facing the Company, including strategic, operational, legal and regulatory risks, to assess whether management has reasonable controls in place to address these risks. In addition, the Compensation Committee is charged with reviewing and discussing with management whether the Company’s compensation arrangements are consistent with effective controls and sound risk management. Finally, risk management is a factor that the Board and the Nominating and Corporate Governance Committee consider when determining who to nominate for election as a director of the Company and which directors serve on the Audit Committee. The Board believes this division of responsibilities provides an effective and efficient approach for addressing risk management.

Code of Business Conduct and Ethics and other governance documents

The Board has adopted a Code of Business Conduct and Ethics (the “CodeofEthics”) that applies to all officers, directors and employees and consultants. The Code of Ethics, as well as any amendments to, or waivers under, the Code of Ethics as it applies to the Company’s officers, can be accessed in theInvestorRelations—CorporateGovernancesection of our website atwww.altimmune.comwww.altimmune.com.

You may also obtain a copy of these documents by writing to Altimmune, Inc., 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878, Attention: Investor Relations.

Copies of the charters of our Board’s Audit Committee, Compensation Committee, and Nominating and Corporate Governance Committee, as well as a copy of the Company’s Corporate Governance Guidelines, can be accessed in theInvestorRelations—CorporateGovernance section of our website.

PROPOSAL 2 — RATIFICATION OF SELECTION OF INDEPENDENT REGISTERED PUBLIC

ACCOUNTING FIRM

The Audit Committee of our Board has appointed the firm of Ernst & Young LLP (“E&Y”) as our independent registered public accounting firm to audit our consolidated financial statements for the year ending December 31, 2018,2019, and the Board is asking stockholders to ratify that selection. Although current law, rules and regulations, as well as the charter of the Audit Committee, require our independent registered public accounting firm to be engaged, retained and supervised by the Audit Committee, the Board considers the selection of our independent registered public accounting firm to be an important matter of stockholder concern and considers a proposal for stockholders to ratify such selection to be an important opportunity for stockholders to provide direct feedback to the Board on an important issue of corporate governance. In the event that stockholders fail to ratify the appointment, the Audit Committee will reconsider whether or not to retain E&Y, but may ultimately determine to retain E&Y as our independent registered public accounting firm. Even if the appointment is ratified, the Audit Committee, in its sole discretion, may direct the appointment of a different independent registered public accounting firm at any time during the year if the Audit Committee determines that such a change would be in the best interests of the Company and its stockholders.

Vote Required

Unless proxy cards are otherwise marked, the persons named as proxies will vote FOR the ratification of E&Y as the Company’s independent registered public accounting firm for the year ending December 31, 2018.2019. Approval of this proposal requires the affirmative vote of a majority of the votes cast on the proposal. Abstentions will have no effect on Proposal 2. Because Proposal 2 is considered “routine” for these purposes, there will not be any brokernon-votes for this proposal.

Board recommendation

THE BOARD UNANIMOUSLY RECOMMENDS THAT STOCKHOLDERS VOTE “FOR” RATIFICATION OF ERNST & YOUNG LLP AS THE COMPANY’S INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM FOR THE YEAR ENDING DECEMBER 31, 2018.2019.

Information about our independent registered public accounting firm

E&Y served as the principal accountant for the audit of PharmAthene’s consolidated financial statements for the fiscal year ended December 31, 2016 and BDO LLP (“BDOLLP”) served as the principal accountant for the audit of Private Altimmune’s consolidated financial statements for the fiscal year ended December 31, 2016. On June 22, 2017 (the “EngagementDate”), the Company engaged E&Y to serve as the independent registered public accounting firm for the combined Company for the year ending December 31, 2017. Representatives of E&Y will be present at our Annual Meeting, will have the opportunity to make a statement if they desire to do so, and will be available to respond to appropriate questions from stockholders.

Change in Accountants and Engagement of E&Y

During the fiscal years ended December 31, 20172018 and 2016,2017, E&Y’s reports on the Company’s financial statements did not contain an adverse opinion or disclaimer of opinion, and were not qualified or modified as to uncertainty, audit scope or accounting principles, except that eachthe report on the Company’s financial statements for the fiscal year ended December 31, 2017 contained a modification to the effect that there was substantial doubt as to the Company’s ability to continue as a going concern.

During the fiscal years ended December 31, 20172018 and 20162017 (i) there were no disagreements between the Company and E&Y on any matter of accounting principles or practices, financial statement disclosure, or

auditing scope or procedure which, if not resolved to the satisfaction of E&Y, would have caused E&Y to make reference to the subject matter of the disagreement in connection with its reports on the Company’s financial statements, and (ii) there were no reportable events as that term is described in Item 304(a)(1)(v) of RegulationRegulation S-K.

Immediately following the completion of each fiscal year, the Company’s independent registered public accounting firm shall submit to the Audit Committee (and the Audit Committee shall request from the

Company’s senior management, including particularly its senior financial management, to prepare financial statements with integrity and objectivity and in accordance with generally accepted accounting principles, and relies upon the Company’s independent registered public accounting firm to review or audit, as applicable, such financial statements in accordance with the standards of the Public Company Accounting Oversight Board (United States) (“PCAOB”).

PROPOSAL 3 — ADVISORY VOTE ON EXECUTIVE COMPENSATION

In accordance with the requirements of Section 14A of the Securities Exchange Act of 1934 (which was added by the Dodd-Frank Wall Street Reform and Consumer Protection Act and the related rules of the SEC (the “Dodd-FrankAct”)), the Company is providing its stockholders the opportunity to cast an advisory vote on the compensation of its named executive officers. This proposal, commonly known as a“say-on-pay” proposal, gives the Company’s stockholders the opportunity to express their views on the named executive officers’ compensation. At our annual meeting of stockholders in 2017,2018, our stockholders overwhelmingly voted in favor of an annual vote on this proposal. Accordingly, we will include an advisory vote on executive compensation on an annual basis at least until the next stockholder advisory vote on the frequency of such votes.

The Company’s post-Merger named executive officer compensation program is designed to attract, reward and retain the caliber of officers needed to ensure the Company’s continued growth and profitability. The primary objectives of the program are to:

The Company seeks to accomplish these goals in a manner that is aligned with the long-term interests of the Company’s stockholders. The Company believes that its named executive officer compensation program is designed to achieve this goal with its emphasis on long-term equity awards and performance-based compensation, in addition to short-term (annual) incentive awards, specifically cash incentives, which are intended to enable the Company to successfully motivate and reward its named executive officers. The Company believes that its ability to retain its named executive officers is critical to its continuing financial success and that its focus on the long-term interests of its named executive officers aligns with the interests of its stockholders. For these reasons, the Board recommends a vote in favor of the following resolution:

“RESOLVED, that the compensation paid to the Company’s named executive officers, as disclosed in Part III of the Company’s annual report onForm 10-K for the year ending December 31, 2018 and its proxy statement for the 20182019 Annual Meeting, pursuant to the compensation disclosure rules of the Securities and Exchange Commission, including the compensation tables and narrative discussion thereto, is hereby APPROVED.”

As an advisory vote, this proposal is not binding upon the Company, our Board or our Compensation Committee. Notwithstanding the advisory nature of this vote, our Board and the Compensation Committee, which is responsible for designing and administering the Company’s named executive officer compensation program, value the opinions expressed by stockholders in their vote on this proposal, and will consider the outcome of the vote when making future compensation decisions for named executive officers. Approval of this proposal requires the affirmative vote of a majority of the votes cast on this Proposal 3.

THE BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT STOCKHOLDERS VOTE “FOR” THE APPROVAL, ON AN ADVISORY BASIS, OF THE COMPENSATION OF THE COMPANY’S NAMED EXECUTIVE OFFICERS AS DISCLOSED IN THIS PROXY STATEMENT.

PROPOSAL 4 — APPROVAL OF THE ADOPTION OF AN AMENDMENT TO THE ALTIMMUNE, INC. 2017 OMNIBUS INCENTIVE2019 EMPLOYEE STOCK PURCHASE PLAN

Stockholders are being asked to approve an amendment (the “Amendment”) tothe adoption of the Altimmune, Inc. 2017 Omnibus Incentive2019 Employee Stock Purchase Plan (the “2017 Incentive Plan2019 ESPP”). Our Board approved the Amendment on July 23, 2018, subject to, and to be effective upon, the approval of the Amendment by our stockholders at the 2018 Annual Meeting. The Amendment, among other things, modifies the 2017 Incentive Plan’s “evergreen” provision to remove the 1,000,000 share maximum amount of the automatic annual increase in the share reserve thereunder. As modified, the aggregate number of shares authorized for issuance under the 2017 Incentive Plan will be automatically increased each year beginning on January 1, 2019 by 4% of the number of shares of Common Stock outstanding on a fully diluted basis as of the immediately preceding December 31, or such lesser number of shares determined, which was previously adopted by our Board of Directors.

