As filed with the Securities and Exchange Commission on March 30, 2017April 4, 2019

RegistrationNo. 333-

REGISTRATION NO. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DCD.C. 20549

FORMS-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Registration Statement Under the Securities Act of 1933

PHARMATHENE,ALTIMMUNE, INC.

(Exact nameName of registrantRegistrant as specifiedSpecified in its charter)Charter)

One Park Place,Delaware

(State or Other Jurisdiction of Incorporation or Organization)

20-2726770

(I.R.S. Employer Identification No.)

910 Clopper Road, Suite 450201S

Annapolis, MD 21401Gaithersburg, Maryland

(410) 269-2600Telephone: (240)654-1450

(Address, including zip code,Including Zip Code, and telephone number, including area

code,Telephone Number, Including Area Code, of Registrant’s principal executive offices)Principal Executive Offices)

John M. Gill

Dr. Vipin K. Garg

Chief Executive Officer

PharmAthene,Altimmune, Inc.

One Park Place,910 Clopper Road, Suite 450201S

Annapolis, MD 21401Gaithersburg, Maryland

(410) 269-2600Telephone: (240)654-1450

(Name, address, including zip code, and telephone number,

including area code, of agent for service)

With a copy

Copies to:

Jeffrey A. Baumel,Ori Solomon, Esq.

Dentons USMorrison & Foerster LLP

1221 Avenue of the AmericasJohn Hancock Tower

New York, New York 10020-1089200 Clarendon Street, Floor 20

(212) 768-6700Boston, MA 02116

Telephone: (617)648-4700

Approximate date of commencement of proposed sale to the public:

From time to time after the effective date of this Registration Statement.registration statement.

If the only securities being registered on this Formform are being offered pursuant to dividend or interest reinvestment plans, please check the following box.¨  ☐

If any of the securities being registered on this Formform are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.x  ☒

If this Formform is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.o  ☐

If this Formform is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.o  ☐

If this Formform is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.o  ☐

If this Formform is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.o

  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, anon-accelerated filer, or a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer” andfiler,” “smaller reporting company” and “emerging growth company” in Rule12b-2 of the Exchange Act. (Check one):Act:

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of Securities Act.  ☐

CALCULATION OF REGISTRATION FEE

CALCULATION OF REGISTRATION FEE

(1) There are being registered an indeterminate number of securities as shall have an aggregate offering price not to exceed $100,000,000. The securities registered hereunder may be sold separately or with other securities registered hereunder. The proposed maximum aggregate offering price per class of security will be determined from time to time by the registrant in connection with the issuance by the registrant of the securities registered under this registration statement and is not specified as to each class of security being registered under this registration statement pursuant to General Instruction II.D. of Form S-3 under the Securities Act of 1933, as amended (the “Securities Act”).

(2)  The proposed maximum offering price has been estimated solely for the purpose of calculating the registration fee in accordance with Rule 457(o) under the Securities Act.

(3)  Subject to note 1 above, there is being registered hereunder an indeterminate number of shares of common and preferred stock of the registrant as may be sold from time to time by the registrant. Pursuant to Rule 416 under the Securities Act, the shares being registered hereunder include such indeterminate number of shares of common and preferred stock as may be issuable from time to time with respect to the shares being registered hereunder as a result of stock splits, stock dividends or similar transactions. Pursuant to Rule 457(i) under the Securities Act, the shares being registered hereunder include such indeterminate number of shares of common and preferred stock as may be issuable from time to time upon conversion, exercise or exchange of any preferred stock or warrants issued under this registration statement.

(4)  Subject to note 1 above, there is being registered hereunder an indeterminate number of warrants to purchase common stock or preferred stock of one or more series. Pursuant to Rule 457(i) under the Securities Act, the warrants being registered hereunder include such indeterminate number of warrants as may be issuable upon conversion or exchange of any preferred stock issued under this registration statement.

The Registrant hereby amends this Registration Statementregistration statement on such date or dates as may be necessary to delay its effective date until the Registrantregistrant shall file a further amendment which specifically states that this Registration Statementregistration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act, or until the Registration Statementregistration statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements, and other information with the SEC. The SEC maintains a later date,web site at http://www.sec.gov that contains reports, proxy statements and other information about issuers, such as us, who file electronically with the SEC. Our SEC filings are available on the SEC’s Internet site. We maintain a website at http://www.altimmune.com. Information found on, or accessible through, our website is not a part of, and is not incorporated into, this prospectus supplement or the accompanying prospectus, and you should not consider it part of this prospectus supplement or part of the accompanying prospectus.