Currently, the “evergreen” provision under the 2017 Incentive Plan provides that the aggregate number of shares authorized for issuance under the 2017 Incentive Plan is automatically increased each year beginningDirectors on January 1, by an amount equal to the least of: (i) 1,000,000 shares of Common Stock; (ii) 4% of the number of shares of Common Stock outstanding on a fully diluted basis as of the immediately preceding December 31, or (iii) such lesser number of shares determined by our Board of Directors. As described above, the Amendment removes the 1,000,000 share maximum amount of such annual automatic increase.March 29, 2019. We believe that the current 1,000,000 share maximum automatic annual increase in the share reserve is limiting and would result in insufficient shares to meet our future needs with respect to attracting, motivating and retaining key executives and employees in a competitive market for talent. If the Amendment is not approved, we may be unable in the future to continue to offer competitive equity packages to attract and retain employees, and we may need to consider other compensation alternatives.

As a resultadoption of the Tax Cuts and Jobs Act, effective January 1, 2018, the exception under Section 162(m) for certain performance-based compensation has been eliminated for taxable years beginning after December 31, 2017 (other than with respect to certain compensation payable under a written binding contract that was in effect on November 2, 2017). As a result, Section 162(m) of the Code no longer operates as a limit to the 2017 Incentive Plan. For that reason, the Amendment also provides for the elimination of the annual award share and dollar limits in the 2017 Incentive Plan (other than with respect to the annual share limit applicable tonon-employee directors), because these limitations were included solely for purposes of qualifying applicable awards for the performance based compensation exception to Section 162(m) of the Code. Thus, if the Amendment is approved, we may make awards in the future under the 2017 Incentive Plan that would not be restricted2019 ESPP will benefit us by these annual award limits (other than with respect to the annual share limit applicable tonon-employee directors).

Finally, the Amendment provides for the increase in the number of shares of Common Stock that may be issued under the 2017 Incentive Plan in respect of incentive stock options from 1,500,000 shares to 5,000,000 shares. We believe that the current 1,500,000 share maximum for incentive stock options (ISO), which option type providesproviding employees with an opportunity to receive a more favorable tax treatment if all statutory requirements are met, is limitingacquire shares of our Common Stock and would result in insufficient ISO shareswill enable us to meetattract, retain and motivate valued employees.

Based solely on the closing price of our future needs for attracting, motivating and retaining employees in a competitiveCommon Stock reported on the Nasdaq Global Market on August     , 2019, the maximum aggregate market for talent. All other existing provisionsvalue of the 2017 Incentive Plan remain in effect.

We designed403,500 shares of Common Stock initially reserved for issuance under the 2017 Incentive Plan to reflect strong equity plan governance practices. The 2017 Incentive Plan has a number of features intended to address stockholder concerns related to equity plans, including:

and August 1 and terminating on the following July 31 or January 31, respectively. The Administrator may establish different offering periods or exercise dates under the 2019 ESPP, provided that no offering period may exceed 27 months in duration. The initial offering period under the ESPP commenced August 1, 2019 and will end on January 31, 2020; provided that no exercise shall occur unless the ESPP is approved by the stockholders.

Exercise Price. On the first day of an offering period, employees participating in that offering period will receive an option to purchase shares of our Common Stock, which will be automatically exercised on the last day of the purchase period. The option exercise price is equal to the lesser of (i) 85% (or such greater percentage as determined by the Administrator) of the fair market value per share of our Common Stock on the first day of the offering period or (ii) 85% (or such greater percentage as determined by the Administrator) of the fair market value per share of our Common Stock on the exercise date.

In general, if an employee is no longer a participant on an exercise date, the employee’s option will be automatically terminated, and the amount of the employee’s accumulated payroll deductions will be refunded.

Terms of Participation. A participant may discontinue participation in the 2019 ESPP or may increase or decrease the rate of payroll deductions during the offering period by submitting notice of a change of status authorizing an increase or decrease in the payroll deduction rate. Any increase or decrease in payroll deductions shall be effective as soon as administratively practicable following the date of the acquisition eventrequest. The Administrator may be canceled by the compensation committee without consideration. Under the 2017 Incentive Plan, an “acquisition event” means (i) a merger or consolidation in which the Company is not the surviving entity, (ii) any transaction that results in the acquisition of all or substantially all of the Company’s outstanding Common Stock by a single person or group of persons, or (iii) the sale or transfer of all or substantially all of the Company’s assets.

Adjustments. In the event of a stock split, reverse stock split, dividend, recapitalization, merger, consolidation, spin off, reorganization, sale or transfer of all or part of the Company’s assets or business, or other certain corporate transactions having a similar effect to the foregoing, the board of directors will make such adjustments to the aggregate number of shares may be issued,limit the number of sharespayroll deduction rate changes during any offering period. A participant’s payroll deduction authorization shall remain in effect for successive offering periods and the Administrator shall be authorized to limit the number of payroll deduction rate changes during any offering period.

Term; Amendments and Termination. The Administrator may, in its discretion, at any time, terminate or other propertyamend the 2019 ESPP. Except as provided in the 2019 ESPP, such termination cannot adversely affect options previously granted.

New Plan Benefits

Since participation in the 2019 ESPP is voluntary and benefits under the 2019 ESPP depend on the fair market value of our Common Stock at various future dates and elections made by participants, the benefits or amounts that will or may be received by or allocated to employees under the 2019 ESPP in the future are not determinable.

Summary of Federal Income Tax Consequences

The following is only a summary of the effect of the United States income tax laws and regulations upon an employee and us with respect to an employee’s participation in the 2019 ESPP. This summary does not purport to be a complete description of all federal tax implications of participation in the 2019 ESPP, nor does it discuss the income tax laws of any municipality, state or foreign country in which a participant may reside or otherwise be subject to tax.

A participant in the 2019 ESPP is still subject to income tax on the amounts deducted from their base pay to participate in the 2019 ESPP, but recognizes no taxable income upon exercise of an award, theoption to purchase or exercise priceshares of awards, or the individual participant limits discussed above as are determined to be necessary or advisable to prevent substantial dilution or enlargement of the rights of participants under the 2017 Incentive Plan.

Non-Transferability of Awards. Except as the compensation committee may permit, at the time of grant or thereafter, awards granted under the 2017 Incentive Plan are generally not transferable by a participant other than by will or the laws of descent and distribution. Shares of the Company’sour Common Stock acquired by a permissible transferee will continue to be subject tounder the terms of the 2017 Incentive Plan and2019 ESPP. If a participant disposes of shares purchased upon exercise of an option granted under the 2019 ESPP within two years from the first day of the applicable award agreement.

Term. Awards underoffering period or within one year from the 2017 Incentive Plan may not be made after March 29, 2027, but awards granted priorexercise date, which we refer to such date may extend beyondas a “disqualifying disposition,” the participant will realize ordinary income in the year of that date.

Amendment and Termination. Subjectdisposition equal to the rules referredamount by which the fair market value of the shares on the date the shares were purchased exceeds the purchase price. The amount of ordinary income will be added to the participant’s basis in the balance of this paragraph,shares, and any additional gain or resulting loss recognized on the Company’s board of directors may at any time amend, in whole or in part, any or alldisposition of the provisions ofshares will be a capital gain or loss. A capital gain or loss will be long-term if the 2017 Incentive Plan,participant’s holding period is more than 12 months, or suspendshort-term if the participant’s holding period is 12 months or terminate it entirely, retroactively or otherwise. Except as required to comply with applicable law, no such amendment may reduce the rights of a participant with respect to awards previouslyless.

granted withoutIf the consent of such participant. In addition, without the approval of stockholders, no amendment may be made that would: (i) increase the aggregate numberparticipant disposes of shares purchased upon exercise of an option granted under the 2019 ESPP at least two years after the first day of the Company’sapplicable offering period and at least one year after the exercise date, the participant will realize ordinary income in the year of disposition equal to the lesser of (1) the amount by which the fair market value of the Common Stock that may be issued underon the 2017 Incentive Plan; (ii) increasefirst day of the maximum individual participant share limitations for a fiscal year or year of a performance period; (iii) changeoffering period in which the classification of individuals eligible to receive awards under the 2017 Incentive Plan; (iv) extend the maximum term of options; (v) alter the performance criteria; (vi) amend the terms of any outstanding stock option or other stock appreciation award to reduceshares were purchased exceeds the exercise price thereof (i.e., reprice); (vii) cancel any outstanding“in-the-money” stock(determined as if the option or other stock appreciation award in exchange for cash, other awards or stock option or other stock appreciation award with a lower exercise price; (viii) require stockholder approval in orderhad been exercised on the first day of the offering period) and (2) the excess of the amount actually received for the 2017 Incentive Plan to continue to comply with Section 422Common Stock over the amount paid. The amount of the Code; or (ix) require stockholder approval under the rules of any exchange or system on which the Company’s securities are listed or traded.

Federal Income Tax Implications of the 2017 Incentive Plan. The federal income tax consequences arising with respect to awards granted under the 2017 Incentive Plan will depend on the type of award. From the recipients’ standpoint, as a general rule, ordinary income will be added to the participant’s basis in the shares, and any additional gain recognized atupon the timedisposition after that basis adjustment will be a long-term capital gain. If the fair market value of paymentthe shares on the date of cash, or delivery of actual shares. Future appreciation on shares held beyonddisposition is less than the exercise price, there will be no ordinary income recognition eventand any loss recognized will be taxable ata long-term capital gains rates when the sharesloss.

Generally, we are sold. The Company, as a general rule, will be entitled to a tax deduction that corresponds in time and amountthe year of a disqualifying disposition equal to the amount of ordinary income recognized by the recipient, and the Company will not be entitled to any tax deduction in respect of capital gain income recognized by the recipient. Exceptions to these general rules may arise under the following circumstances: (i) if shares, when delivered, are subject to a substantial risk of forfeiture by reason of failure to satisfy any employment or performance-related condition, ordinary income taxation and the Company’s tax deduction will be delayed until the risk of forfeiture lapses (unless the recipient makes a special election to ignore the risk of forfeiture); (ii) if an employee is granted an ISO, no ordinary income will be recognized, and the Company will not be entitled to any tax deduction, if shares acquired upon exercise of the ISO are held longer than the later of one year from the date of exercise and two years from the date of grant; (iii) the Company may not be entitled to a tax deduction for compensation attributable to awards granted to certain current and former employees if and to the extent such compensation exceeds $1 million per taxable year; and (iv) an award may be taxable at 20% above ordinary income tax rates at the time it becomes vested, even if that is prior to the delivery of the cash or stock in settlement of the award, if the award constitutes “deferred compensation” under Section 409A of the Code, and the requirements of Section 409A of the Code are not satisfied.

The foregoing provides only a general description of the application of federal income tax laws to certain awards under the 2017 Incentive Plan, and is not intended as tax guidance to participants in the 2017 Incentive Plan, as the tax consequences may vary with the types of awards made, the identity of the recipients and the method of payment or settlement. This summary does not address the effects of other federal taxes (including possible “golden parachute” excise taxes) or taxes imposed under state, local, ornon-U.S. tax laws.

Future Plan Awards.The grant of additional stock-based awards under the 2017 Incentive Plan in the future and the nature of any such awards are subject to the discretion of the compensation committee (or, in the case of awards tonon-employee directors, the Board). Accordingly, other than the annual grants to ournon-employee directors, it is not possible to determine the number, amount and type of awards to be granted under the 2017 Incentive Planparticipant as a result of the proposed amendment (or that would have been granted in 2018 had this proposed amendment of the 2017 Incentive Plan then been in effect). Information regarding our recent practices with respect to annual and long-term incentive awards and stock-based compensation under the 2017 Incentive Plan is presented in the “Summary Compensation” and “Outstanding Equity Awards at Fiscal Year End��� tables.

The annual equity grants fornon-employee directors in connection with our Annual Meeting generally vest as follows: one half on or about the six month anniversary of the Annual Meeting and one half on or about the first anniversary of the Annual Meeting. The following table shows the aggregate amount of such awards. All

PROPOSAL 5 — APPROVAL OF THE ISSUANCE OF OUR COMMON STOCK IN CONNECTION WITH MILESTONE PAYMENTS THAT MAY BECOME PAYABLE IN THE FUTURE TO FORMER EQUITYHOLDERS OF SPITFIRE PHARMA, INC. PURSUANT TO NASDAQ LISTING RULES 5635(d)5635(A) AND 5635(b)5635(B)

Background

On June 22, 2018,July 8, 2019, the Company entered into substantially similar privately negotiated exchange agreementsan Agreement and Plan of Merger and Reorganization (the “First Exchange AgreementsMerger Agreement”) with certain investorsSpringfield Merger Sub, Inc., a Delaware corporation and wholly-owned subsidiary of the Company (“Merger SubI”), Springfield Merger Sub, LLC, a Delaware limited liability company and wholly-owned subsidiary of the Company (“Merger Sub II”), Spitfire Pharma, Inc., a Delaware corporation (“Spitfire”), and David Collier, as the stockholder representative. Spitfire was a preclinical pharmaceutical company developing a noveldual GLP-1/glucagon receptor agonist for the treatmentof non-alcoholic steatohepatitis.

On July 12, 2019 (the “First InvestorsClosing Date”). Pursuant to, Merger Sub I merged with and into Spitfire, with Spitfire continuing as the termssurviving entity (the “FirstMerger”), and, as a part of the same overall transaction, the surviving entity of the First Exchange Agreements,Merger merged with and into Merger Sub II, with Merger Sub II continuing as the Company (i) issued 2,560,693 shares of Common Stock, (ii) issued convertible notes (the “Exchange Notes”) with an aggregate principal value of $1,500,000, which are initially convertible into up to 2,205,883 shares of Common Stock upon the default by the Company, based onsurviving entity and a conversion price assuming conversionwholly-owned subsidiary of the Exchange Notes on the date the First Exchange Agreements were signed, subject to adjustment under certain circumstances in accordance with the terms of the Exchange Notes, and (iii) paid $1,100,000 in aggregate cash consideration, all in exchange for certain warrants to purchase shares of the Common Stock held by the First Investors.

On July 11, 2018, the Company entered into substantially similar privately negotiated exchange agreements (the “Second Exchange Agreements”) with certain investors (the “Second InvestorsMerger,” and, together with the First Investors,Merger, the “InvestorsMerger”). PursuantImmediately following the consummation of the Second Merger, the name of Merger Sub II was changed to “Spitfire Pharma, LLC”.

In connection with the closing, the Company issued 1,887,250 unregistered shares of its Common Stock as upfront consideration to certain former equityholders of Spitfire (collectively, the “Spitfire Equityholders”), representing an amount equal to $5.0 million less working capital and transaction expense adjustment amounts (the “Closing Consideration”). The number of shares issued as payment of the Closing Consideration was determined based on the average of the closing prices of the Company’s Common Stock as reported on the Nasdaq Global Market for the twenty (20) consecutive trading days prior to and including July 8, 2019, the date on which the parties entered into the Merger Agreement.

In addition, pursuant to the terms of the Second Exchange Agreements, the Company issued an aggregate of 963,711 shares of Common Stock to the Second Investors and paid $22,241 in cash in exchange for all of the shares of Series B Redeemable Convertible Preferred Stock held by the Second Investors. Subject to the approval by the Company’s stockholders of the issuance of the Company’s shares of Common Stock pursuant to the Second Exchange Agreements,Merger Agreement, the Company will issue an additional 4,351,136 shares of Common Stock at the second closingbe required to pay future contingent payments (the “Second ClosingMilestone Payments”) of the Second Exchange Agreementsup to $88.0 million in exchange for the warrants to purchase shares of Common Stock held by the Second Investors. The Company may in the future, but in no event more than 180 days after the date of the Annual Meeting, negotiatecash and execute additional exchange agreements with certain of its remaining warrant holders (the “Future Exchange Agreements,” and together with the First Exchange Agreements and the Second Exchange Agreements, the “Exchange Agreements”), which, depending on the outcome of such negotiations, could involve the issuance of additional shares of the Company’s Common Stock. The shares of our Common Stock that we have issued or will issue pursuantpayable by the Company to the Exchange Agreements,Spitfire Equityholders upon the achievement of the following milestone events (each, a “Milestone Event”):

Reasons for Seeking Stockholder Approval

Our Common Stock is listed on Thethe Nasdaq Global Market, and as such, we are subject to the Nasdaq listing standards set forth in its Marketplace Rules. Rules (the “Marketplace Rules”). Although we were not required to obtain stockholder approval in connection with the issuance of the Closing Consideration because the shares of our Common Stock issued at closing constituted less than 20% of our outstanding shares and did not constitute a

change of control, we are required under Marketplace Rules 5635(a) and 5635(b) to seek stockholder approval for the issuance of shares of Common Stock in connection with the Milestone Payments as further described below.

Nasdaq Marketplace Rule 5635(d)5635(a) requires that an issuer obtain stockholder approval prior to the issuance of Common Stocksecurities in certain circumstances,connection with the acquisition of the stock or assets of another company, including forpursuant to an“earn-out” or similar provision, where due to the present or potential issuance of Common Stock to(or securities convertible into or exercisable for Common Stock), other than a limited number of investors if such issuance ispublic offering for less than the greater of book or market value ofcash, the Common Stock and would equal 20% or more of the Common Stock orto be issued (a) constitutes voting power in excess of 20% of the issuer’s outstanding voting power beforeprior to the issuance (as determinedor (b) is or will be in excess of 20% of the outstanding Common Stock prior to the issuance. The Closing Consideration that we have already issued to the Spitfire Equityholders did not constitute more than 20% of our total shares of Common Stock outstanding, so we were not required to obtain stockholder approval for the issuance of these shares. The Closing Consideration constituted an amount of our Common Stock equivalent to approximately 12.3% of our outstanding Common Stock as of July 8, 2019, the execution date of the Merger Agreement. The issuance of additional shares of Common Stock to former the Spitfire Equityholders in connection with the Milestone Payments would be aggregated with the shares we issued as Closing Consideration for purposes of Nasdaq Marketplace Rule 5635(a). Accordingly, issuing additional shares of Common Stock as Milestone Payments to the Spitfire Equityholders may result in the aggregate number of shares issued by us in connection with the Spitfire Acquisition exceeding 20% of our total shares outstanding prior to the Spitfire Acquisition. Therefore, we are requesting stockholder approval for Proposal 5 under this Nasdaq listing standard to ensure that we have stockholder approval to issue shares of Common Stock as Milestone Payments to the extent that any such shares issued, when aggregated with shares previously issued in connection with the Spitfire Acquisition, exceed 20% of our Common Stock outstanding prior to the Spitfire Acquisition. To the extent a Milestone Event is achieved and we pay the corresponding Milestone Payment in shares of our Common Stock, the shares would be valued pursuant to a formula based on the then-market price of our Common Stock.