Our common stock is listed on the Nasdaq Global Market under the ticker “ALT.”

We have filed with the SEC a registration statement on FormS-3 under the Securities Act of 1933, as amended (the “Securities Act”), with respect to the securities covered by this prospectus. This prospectus, which is a part of the registration statement, does not contain all of the information set forth in the document havingregistration statement or the later date modifiesexhibits and schedules filed therewith. For further information with respect to us and the securities covered by this prospectus, please see the registration statement and the exhibits filed with the registration statement. A copy of the registration statement and the exhibits filed with the registration statement may be inspected without charge from the SEC as indicated above, or supersedesfrom us as indicated under “Incorporation by Reference.”

INCORPORATION BY REFERENCE

The SEC allows us to “incorporate by reference” into this prospectus the earlier statement.

Unless otherwise mentioned or unlessinformation that we file with the context requires otherwise, all referencesSEC. This permits us to “PharmAthene,” “the Company,” “we,” “us,” “our,” and similar terms referdisclose important information to PharmAthene, Inc. and its subsidiaries on a consolidated basis. The phrase “this prospectus” refersyou by referring to these filed documents. Any information referred to in this way is considered part of this prospectus, and any applicable prospectus supplement, unless the context otherwise requires. Whenever we refer to “you” or “yours”, we mean the persons to whom offers are made under this prospectus.

SUMMARY

We are a biodefense company engaged in developing next generation anthrax vaccine. These next generation vaccines are intended to have more rapid time to protection, fewer doses for protection and less stringent requirements for temperature controlled storage and handling than the currently used vaccine.

On September 9, 2014, we signed a contractinformation filed with the National InstitutesSEC by us after the date of Allergythis prospectus will automatically be deemed to update and Infectious Diseases (“NIAID") forsupersede this information. We incorporate by reference the development of a next generation lyophilized anthrax vaccine (“SparVax-L”) based onfollowing documents that have been filed with the Company’s proprietary technology platform which contributes the recombinant protective antigen (“rPA”) bulk drug substance that is used in the liquid SparVax® formulation. The contract is incrementally funded. Over the base period of the contract, we were awarded initial funding of approximately $5.2 million, which includes a cost reimbursement component and a fixed fee component payable upon achievement of certain milestones. NIAID has exercised four options under this agreement to provide additional funding of approximately $8.8 million and an extension of the period of performance through December 31, 2017. The contract has a total value of up to approximately $28.1 million, if all technical milestones are met and all eight contract options are exercised by NIAID. If NIAID exercises all options, the contract would last approximately five years. If NIAID does not exercise any additional options, the contract would expire by its terms on December 31, 2017.

From 2006 through 2016, we were engaged in legal proceedings with SIGA Technologies, Inc. (“SIGA”). On December 23, 2015, the Delaware Supreme Court affirmed the Delaware Court of Chancery’s judgment against SIGA. On November 16, 2016, the Company received a final payment from SIGA which fully satisfied the judgment owed to PharmAthene. In total, the Company received payment of approximately $217.1 million (including interest) from SIGA.

On November 17, 2016, PharmAthene declared a special one-time cash dividend of $2.91 per share of common stock. PharmAthene funded the one-time special dividend with approximately 98% of the after tax net cash proceeds that PharmAthene received from SIGA in satisfaction of the judgment owed to PharmAthene. On February 3, 2017, PharmAthene paid the one-time dividend in an aggregate amount of $200.3 million.

The receipt of the award from SIGA generated substantial taxable income to the Company, a portion of which was offset by the Company’s tax net operating loss carryforwards (“NOLs”). At December 31, 2016, we had available $176.1 million in accumulated domestic losses available to offset income, subject to limitations imposed by the Internal Revenue Code of 1986 (the “Code”) and a 382 limitation of approximately $1 million, of which approximately $175 million was utilized to offset current year income. On November 25, 2015, the Company had adopted a Shareholders Rights Plan to help ensure that the NOLs remained available to help maximize the value for our shareholders of any amount received from the SIGA litigation.