Pursuant to the Merger Agreement, we agreed to seek stockholder approval at our next stockholders meeting following the execution of the Merger Agreement for the possible issuance of shares of our Common Stock pursuant to the Nasdaq Rules). Merger Agreement in excess of 19.99% of our outstanding shares.

Nasdaq Rule 5635(b) requires stockholder approval for issuances of securities that will result in a “change of control” of the issuer, and Nasdaq may deem a change of control to occur when, as a result of an issuance, an investor or a group of investors, acting together, would own, or have the right to acquire, 20% or more of our shares of Common Stock or voting power then issued and outstanding and such ownership or voting power would be the largest ownership position of the Company.

On June 22, 2018,Pursuant to the dateMarketplace Rules, the Company executed1,887,250 shares issued to the First Exchange Agreements, there were 34,746,022 shares of the Company’s Common Stock issuedSpitfire Equityholders are not entitled to vote on this Proposal 5 and outstanding. In the aggregate, after the Second Closing of the Second Exchange Agreements (assuming the approvalare not counted in determining votes cast for purposes of this Proposal 5), the Company will issue a total of 7,875,539 shares of Common Stock pursuant to the terms of the First Exchange Agreements and the Second Exchange Agreements, which would be greater than 20% of the Company’s outstanding shares of Common Stock as of June 22, 2018. Accordingly, the terms of the Second Exchange Agreements prohibit the Company from completing the Second Closing unless the Company obtains stockholder approval of the issuance of the shares pursuant to the Second Exchange Agreements. Additionally, if the Company were to default on the terms5.

Consequences of Approving the Proposal

If our stockholders approve this Proposal 5, current stockholders will experience immediate and significant dilution to their current equity ownership in the Company. If this Proposal 5 is approved and we obtain stockholder authorization to issue in connection with the Company will issueMerger Agreement shares of Common Stock in excess of 20% of our outstanding shares, pursuant to satisfy its obligationsthe terms of the Merger Agreement, no less than 41% of the consideration payable to the Spitfire Equityholders will be paid by the Company in shares of our Common Stock. The actual number of shares that may become issuable as Milestone Payments will depend on multiple factors including the Milestone Events that are actually achieved, the amount of the corresponding Milestone Payments that are paid in shares of our Common Stock, and the market price of our Common Stock at the Second Closingtime that we pay the corresponding Milestone Event in shares of our Common Stock. While we believe that having the Second Exchange Agreements, and would be ableability to issuepay Milestone Payments in shares of Common Stock pursuantoffers benefits to additional exchange agreements it may negotiatethe Company and its stockholders, including conservation of cash, the payment of Milestone Payments in separate, privately negotiated transactions with its current warrant holders. The Company would also be able to issue shares of Common Stock may cause substantial dilution to satisfy its obligations under the Exchange Notes if the Company were to default upon its obligations under the notes.equity interest of our current stockholders.

Interests of Directors and Executive Officers

Our directors and executive officers have no substantial interests, directly or indirectly, in the matters set forth in this proposal except to the extent of their ownership of shares of Common Stock. None of our officers or directors hold any of the securities that were exchanged in the First Exchange Agreements or the Second Exchange Agreements, or were otherwise a party to any such agreements.

The Investors include entities affiliated with Hudson Bay Capital Management LP, Novartis Bioventures Ltd., entities affiliated with HealthCap V LP, and entities affiliated with Truffle Capital S.A.S, each of whom beneficially owned greater than five percent of our outstanding Common Stock prior to the execution of the Exchange Agreements.

No Appraisal Rights

Under Delaware law, stockholders are not entitled to appraisal rights with respect to this proposal and the Company will not independently provide stockholders with any such rights.

Vote Required

Unless proxy cards are otherwise marked, the persons named as proxies will vote FOR the approval of this Proposal 5. The affirmative votes of a majority of the votes cast by our stockholders is required to approve this Proposal 5. This means that the majority of the shares voted “for” the proposal must exceed the number of shares voted “against” the proposal. Abstentions and brokernon-votes are not considered votes cast for the foregoing purpose, and will have no effect on the vote for this proposal. Pursuant to Nasdaq Listing Rule 5635(d)5635(a) and applicable guidance, the InvestorsSpitfire Equityholders are not entitled to vote the shares of our Common Stock that have been issued pursuant to the Exchange AgreementsMerger Agreement with respect to this Proposal 5.

Board Recommendation

Our Board of Directors has determined that is in the best interests of Altimmune and its stockholders to approve the issuance of our Common Stock to satisfy the Company’s obligations under the Exchange Agreements. Accordingly, our Board unanimously recommends that you vote “FOR” this proposal.

THE BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT STOCKHOLDERS VOTE “FOR” THE APPROVAL OF THE ISSUANCE OF OUR COMMON STOCK PURSUANT TO NASDAQ LISTING RULES 5635(d)5635(A) AND 5635(b)5635(B).

PROPOSAL 6 — APPROVAL OF AN AMENDMENT TO OUR AMENDED AND RESTATED CERTIFICATE OF INCORPORATION TO INCREASE THE AUTHORIZED SHARES OF COMMON STOCK

The Board of Directors has approved an amendment to the Company’s Amended and Restated Certificate of Incorporation (the “Certificate of Incorporation”), that would increase the number of authorized shares of Common Stock from 100,000,000 to 200,000,000, and has directed the submission of the amendment for approval by the Company’s stockholders at the Annual Meeting. The Board of Directors recommends that the stockholders vote “FOR” the proposal to approve the amendment to the Certificate of Incorporation to increase the number of authorized shares of Common Stock.

If the proposal to amend the Certificate of Incorporation described in this Proposal 6 is approved by the requisite vote of the Company’s stockholders, we intend to file a Certificate of Amendment with the Secretary of State of the State of Delaware amending the first paragraph of Article FOURTH of our Certificate of Incorporation as follows:

“FOURTH: The total number of shares of all classes of capital stock which the Corporation shall have authority to issue is 201,000,000 of which 200,000,000 shares shall be Common Stock of the par value of $0.0001 per share and 1,000,000 shares shall be Preferred Stock of the par value of $0.0001 per share.”

The Certificate of Amendment would be effective when it is filed. A copy of the Certificate of Amendment is attached to this Proxy Statement as Appendix B.

Reasons for the Proposed Amendment

The Company’s Certificate of Incorporation as currently in effect provides that the Company’s authorized capital stock consists of 101,000,000 shares of capital stock, of which 100,000,000 shares are designated as Common Stock and the remaining 1,000,000 shares are designated as preferred stock. As of July 23, 2018, 38,506,650 shares of Common Stock were issued and outstanding, and 2,779,649 shares of Common Stock were issuable or available for future issuance under the Company’s incentive plans. In addition, as of July 23, 2018, the Company has outstanding warrants that may be exercised for up to 756,332 shares of Common Stock (“Warrant Shares”), and convertible notes that are convertible into an aggregate of 2,205,833 shares of Common Stock (“Convertible Note Shares”).

As discussed in more detail above in Proposal 5, on July 11, 2018, the Company entered into the substantially similar privately negotiated Second Exchange Agreements with the Second Investors. Pursuant to the terms of the Second Exchange Agreements, the Company has agreed to issue an aggregate of 4,351,136 shares of Common Stock in exchange for the Company’s outstanding warrants to purchase shares of Common Stock held by the Second Investors (the “Warrant Exchange Shares”). As described in more detail in Proposal 5, the Warrant Exchange Shares may only be issued upon the receipt by the Company of stockholder approval of the issuance of the Warrant Exchange Shares pursuant to Proposal 5.

If the Company were to issue all of the shares available for issuance under the incentive plans, the Warrant Shares, the Convertible Note Shares and the Warrant Exchange Shares (after receiving stockholder approval of Proposal 5 contained in this Proxy Statement), the Company would have a total of 47,923,921 shares of Common Stock outstanding, and 52,076,079 shares authorized and available for future issuance pursuant to the Certificate of Incorporation.

As described above in Proposal 5, the Company currently intends to conduct the Public Offering, which it contemplates will consist of the offer and sale of shares of Common Stock and securities exercisable or convertible into shares of Common Stock. We intend to use the proceeds from the anticipated public offering for the continued advancement of development activities for our clinical-stage product pipeline, general corporate

SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

The following table sets forth certain information regarding the beneficial ownership of the Company’s Common Stock as of July 23, 2018August 1, 2019 by (i) each person or group of persons known by us to beneficially own more than five percent of our Common Stock, (ii) each of our named executive officers, (iii) each of our directors and nominees for director and (iv) all of our directors and executive officers as a group.

The following table gives effect to the shares of Common Stock issuable within 60 days of July 13, 2018August 1, 2019 upon the exercise of all options and other rights beneficially owned by the indicated stockholders on that date. Beneficial ownership is determined in accordance withRule13d-3 promulgated under Section 13 of the Securities Exchange Act of 1934, as amended, and includes voting and investment power with respect to shares. Percentage of beneficial ownership is based on 38,506,65015,338,001 shares of Common Stock outstanding at the close of business on July 23, 2018.August 1, 2019. Except as otherwise noted below, each person or entity named in the following table has sole voting and investment power with respect to all shares of our Common Stock that he, she or it beneficially owns.