On January 18, 2017, PharmAthene entered into an Agreement and Plan of Merger and Reorganization (as amended from time to time, the “Merger Agreement”), pursuant to which Altimmune, Inc. (“Altimmune”) will merge into Mustang Merger Sub Corp I Inc., a Delaware corporation and a direct wholly owned subsidiary of PharmAthene (“Merger Sub Corp”), with Altimmune as the surviving entity in such merger (“Merger 1”), and immediately thereafter, Altimmune will be merged with and into Mustang Merger Sub II LLC, a Delaware limited liability company and a direct wholly owned subsidiary of PharmAthene (“Merger Sub LLC”), with Merger Sub LLC as the surviving entity in such merger (“Merger 2”, and together with Merger 1, the “Mergers”). Upon consummation of the Mergers, Merger Sub Corp and Altimmune will cease to exist, and Merger Sub LLC will continue as a direct wholly owned subsidiary of PharmAthene. Upon completion of the Mergers, former PharmAthene security holders will own approximately 41.8% of the outstanding equity of the combined company, and former Altimmune security holders will own approximately 58.2% of the outstanding equity of the combined company,SEC (other than, in each case, documents or information deemed furnished and not filed in accordance with SEC rules, including pursuant to Item 2.02 or Item 7.01 or any related exhibit furnished under Item 9.01(d) of Form8-K, and no such information shall be deemed specifically incorporated by reference hereby):

We also incorporate by reference any future filings (other than information in such documents that is not deemed to be filed) made with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act until we file a post-effective amendment which indicates the termination of the offering of the securities made by this prospectus.

We will change its nameprovide to “Altimmune,each person, including any beneficial owner, to whom a prospectus is delivered a copy of any or all of the documents that are incorporated by reference into this prospectus supplement but not delivered with this prospectus, including exhibits that are specifically incorporated by reference in such documents. You may request a copy of such documents at no cost, by writing or telephoning us at the following address or telephone number: Altimmune, Inc.”., 910 Clopper Road, Suite 201S, Gaithersburg, Maryland, Attention: Chief Financial Officer. Our telephone number is(240) 654-1450.

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

On March 29, 2017, PharmAtheneThis prospectus and Altimmune entered into amendment no. 1the documents incorporated by reference herein contain forward-looking statements within the meaning of Section 27A of the Securities Act and Section 21E of the Exchange Act. Some of these statements can be identified by use of forward-looking words such as “believes,” “expects,” “anticipates,” “may,” “will,” “should,” “seeks,” “approximately,” “intends,” “plans” or “estimates,” or the negative of these words, or other comparable terminology. The discussion of financial trends, strategy, plans or intentions may also include forward-looking statements. Forward-looking statements involve risks and uncertainties that could cause actual results to differ materially from those projected, anticipated, or implied. Although it is not possible to predict or identify all such risks and uncertainties, they may include, but are not limited to, the Merger Agreement, pursuantfactors discussed under “Risk Factors” in our most recent Annual Report on Form10-K, in our Quarterly Reports on Form10-Q, in any prospectus supplement related hereto, and in other information contained in our publicly available SEC filings and press releases.

You should not consider this list to be a complete statement of all risks and uncertainties. You are cautioned not to place undue reliance on any such forward-looking statements, which PharmAthene and Altimmune agreed that: (i)speak only as of the certificatedate such statements were first made. Except to the extent required by federal securities laws, we undertake no obligation to publicly release the result of incorporation of PharmAtheneany revisions to these forward-looking statements to reflect events or circumstances after the effective timedate hereof or to reflect the occurrence of the mergers will be the certificateunanticipated events.

RISK FACTORS

Investing in our securities involves a high degree of incorporation of PharmAthene immediately priorrisk. Before making a decision to invest in our securities, in addition to the effective time ofother information contained in this prospectus, any accompanying prospectus supplement or any related free writing prospectus, or incorporated by reference herein or therein, you should carefully consider the mergersrisks discussed under “Risk Factors” in our most recent Annual Report on Form10-K, in our Quarterly Reports on Form10-Q, in any prospectus supplement related hereto, and that, at that time PharmAthene, will file an amendment to its certificate of incorporation to change its name to “Altimmune, Inc.”in other information contained in our publicly available SEC filings and (ii) PharmAthene’s bylaws will be amended and restated immediately after the effective time of the mergers, as agreed therein (which amended and restated Exhibit C to the Merger Agreement).press releases. See “Where You Can Find Additional Information.”

ALTIMMUNE, INC.

Altimmune, Inc. is a privately-held clinical stage immunotherapeutics company focused on the discovery and development of products to stimulate robust and durable immune responses for the prevention and treatment of disease. Altimmune hasdiseases. Our product candidates are focused in two key areas for which we have unique proprietary platform technologies, RespirVecapproaches; i) our synthetic peptide technology for cancer and Densigen, each of which has been shown, in preclinical studiesinfectious disease, and early clinical trials, to activate the immune system in distinctly different ways than traditional vaccine methods.ii) intranasal vaccines for infectious disease. Using these technologies, Altimmune haswe have generated preclinical and clinical product candidates whichthat potentially represent an entirely new approach to harnessing the immune system. Altimmune’s most advanceIn addition, we plan to acquire or in-license product candidate, NasoVAX, an intranasally administered recombinant influenza vaccine, uses an adenovectorcandidates in immunotherapeutic indications that are either synergistic or complementary to achieve expressionour capabilities to expand our pipeline.

We were incorporated in Delaware in April 2005 and subsequently changed our name to Altimmune, Inc. in May 2017 upon the completion of the influenza antigen in the target cell, thereby potentially stimulating a broader and more rapid immune response than traditional influenza vaccines. Altimmune’s planned Phase 2 program for NasoVAX is expected to start in third quarter of 2017,our merger with initial data anticipated approximately six months following the start of enrollment. Altimmune’s second most advanced product candidate, HepTcell, is being tested as an immunotherapy for patients chronically infected with the hepatitis B virus (‘‘HBV’’), and has the potential to provide a functional cure, something that is not achievable with current treatments. HepTcell is currently in a Phase 1 trial in the United Kingdom in patients with chronic HBV. Initial results from this trial are expected by the end of 2017. With the support of the U.S. Biomedical Advanced Research and Development Authority (‘‘BARDA’’), Altimmune is developing a third product candidate, NasoShield, an anthrax vaccine designed to provide rapid, stable protection after one intranasal administration. Subject to continued financial and other support from BARDA, Altimmune anticipates launching a Phase 1 trial for NasoShield in the first quarter of 2018. The combined company will be a fully-integrated and diversified immunotherapeutics company with one preclinical stage and four clinical stage programs.

PharmAthene, Inc. Our principal executive offices are located at One Park Place,910 Clopper Road, Suite 450, Annapolis,201S, Gaithersburg, Maryland 2140120878, and ourits telephone number is (410) 269-2600. After the consummation of the Mergers,(240)654-1450. Our Internet website is www.altimmune.com and our executive offices will beinvestor relations website is located at 19 Firstfield Road, Suite 200, Gaithersburg, Maryland 20878.

RISK FACTORS

Before you invest in any of our securities, in addition to the other information in this prospectus and the applicable prospectus supplement, you should carefully consider the risk factors under the heading “Risk Factors” contained in Part I, Item 1A in our most recent Annual Report“Investors” tab. The information on, Form 10-K and any risk factors disclosed under the heading “Risk Factors” in Part II, Item 1A in any Quarterly Report on Form 10-Q or in any Current Report on Form 8-K that we file after our most recent Annual Report on Form 10-K, which are incorporated by reference into this prospectus and the applicable prospectus supplement, as the same may be updated from time to time by our future filings under the Exchange Act.

The risks and uncertainties we describe are not the only ones facing us. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also impaircan be accessed through, our business or operations. Any adverse effect on our business, financial condition or operating results could result in a decline in the value of the securities and the loss of all orwebsite is not part of your investment.

SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION

This prospectus and any relatedthis prospectus supplement, and you should not rely on any such information in making the information incorporated by reference herein and therein contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). This information may involve known and unknown risks, uncertainties and other factors that are difficultdecision whether to predict and may causepurchase our actual results, performance or achievements to be materially different from future results, performance or achievements expressed or implied by any forward-looking statements. These risks, uncertainties and other factors include, but are not limited to, risk associated with the following:common stock.

DESCRIPTION OF COMMON STOCK

Under our Amended and Restated Certificate of Incorporation, as amended, to which we refer as our “charter,” we are currently authorized to issue 100,000,000200,000,000 shares of common stock, par value $.0001 per share. As of March 22, 2017,27, 2019, we had 68,815,19513,451,105 shares of common stock outstanding.

Holders of our common stock are entitled to one vote for each share of common stock held of record on all matters to be voted on by stockholders, except as otherwise provided by law or in any preferred stock designation. Our bylaws specify that, except as otherwise required by law or our charter, the presence in person or by proxy of holders of a majority of the shares entitled to vote at a meeting of stockholders will be necessary, and will constitute a quorum, for the transaction of business at such meeting. Our bylaws furthermore specify that all elections of directors will be determined by a plurality of the votes and that, except as otherwise provided by law or in the charter or bylaws, any other matter will be determined by the vote of a majority of the shares which are voted with regard to it. Holders of our common stock have no conversion, preemptive or other subscription rights and there are no sinking fund or redemption provisions applicable to the common stock.