Unless otherwise indicated, the address of each beneficial owner listed below is c/o Altimmune, Inc., 910 Clopper Road, Suite 201S, Gaithersburg, Maryland 20878.

EXECUTIVE OFFICERS

Executive Officers

The names and ages of our executive officers as of July 23, 2018August 1, 2019 are set forth below:

William J. EnrightVipin K. Garg, Ph.D. is our President, Chief Executive Officer and a Director. SeeProposal1—ElectionofDirectors—Directorinformation for a discussion of Mr. Enright’sDr. Garg’s business experience.

WilliamBrowncurrently serves as our Acting Chief Financial Officer, a position he has held since June 2019. Mr. Brown previously served as the Company’s Acting Chief Financial Officer and Principal Accounting Officer since May 2018. Since October 2016,Prior to that, Mr. Brown hasacted as an independent consultant to the Company since February 2018. Mr. Brown had been a consultant to several private and public companies in a variety of accounting and tax matters, both independently and as the managing partner of Redmont CPAs.CPAs since October 2016. Prior to his current roles,that, he was an audit manager at PwCPricewaterhouseCoopers LLP in both Montgomery, Alabama (from June 2012 through July 2013) and Denver, Colorado (from November 2014 through September 2016). From August 2013 through October 2014, Mr. Brown was the Water Heater Division Controller at Rheem Manufacturing, a private company located in Montgomery, Alabama. Mr. Brown is a CPA licensed in Colorado and Alabama.certified public accountant. He has a Bachelor of Science and a Master of Business Administration from Auburn University at Montgomery.

M. M.ScotRoberts Ph.D.,Ph.D. currently serves as Chief Scientific Officer of the Company. Dr. Roberts joined Altimmune in December 2012 and has nearlyover 20 years of senior technical leadershipbiologics development experience, most recently at ImQuest BioSciences, Inc., where as Chief Scientific Officer from November 2010 until November 2012, he was responsible for managing scientific operations as well as business development opportunities in cancer and antivirals.operations. Dr. Roberts held key positions at Wellstat Biologics Corporation from August 1996 until October 2010, including Director of Research and Development where he was responsible for development of a portfolio of biologic candidates in oncology including a clinical stage oncolytic virus asset. He also led bioassay development efforts for the companyis an inventor on twelve patent and assumed leadership roles in upstream process developmentpatent application families, and animal pharmacology while at Wellstat. Dr. Roberts has significant experience in both small molecule and biologics drug development with a focus on viral vectors and antiviral therapies. Dr. Roberts completed a post-doctoral fellowship at the National Cancer Institute, Laboratoryauthor of Molecular Virology and has numerous patents and publications in peer-reviewed journals, andjournals. Dr. Roberts has been an invited speaker and Chair at numerous international conferences.conferences where he chaired a variety of scientific sessions. Dr. Roberts received hisa Masters in Chemistry from Illinois State University and a Ph.D. from the Johns Hopkins School of Medicine, Department of Pharmacology and Molecular Sciences.

Sybil Tasker, M.D.,MPH, FACP, FIDSAserves as Chief Medical Officer of the Company. Dr. Tasker joined Altimmune as Senior Vice President of Clinical Research and Development in April 2016 and was promoted to Chief Medical Officer in May 2017, and is an experienced infectious disease clinician and fellow of the American College of Physicians and the Infectious Diseases Society of America. Prior to joining Altimmune, she led development of a therapeutic herpes simplex vaccine at Genocea Biosciences (July 2014-March 2016) and had positions of increasing responsibility in infectious disease product development strategy at two global CROs. A prior career military officer, she was the senior U.S. Navy infectious disease physician and technical advisor to Department of Defense leaders about a wide variety of infectious disease policy issues, including HIV, tropical disease, vaccination, infection control, bioterrorism and pandemic preparedness. She has extensive antimicrobial, vaccine and infectious disease-related device and diagnostic development experience across all phases of the clinical development process. Dr. Tasker earned an A.B. degree in Biochemistry from Princeton University, an M.D. degree from Columbia University and an M.P.H. degree from Johns Hopkins University School of Public Health and holds a California medical license.

EXECUTIVE COMPENSATION

Our named executive officers (“NamedExecutiveOfficers”) for the year ended December 31, 20172018 are:

Mr. Gill resigned as Chief Executive Officer in connection with the Mergers effective May 4, 2017. On May 8, 2018, Ms.

Elements of Compensation

The compensation arrangement for each Named Executive Officer is intended to encourage performance and to align the Named Executive Officers’ interests with those of our stockholders. In setting compensation for our Named Executive Officers, the Compensation Committee and the Board takes into account the relative amount of compensation that is delivered on a current and long-term basis and in the form of cash and equity. The combination of performance measures for annual bonuses and the equity compensation programs for executive officers, as well as the multi-year vesting schedules for equity awards encourage employees to maintain both a short-term and a long-term view with respect to Company performance.

The Company’s executive compensation program consists of the following elements:

Base Salary

The Named Executive Officers receive a base salary to compensate them for services rendered to our Company. The base salary payable to each Named Executive Officer is intended to provide a fixed component of compensation reflecting the executive’s skill set, experience, roles and responsibilities.

Annual Performance-Based Bonus

The Named Executive Officers are entitled to receive annual performance-based cash bonuses, the amount of which is based on satisfaction of corporate and personal objectives that are established by the Board of Directors or the Compensation Committee. The annual bonuses are intended to encourage the Named Executive Officers to promote the growth of the Company’s business.

Equity Awards

The Named Executive Officers are eligible to receive equity awards under the Altimmune, Inc. 2017 Omnibus Incentive Plan.Plan (as amended, the “2017Plan”). Awards under the 2017 Plan are intended to align the interests of the Named Executive Officers with those of our stockholders and to create a link between executive pay and the long-term performance of our Common Stock.

Employee Benefits

The Named Executive Officers, like our other employees, participate in health and welfare benefit plans, subject to satisfying eligibility requirements. We believe these benefits are necessary and appropriate to provide a competitive compensation package to our Named Executive Officers.

401(k) Plan

The Company maintains atax-qualified retirement plan (the “401(k) 401(k)Plan”) that provides eligible employees (including the Named Executive Officers) with an opportunity to save for retirement on atax-advantaged basis. Eligible employees are able to participate in the 401(k) Plan as of the first day of the month following the date they meet the 401(k) Plan’s eligibility requirements, and participants are able to defer up to 100% of their eligible compensation subject to applicable annual limits under the Internal Revenue Code of 1986, as amended (the “Code”“Code”). All participants’ interests in their deferrals are 100% vested when contributed. The 401(k) Plan permits Altimmune to make matching contributions and profit sharing contributions to eligible participants. Altimmune matches contributions 100% on the first 4% of contributions made by participants.

We believe the benefits described above are necessary and appropriate to provide a competitive compensation package to our Named Executive Officers.

Summary Compensation Table

The following table sets forth the total compensation that was paid to or earned by the Named Executive Officers for the 20162017 and 20172018 fiscal years.

Narrative to Summary Compensation Table

Agreements with Named Executive Officers

We have entered into employment agreements with each of Mr. Enright and Dr. Tasker and a consulting arrangement withGarg, Mr. Brown and previously entered into an employment agreement with Ms. Czerepak, our former Chief Financial Officer,.Dr. Tasker. The material terms of such agreements are summarized below.

Employment Agreement with William J. EnrightVipin K. Garg, Ph.D.

TheOn November 16, 2018, the Company entered into an amended and restated employment agreement with William J. Enright,Dr. Garg in connection with his employment as the President and Chief Executive Officer of the Company which became effective on May 4, 2017,(the “Employment Agreement”). Pursuant to the date of the closing of the Mergers. The amended agreement has an initial term that will expire on December 31, 2018. Unless eitherEmployment Agreement, Dr. Garg commenced employment with the Company or Mr. Enright elect not to renew the agreement at least 90 days prior to expiration, Mr. Enright’s agreement will automatically renew for successiveone-year terms effective January 1, 2019 and each January 1 thereafter.on November 30, 2018.

Under the agreement, Mr. Enright will receiveEmployment Agreement, Dr. Garg receives a base salary of $375,000$500,000 and, from January 1, 2019, will be eligible to receive an annual discretionary incentive bonus of up to 50%55% of his base salary based on achievement of performance goals established by the compensation committeeCompensation Committee. In addition, Dr. Garg received a lump sum cash signing bonus of the Company’s Board (the “Compensation Committee”). Mr. Enright$100,000, which will be subject to claw-back if Dr. Garg’s employment with the Company terminates for any reason other than by the Company without cause or by Dr. Garg for good reason on or prior to November 30, 2019.

Dr. Garg is eligible to participate in the Company’s employee benefit plans made available to its similarly situated senior executives. In addition, the Company will paypays the premium costs for a term life insurance policy for Mr. EnrightDr. Garg with a benefit equal to Mr. Enright’sDr. Garg’s base salary and for short- and long-term disability plans that provide for an annual benefit of at least 60% of Mr. Enright’sDr. Garg’s base salary for as long as the disability continues. In addition, during the term of Dr. Garg’s employment, so long as Dr. Garg’s primary residence is located within 50 miles of his current residence in North Carolina, the Company will reimburse Dr. Garg an amount not to exceed $36,000 duringany 12-month period to cover Dr. Garg’s commuting expenses, which amount will be grossed up for taxes. During the term of Mr. Enright’sDr. Garg’s employment, and subject to applicable securities laws or listing standards, the Company will use its best efforts to cause Mr. EnrightDr. Garg to be nominated for election as a member of the Company’s board of directors at each annual meeting of stockholders at which Mr. EnrightDr. Garg is up for election. Mr. Enright received a base salary increase to $425,000 in September 2017.