There is no cumulative voting with respect to the election of directors, with the result that the holders of more than 50% of the shares voting for the election of directors can elect all of the directors then up for election. Holders of our common stock are entitled to receive dividends when, as and if declared by our board of directors out of funds legally available therefor. In the event of our liquidation, dissolution or winding up, the holders of common stock are entitled to share in all assets remaining which are available for distribution to them after payment of liabilities and after provision has been made for each class of stock, if any, having preference over the common stock.

Transfer Agent

The transfer agent and registrar for the common stock is Continental Stock Transfer & Trust Company, New York, New York.

DESCRIPTION OF PREFERRED STOCK

Under our charter, we are currently authorized to issue 1,000,000 shares of preferred stock, par value $.0001 per share. As of the date of this prospectus, we had no shares of preferred stock outstanding.

Under our charter, our board of directors is expressly granted authority to issue shares of preferred stock, in one or more series, and to fix for each series such voting powers, full or limited, and such designations, preferences and relative, participating, optional or other special rights and such qualifications, limitations or restrictions as it may determine in the resolution or resolutions providing for the issue of such series (to which we also refer as a “preferred stock designation”) and as may be permitted by the Delaware General Corporation Law. The number of authorized shares of preferred stock may be increased or decreased (but not below the number of shares of preferred stock then outstanding) by the affirmative vote of the holders of a majority of the voting power of all of the then outstanding shares of our capital stock entitled to vote generally in the election of directors, voting together as a single class, without a separate vote of the holders of the preferred stock, or any series of preferred stock, unless a vote of any such holders is required pursuant to any preferred stock designation.

This section describes the general terms of our preferred stock to which any prospectus supplement may relate. A prospectus supplement will describe the terms relating to any preferred stock to be offered by us in greater detail, and may provide information that is different from the information described in this prospectus. If the information in the prospectus supplement with respect to the particular preferred stock being offered differs from the information in this prospectus, you should rely on the information in the prospectus supplement. A copy

of our charter has been incorporated by reference from our filings with the SEC as an exhibit to the registration statement of which this prospectus is a part. A certificate of designations will specify the terms of the preferred stock being offered, and will be filed as an exhibit to the registration statement of which this prospectus is a part or incorporated by reference from a report that we file with the SEC.

The rights and terms relating to any new series of preferred stock could adversely affect the voting power or other rights of the holders of the common stock or could be utilized, under certain circumstances, as a method of discouraging, delaying or preventing a change in control of the Company.

The following description of our preferred stock, together with any description of our preferred stock in a related prospectus supplement, summarizes the material terms and provisions of the preferred stock that we may sell under this prospectus. We urge you to read the applicable prospectus supplement(s) related to the particular series of preferred stock that we sell under this prospectus and to the actual terms and provisions contained in our charter (certificate of designations) and bylaws, each as amended from time to time.

Terms

Our board of directors will fix the rights, preferences, privileges, qualifications and restrictions of the preferred stock of each series that we sell under this prospectus and applicable prospectus supplements in the certificate of designations relating to that series. We will file as an exhibit to the registration statement of which this prospectus is a part or incorporate by reference the form of any certificate of designations that describes the terms of the series of preferred stock we are offering in connection with the issuance of the related series of preferred stock. This description of the preferred stock in the certificate of designations and any applicable prospectus supplement may include:

 Rank

Rank

As set forth in the applicable supplement to this prospectus, shares of our preferred stock may rank, with respect to payment of distributions and rights upon our liquidation, dissolution or winding up, and allocation of our earnings and losses:

Distributions

Subject to any preferential rights of any outstanding stock or series of stock, holders of our preferred stock may be entitled to receive distributions, when and as authorized by our board of directors, out of legally available funds, and share pro rata based on the number of shares of preferred stock, common stock and other equity securities outstanding.

Voting Rights

As indicated in the applicable supplement to this prospectus, and as otherwise required under Delaware law, holders of our preferred stock may or may not have voting rights.