On May 4, 2017, the effective date of the agreement, the Compensation Committee granted Mr. Enright an option to purchase 99,927 shares of Common Stock of the Company at an exercise price of $6.50 per share (which is equalPursuant to the closingEmployment Agreement, Dr. Garg received the following equity-based awards:

In the event of an employment termination, the Company will pay Mr. EnrightDr. Garg his earned but unpaid base salary through the date of termination, accrued but unused vacation pay, unreimbursed business expenses and such employee benefits as may be due to Mr. EnrightDr. Garg under the terms of the applicable benefit plans (the “Accrued Benefits”). In addition, if the Company terminates Dr. Garg’s employment for “cause” (as defined below), Dr. Garg will be entitled to payment of any unpaid prior year’s annual bonus.

If the Company terminates Mr. Enright’sDr. Garg’s employment without cause or Mr. EnrightDr. Garg resigns his employment for good reason,“good reason” (as defined below), in addition to the Accrued Benefits, Mr. EnrightDr. Garg will be entitled to receive 12 months of base salary continuation payments, 12 months of continued coverage under the health insurance plans in which Mr. EnrightDr. Garg participates at the time of the termination and payment of any unpaid prior year’s annual bonus. If such employment termination or resignation occurs within one year following a change of control, Mr. Enright“change in control” (as defined in the Employment Agreement), Dr. Garg is entitled to receive an amount equal to the sum of 18 months of his base salary plus his target annual discretionary incentive bonus for the year of termination, 1218 months of continued coverage under the health insurance plans in

and not revoking a general release of claims against the Company and its affiliates in a form provided by the Company, he will receive $84,000.

Employment AgreementsAgreement with Sybil Tasker, M.D., MPH

The Company entered into an employment agreement with Sybil Tasker, M.D., MPH, the Chief Medical Officer. Upon the closing of the Mergers, the agreement have become agreements of the Company. The agreement provided for an initial term that expired on December 31, 2017. However, unless either party elects not to renew the agreement by providing at least 90 days prior notice to the other party, the agreement will automatically renew for successiveone-year terms effective January 1, 2019 and each January 1 thereafter. The current renewal period is set to expire on December 31, 2019.

The agreement provided Dr. Tasker with an initial base salary of $290,000, which was most recently increased to $397,000 on January 1, 2018. In addition, Dr. Tasker was initially was to receive an annual discretionary incentive bonus of up to 30% of her respective base salary based on achievement of performance goals previously established by the Compensation Committee. Dr. Tasker is eligible to participate in the Company’s employee benefit plans made available to its similarly situated senior executives.

If, prior to a “change in control” (as defined in the employment agreement), the Company terminates the employment of Dr. Tasker without “cause” or if such she resigns for “good reason” (as defined below), in addition to accrued benefits (to which she is entitled on any termination of employment), Dr. Tasker will be entitled to receive severance equal to six months of base salary continuation payments, six months of continued coverage under the health insurance plans in which the executive participated at the time of the termination and payment of any unpaid prior year’s annual bonus. If such employment termination or resignation occurs within theone-year period following a change in control, she would be entitled to receive a severance amount equal to the sum of 12 months of her base salary plus her target annual discretionary incentive bonus for the year of termination, six months of continued coverage under the health insurance plans in which she participates at the time of termination, payment of any unpaid prior year’s annual bonus and, all of her outstanding unvested equity awards will become vested. The agreement also provides that if any payments, whether under the agreement or otherwise, payable to her would be subject to the golden parachute excise tax under Section 4999 of the Code, such payments will be reduced to the extent necessary to avoid the excise tax if doing so would result in a greater net after tax payment to the her. Dr. Tasker is required to execute and not revoke a release of claims in Altimmune’s favor in order to be eligible to receive the severance payments and benefits.

Under the agreement with Dr. Tasker, “cause” generally means her (i) material breach of her fiduciary duties to us, (ii) material breach of the agreement, (iii) willful failure or refusal to follow Altimmune’s written policies, (iv) conviction of, or plea of guilty or nolo contendere to, a felony or (v) continuing and willful failure to act as directed by Altimmune’s board of directors or its chief executive officer. Under the agreement, “good reason” generally means (i) a reduction in the Dr. Tasker’s base salary or target annual bonus opportunity, (ii) a material diminution in authority, duties or responsibilities or (iii) a relocation of her principal place of employment more than 50 miles from Gaithersburg, Maryland.

Under the agreement, Dr. Tasker is subject to restrictive covenants during the term of her employment and for a period of six months following termination of employment. In particular, she is prohibited from soliciting the Company’s customers, clients and employees and from engaging in sales, marketing or related activities on the executive’s behalf or another entity that directly competes with the Company. Dr. Tasker resigned as the Chief Medical Officer effective June 30, 2019.

Employment Agreement with Elizabeth A. Czerepak and Sybil Tasker, M.D., MPH

The Company entered into an employment agreement with Elizabeth A. Czerepak, the Chief Financial Officer and Executive Vice President, Corporate Development and with Sybil Tasker, M.D., MPH, the Chief Medical Officer.Development. Upon the closing of the Mergers, each of these agreements has become agreementsthe agreement

became an agreement of the Company. Each of these agreements providedThe agreement provides for an initial term that expired on December 31, 2017, and a renewal term that is set to expire on December 31, 2018. However, unless either party elects not to renew the agreement by providing at least 90 days prior notice to the other party, the agreement will automatically renew for successivesuccessive one-year terms effective January 1, 2019 and each January 1 thereafter. As previously disclosed, on May 8, 2018, Ms. Czerepak resigned from her position with the Company.

The agreements provideagreement provides Ms. Czerepak and Dr. Tasker with an initial base salary of $290,000. Upon the closing of the Mergers, the base salary amountsamount for Ms. Czerepak and Dr. Tasker werewas increased to $325,000 and $350,000, respectively, and werewas increased to $360,000 and $380,000 in September 2017, respectively, and Dr. Tasker was increased to $397,000 on January 1, 2018.2017. In addition, Ms. Czerepak and Dr. Tasker are eachis eligible to receive an annual discretionary incentive bonus of up to 30% of their respectiveher base salariessalary based on achievement of performance goals previously established by the Compensation Committee. Ms. Czerepak’s bonus target increased from 30% to 40% of base salary and Dr. Tasker’s bonus target increased from 30% to 35% of base salary on January 1, 2018. Ms. Czerepak and Dr. Tasker will be eligible to participate in the Company’s employee benefit plans made available to its similarly situated senior executives. Given Ms. Czerepak’s resignation from the Company on May 8, 2018, she will not be entitled to an annual discretionary incentive bonus for 2018.

If, prior to a change in control, the Company terminates the employment of Ms. Czerepak or Dr. Tasker without cause or if such executive resigns for good reason, in addition to the executive’s Accrued Benefits (to which the executive is entitled on any termination of employment), the executive will be entitled to receive severance equal to six months of base salary continuation payments, six months of continued coverage under the health insurance plans in which the executive participated at the time of the termination and payment of any unpaid prior year’s annual bonus. If such employment termination or resignation occurs within theone-year

Outstanding Equity Awards at 20172018 FiscalYear-End

The following table sets forth certain information with respect to outstanding equity awards of our Named Executive Officers as of December 31, 2017.2018.

CERTAIN RELATIONSHIPS AND RELATED PARTY TRANSACTIONS

Review and approval of related party transactions

Our related parties include our directors, director nominees, executive officers, holders of more than five percent of the outstanding shares of our Common Stock and the foregoing persons’ immediate family members. We review relationships and transactions in which the Company and our related parties are participants to determine whether such related persons have a direct or indirect material interest. As required under SEC rules, transactions since January 1, 2017 that are determined to be directly or indirectly material to a related party are disclosed in this Proxy Statement. In addition, the Audit Committee reviews and approves any related party transaction that is required to be disclosed. Set forth below is information concerning transactions with our related parties that is required to be disclosed under SEC rules.

Indemnification agreements

We have entered into an indemnification agreement with each of our outside directors. The indemnification agreements and our certificate of incorporation and bylaws require us to indemnify our directors and officers to the fullest extent permitted by Delaware law.

Financing Agreement

In connection with the Merger Agreement, on January 28, 2017, Private Altimmune entered into a financing agreement with certain of its stockholders and directors (the “Altimmune Financing Agreement”), including Novartis Bioventures Ltd., HealthCap V LP, OFCO Club V, UFF Innovation 14 FCPI and UFF Innovation 15 FCPI, pursuant to which such stockholders irrevocably committed to: (i) participate in the private placement of Private Altimmune’s convertible securities (the “Altimmune Private Placement”) in an aggregate amount of not less than $3.5 million of gross proceeds, and (ii) participate in a private placement after the closing of the Merger (the “Post-Closing Private Placement”) to raise an aggregate of not less than $5.0 million of gross proceeds within 135 days of the closing date of the Merger. However, if the Company completes a public offering of Common Stock during such135-day period, then the purchase price of the shares acquired in the Post-Closing Private Placement will be at the same price as the shares sold in such public offering.

Pursuant to the terms of the Altimmune Financing Agreement:

OTHER MATTERS

Section 16(a) beneficial ownership reporting compliance

The members of our Board of Directors, our executive officers and persons who hold more than 10% of our outstanding Common Stock are subject to the reporting requirements of Section 16(a) of the Securities Exchange Act of 1934, as amended, which requires them to file reports with respect to their ownership of our Common Stock and their transactions in such Common Stock. Based solely upon a review of (i) the copies of Section 16(a) reports that the Company has received from such persons for transactions in our Common Stock and their Common Stock holdings for the 2017 fiscal year and (ii) the written representations of such persons that no annual Form 5 reports were required to be filed by them for the fiscal year, the Company believes that all reporting requirements under Section 16(a) for such fiscal year were met in a timely manner by its directors, executive officers and beneficial owners of more than 10% of its Common Stock.

Other matters

As of the date of this Proxy Statement, the Company knows of no other matters that will be presented for consideration at the Annual Meeting. If any other matters properly come before the Annual Meeting, it is the intention of the persons named in the enclosed proxy card to vote the shares they represent as such persons deem advisable. Discretionary authority with respect to such other matters is granted by the execution of the enclosed proxy card.

Stockholder proposals for 2019 Annual Meeting

In order to be considered for inclusion in the Company’s proxy statement and proxy card relating to the 2019 Annual Meeting of Stockholders (the “2019AnnualMeeting”), any proposal by a stockholder submitted pursuant to Rule14a-8 of the Securities Exchange Act of 1934, as amended, must be received by the Company at its principal executive offices in Gaithersburg, Maryland, on or before                 March 28, 2019,, 2020, unless the 20192020 Annual Meeting is held on a date more than 30 days from the anniversary of the 20182019 Annual Meeting, in which case such proposals must be submitted a reasonable time before the Company prints and mails the proxy materials for the 20192020 Annual Meeting.

In addition, under the Company’s bylaws, any director nominee or proposal for consideration at the 20192020 Annual Meeting submitted by a stockholder other than pursuant to Rule14a-8 will be considered timely if such proposal or director nomination is received by the Corporate Secretary of the Company at its principal executive offices within the time periods set forth in the Company’s bylaws. If the 20192020 Annual Meeting is held on a date no more than 30 days before or after August 30, 2019,September 26, 2020, the anniversary date of the 20182019 Annual Meeting, then a stockholder proposal or director nominee must be received no earlier than                 March 28, 2019, 2020 and no later than                 April 27, 2019., 2020. However, if the Company’s 20192020 Annual Meeting is held on a date more than 30 days before or after August 30, 2019,September 26, 2020, such proposals or director nominees must be received no later than 10 days after the day on which the date of the 20192020 Annual meeting is first disclosed by the Company.

APPENDIX A

AMENDMENT

To

ALTIMMUNE, INC.

2017 OMNIBUS INCENTIVE2019 EMPLOYEE STOCK PURCHASE PLAN

(Adopted byThe following constitute the Board on March 29, 2017)

(Approved byprovisions of the Stockholders on May 4, 2017)

WHEREAS,2019 Employee Stock Purchase Plan of Altimmune, Inc. (the “Company”) maintains the Altimmune, Inc. 2017 Omnibus Incentive Plan, having an effective date of May 4, 2017 (the “Plan”);

WHEREAS, pursuant to Article XI1. Purpose. The purpose of the Plan is to provide Employees of the and its Designated Parents or Subsidiaries with an opportunity to purchase Common Stock (as defined below) of the Company through accumulated payroll deductions. It is the intention of the Company to have the Plan qualify as an “employee stock purchase plan” under Section 423 of the Code. The provisions of the Plan, accordingly, shall be construed so as to extend and limit participation in a manner consistent with the requirements of Section 423 of the Code.

2. Definitions. As used herein, the following definitions shall apply:

(a) “Acquisition Event” means (i) a merger or consolidation in which the Company is not the surviving entity, (ii) any transaction that results in the acquisition of all or substantially all of the Company’s outstanding Common Stock by a single person or entity or by a group of persons or entities acting in concert or (iii) the sale or transfer of all or substantially all of the Company’s assets.

(b) “Administrator” means either the Board or a committee of the Board that is responsible for the administration of the Plan as is designated from time to time by resolution of the Board.

(c) “Applicable Laws” means the legal requirements relating to the administration of employee stock purchase plans, if any, under applicable provisions of federal securities laws, state corporate and securities laws, the Code and the applicable regulations thereunder, the rules of any applicable stock exchange or national market system, and the rules of any foreign jurisdiction applicable to participation in the Plan by residents therein.

(d) “Board” means the Board of Directors of the Company.

(e) “Code” means the Internal Revenue Code of 1986, as amended, and the regulations thereunder.

(f) “Common Stock” means the common stock of the Company, (thepar value $0.001 per share.

(g) “BoardCompany”) may at means Altimmune, Inc., a Delaware corporation, and its successors by operation of law.

(h) “Compensation” means an Employee’s base salary from the Company or one or more Designated Parents or Subsidiaries, including such amounts of base salary as are deferred by the Employee (i) under a qualified cash or deferred arrangement described in Section 401(k) of the Code or (ii) to a plan qualified under Section 125 of the Code. Compensation does not include overtime, bonuses, annual awards, other incentive payments, reimbursements or other expense allowances, fringe benefits (cash or noncash), moving expenses, deferred compensation, contributions (other than contributions described in the first sentence) made on the Employee’s behalf by the Company or one or more Designated Parents or Subsidiaries under any employee benefit or welfare plan now or hereafter established, and any other payments not specifically referenced in the first sentence.

(i) “Designated Parents or Subsidiaries” means the Parents or Subsidiaries which have been designated by the Administrator from time amendto time as eligible to participate in the Plan, subject to stockholder approvalPlan.

(j) “Effective Date” means March 29, 2019. However, if any Parent or Subsidiary becomes a Designated Parent or Subsidiary after such date, then the Administrator, in certain instances; and

WHEREAS, the Board has authorized an amendmentits discretion, shall designate a separate Effective Date with respect to the Plan, effective asemployee-participants of July 23, 2018, subject to stockholder approval atsuch Designated Parent or Subsidiary.

(k) “Employee” means any individual, including an officer or director, who is an employee of the Company’s annual meeting of stockholders occurring on August 30, 2018 (the “2018 Annual Meeting”).

NOW, THEREFORE, the Plan is hereby amended, subject to stockholder approval at the 2018 Annual Meeting, such that the last sentenceCompany or a Designated Parent or Subsidiary for purposes of Section 4.1(a)(i)423 of the Code. For purposes of the Plan, the employment relationship shall be treated as continuing intact while the individual is hereby amendedon sick leave or other leave of absence approved by the individual’s employer. Where the period of leave exceeds three months and restatedthe individual’s right to reemployment is not guaranteed either by statute or by contract, the employment relationship will be deemed to have terminated on the day that is three months and one day following the start of such leave, for purposes of determining eligibility to participate in its entirety to readthe Plan.

(l) “Exchange Act” means the Securities Exchange Act of 1934, as amended.

(m) “Exercise Date” means the last day of each Purchase Period.

(n) “Fair Market Value” means, as of any date, the value of Common Stock determined as follows:

“(i) If the Common Stock is listed on one or more established stock exchanges, including without limitation The aggregate share reserve specifiedNasdaq Global Select Market, The Nasdaq Global Market or The Nasdaq Capital Market of the Nasdaq Stock Market, LLC or The New York Stock Exchange, its Fair Market Value shall be the closing sales price for such stock (or the closing bid, if no sales were reported) as quoted on the principal exchange or system on which the Common Stock is listed (as determined by the Administrator) on the date of determination (or, if no closing sales price or closing bid was reported on that date, as applicable, on the last trading date such closing sales price or closing bid was reported), as reported in thisThe Wall Street Journal or such other source as the Administrator deems reliable;

(ii) If the Common Stock is regularly quoted on an automated quotation system (including the OTC Bulletin Board) or by a recognized securities dealer, but selling prices are not reported, its Fair Market Value shall be the mean between the high bid and low asked prices for the Common Stock on the date of determination (or, if no such prices were reported on that date, on the last date such prices were reported), as reported in The Wall Street Journal or such other source as the Administrator deems reliable; or

(iii) In the absence of an established market for the Common Stock of the type described in (i) and (ii), above, its Fair Market Value thereof shall be determined by the Administrator in good faith.

(o) “New Exercise Date” has the meaning set forth in Section 4.1(a)(i)18(b).

(p) “Offering” means an offer under the Plan of an Option that may be exercised during an Offering Period. For purposes of the Plan, all Employees eligible to participate pursuant to Section 3 will be increaseddeemed to participate in the same Offering unless the Administrator otherwise determines that Employees of the Company or one or more Designated Parents or Subsidiaries will be deemed to participate in separate Offerings, in which case the Offerings will be considered separate even if the dates of each such Offering are identical and the provisions of the Plan will separately apply to each Offering. To the extent permittedby Section 1.423-2(a)(1) of the Treasury regulations issued under Section 423 of the Code, the terms of each Offering need not be identical provided that the terms of the Plan and the Offering togethersatisfy Sections 1.423-2(a)(2) and (a)(3) of such Treasury regulations.

(q) “Offering Date” means the first day of each Offering Period.

(r) “Offering Period” means an Offering Period established pursuant to Section 4.

(s) “Option” means, with respect to each Offering Period, a right to purchase shares of Common Stock on Januarythe Exercise Date for such Offering Period in accordance with the terms and conditions of the Plan.

(t) “Parent” means a “parent corporation” of the Company, whether now or hereafter existing, as defined in Section 424(e) of the Code.

(u) “Participant” means an Employee of the Company or Designated Parent or Subsidiary who has enrolled in the Plan as set forth in Section 5(a).

(v) “Plan” means this 2019 Employee Stock Purchase Plan.

(w) “Purchase Period” means a period of six months, commencing on February 1 and August 1 of each year commencing in 2019 and endingterminating on (and including)the next following July 31 or January 1, 2027 in31, respectively.

(x) “Purchase Price” means an amount equal to the lesser of:lower of (i) four (4) percent (4%)85% (or such greater percentage as designated by the Administrator) of the totalFair Market Value of a share of Common Stock on the Offering Date or (ii) 85% (or such greater percentage as designated by the Administrator) of the Fair Market Value of a share of Common Stock on the Exercise Date.

(y) “Reserves” means, as of any date, the sum of (i) the number of shares of Common Stock covered by each then outstanding on a fully diluted basis as of December 31 ofOption under the immediately preceding calendar year,Plan which has not yet been exercised and (ii) suchthe number of shares of Common Stock if any, determined by the Board.”

RESOLVED, FURTHER,which have been authorized for issuance under the Plan is hereby further amended,but not then subject to stockholder approval atan outstanding Option.

(z) “Subsidiary” means a “subsidiary corporation” of the 2018 Annual Meeting,Company, whether now or hereafter existing, as defined in Section 424(f) of the Code.

3. Eligibility.

(a) General. Subject to the further limitations in Sections 3(b) and 3(c), any individual who is an Employee on a given Offering Date shall be eligible to participate in the Plan for the Offering Period commencing with such thatOffering Date. No individual who is not an Employee shall be eligible to participate in the second sentence of Section 4.1(a)(i)Plan.

(b) Limitations on Grant and Accrual. Any provisions of the Plan is hereby amended and restated in its entirety to read as follows:

“Subject to Section 4.2,the contrary notwithstanding, no more than 5,000,000 shares of Common Stock in the aggregate mayEmployee shall be issuedgranted an Option under the Plan (i) if, immediately after the grant, such Employee (taking into account stock owned by any other person whose stock would be attributed to such Employee pursuant to Section 424(d) of the Code) would own stock and/or hold outstanding options to purchase stock possessing 5% or more of the total combined voting power or value of all classes of stock of the Company or of any Parent or Subsidiary or (ii) which permits the Employee’s rights to purchase stock under all employee stock purchase plans of the Company and its Parents or Subsidiaries to accrue at a rate which exceeds $25,000 worth of stock (determined at the Fair Market Value of the shares at the time such Option is granted) for each calendar year in respectwhich such Option is outstanding at any time. The determination of Incentive Stock Options.”the accrual of the right to purchase stock shall be made in accordance with Section 423(b)(8) of the Code.

RESOLVED, FURTHER(c) Other Limits on Eligibility. Notwithstanding Section 3(a), unless otherwise determined prior to the applicable Offer Date, the following Employees shall not be eligible to participate in the Plan for any relevant Offering Period: (i) Employees whose customary employment is hereby further amended, subject20 hours or less per week; (ii) Employees whose customary employment is for not more than five months in any calendar year; (iii) Employees who have not been employed for such continuous period preceding the Offering Date as the Administrator may require, but in no event shall the required period of continuous employment be equal to stockholder approval at the 2018 Annual Meeting, such that Section 4.1(b)or greater than two years; and (iv) Employees who are citizens or residents ofa non-U.S. jurisdiction (without regard to whether they are also citizens of the United States or resident aliens (within the meaning of Section 7701(b)(1)(A) of the Code)) if their participation is prohibited under the laws of theapplicable non-U.S. jurisdiction or if complying with the laws of theapplicable non-U.S. jurisdiction would cause the Plan is hereby amended and restated in its entiretyor an Offering to readviolate Section 423 of the Code.

4. Offering Periods.

(a) The Plan shall be implemented through overlapping or consecutive Offering Periods until such time as follows:

“Individual Participant Limitations(Non-Employee Directors). Except as otherwise provided herein, at all times,(i) the maximum number of shares of Common Stock available for issuance under the Plan shall have been

purchased or (ii) the Plan shall have been sooner terminated in accordance with Section 19. The maximum duration of an Offering Period shall be 27 months. Initially, unless otherwise determined by the Administrator, the Plan shall be implemented through consecutive Offering Periods of six months’ duration commencing each February 1 and August 1 following the Effective Date.

(b) A Participant shall be granted a separate Option for each Offering Period in which he or she participates. The Option shall be granted on the Offering Date and shall be automatically exercised on the Exercise Date ending within the Offering Period.

(c) Except as specifically provided herein, the acquisition of Common Stock through participation in the Plan for any Offering Period shall neither limit nor require the acquisition of Common Stock by a Participant in any subsequent Offering Period.

5. Participation.

(a) An eligible Employee may become a Participant in the Plan by submitting an authorization of payroll deduction (using such form or method (including electronic forms) as the Administrator may designate from time to time) as of a date in advance of the Offering Date for the Offering Period in which such participation will commence, as required by the Administrator for all eligible Employees with respect to a given Offering Period.

(b) Payroll deductions for a Participant shall commence with the first partial or full payroll period beginning on the Offering Date and shall end on the last complete payroll period during the Offering Period, unless sooner terminated by the Participant as provided in Section 10.

6. Payroll Deductions.

(a) At the time a Participant enrolls in the Plan, the Participant shall elect to have payroll deductions made during the Offering Period in amounts between 1% and not exceeding 10% of the Compensation which the Participant receives during the Offering Period.

(b) All payroll deductions made for a Participant shall be credited to the Participant’s account under the Plan and will be withheld in whole percentages only. A Participant may not make any additional payments into such account.

(c) A Participant may discontinue participation in the Plan as provided in Section 10, or may increase or decrease the rate of payroll deductions during the Offering Period by submitting notice of a change of status (using such form or method (including electronic forms) as the Administrator may designate from time to time) authorizing an increase or decrease in the payroll deduction rate. Any increase or decrease in the rate of a Participant’s payroll deductions shall be effective as soon as administratively practicable following the date of the request. A Participant’s payroll deduction authorization (as modified by any change of status notice) shall remain in effect for successive Offering Periods unless terminated as provided in Section 10. The Administrator shall be authorized to limit the number of payroll deduction rate changes during any Offering Period.

(d) Notwithstanding the foregoing, to the extent necessary to comply with Section 423(b)(8) of the Code and Section 3(b), a Participant’s payroll deductions shall be decreased to 0%. Payroll deductions shall recommence at the rate provided in such Participant’s payroll deduction authorization, as amended, when permitted under Section 423(b)(8) of the Code and Section 3(b), unless such participation is sooner terminated by the Participant as provided in Section 10.

7. Grant of Option. On the Offering Date, each Participant shall be granted an Option to purchase (at the applicable Purchase Price) shares of Common Stock; providedthat such Option shall be subject to (i) the limitations set forth in Sections 3(b), 6 and 12 and (ii) such other terms and conditions (applied on a uniform and nondiscriminatory basis) as the Administrator shall determine from time to time. Exercise of the Option shall

occur as provided in Section 8, unless the Participant has withdrawn pursuant to Section 10, and the Option, to the extent not exercised, shall expire on the last day of the Offering Period with respect to which the Option was granted.

8. Exercise of Option. Unless a Participant withdraws from the Plan as provided in Section 10, the Participant’s Option for the purchase of shares of Common Stock will be exercised automatically on the Exercise Date, by applying the accumulated payroll deductions in the Participant’s account to purchase the number of full shares subject to the Option by dividing the Participant’s payroll deductions accumulated prior to the Exercise Date and retained in the Participant’s account as of the Exercise Date by the applicable Purchase Price. No fractional shares will be purchased; any payroll deductions accumulated in a Participant’s account which are not sufficient to purchase a full share shall be carried over to the next Purchase Period or Offering Period, whichever applies, or returned to the Participant, if the Participant withdraws from the Plan. In addition, any amount remaining in a Participant’s account following the purchase of shares on the Exercise Date due to the application of Section 423(b)(8) of the Code or Section 7, shall be returned to the Participant and shall not be carried over to the next Offering Period or Purchase Period. During a Participant’s lifetime, a Participant’s Option to purchase shares hereunder is exercisable only by the Participant.

9. Delivery. Upon receipt of a request from a Participant after each Exercise Date on which a purchase of shares occurs, the Company shall arrange for the delivery to such Participant, as soon as administratively practicable, of the shares purchased upon exercise of the Participant’s Option.