Liquidation Preference

As indicated in the applicable supplement to this prospectus, upon the voluntary or involuntary liquidation, dissolution or winding up of our affairs, then, before any distribution or payment shall be made to the holders of any common stock or any other class or series of stock ranking junior to the preferred stock in our distribution of assets upon any liquidation, dissolution or winding up, the holders of each series of our preferred stock may be entitled to receive, after payment or provision for payment of our debts and other liabilities, out of our assets legally available for distribution to shareholders, liquidating distributions in the amount of the liquidation preference per share (set forth in the applicable supplement to this prospectus), plus an amount, if applicable, equal to all distributions accrued and unpaid thereon (which shall not include any accumulation in respect of unpaid distributions for prior distribution periods if the preferred stock does not have a cumulative distribution). After payment of the full amount of the liquidating distributions to which they may be entitled, the holders of preferred stock may have no right or claim to any of our remaining assets. In the event that, upon our voluntary or involuntary liquidation, dissolution or winding up, the legally available assets are insufficient to pay the amount of the liquidating distributions on all of our outstanding preferred stock and the corresponding amounts payable on all of our stock of other classes or series of equity security ranking on a parity with the preferred stock in the distribution of assets upon liquidation, dissolution or winding up, then the holders of our preferred stock and all other such classes or series of equity securities may share ratably in the distribution of assets in proportion to the full liquidating distributions to which they would otherwise be respectively entitled.

DESCRIPTION OF DEPOSITARY SHARES

The following description of the depositary shares does not purport to be complete and is subject to and qualified in its entirety by the Deposit Agreement and the depositary receipt relating to the preferred stock that is attached to the Deposit Agreement. You should read these documents as they, and not this description, define your rights as a holder of depositary shares. Forms of these documents have been filed with the SEC as an exhibit to the registration statement of which this prospectus forms a part.

General

If we elect to offer fractional interests in shares of preferred stock, we will provide for the issuance by a depositary to the public of receipts for depositary shares. Each depositary share will represent fractional interests of preferred stock. We will deposit the shares of preferred stock underlying the depositary shares under a Deposit Agreement between us and a bank or trust company selected by us. The bank or trust company must have its principal office in the United States and a combined capital and surplus of at least $50 million. The depositary receipts will evidence the depositary shares issued under the Deposit Agreement.

The Deposit Agreement will contain terms applicable to the holders of depositary shares in addition to the terms stated in the depositary receipts. Each owner of depositary shares will be entitled to all the rights and preferences of the preferred stock underlying the depositary shares in proportion to the applicable fractional interest in the underlying shares of preferred stock. The depositary will issue the depositary receipts to individuals purchasing the fractional interests in shares of the related preferred stock according to the terms of the offering described in a prospectus supplement.

Dividends and Other Distributions

The depositary will distribute all cash dividends or other cash distributions received for the preferred stock to the entitled record holders of depositary shares in proportion to the number of depositary shares that the holder owns on the relevant record date. The depositary will distribute only an amount that can be distributed without attributing to any holder of depositary shares a fraction of one cent. The depositary will add the undistributed balance to and treat it as part of the next sum received by the depositary for distribution to holders of depositary shares.

If there is anon-cash distribution, the depositary will distribute property received by it to the entitled record holders of depositary shares, in proportion, insofar as possible, to the number of depositary shares owned by the holders, unless the depositary determines, after consultation with us, that it is not feasible to make such distribution. If this occurs, the depositary may, with our approval, sell such property and distribute the net proceeds from the sale to the holders. The Deposit Agreement also will contain provisions relating to how any subscription or similar rights that we may offer to holders of the preferred stock will be available to the holders of the depositary shares.

Conversion, Exchange and Redemption

If any series of preferred stock underlying the depositary shares may be converted or exchanged, each record holder of depositary receipts will have the right or obligation to convert or exchange the depositary shares represented by the depositary receipts.

Whenever we redeem shares of preferred stock held by the depositary, the depositary will redeem, at the same time, the number of depositary shares representing the preferred stock. The depositary will redeem the depositary shares from the proceeds it receives from the corresponding redemption, in whole or in part, of the applicable series of preferred stock. The depositary will mail notice of redemption to the record holders of the depositary shares that are to be redeemed between 30 and 60 days before the date fixed for redemption. The redemption price per depositary share will be equal to the applicable fraction of the redemption price per share on

the applicable series of preferred stock. If less than all the depositary shares are to be redeemed, the depositary will select which shares to be redeemed by lot, proportionate allocation or any other method.

After the date fixed for redemption, the depositary shares called for redemption will no longer be outstanding. When the depositary shares are no longer outstanding, all rights of the holders will end, except the right to receive money, securities or other property payable upon redemption.

Voting

When the depositary receives notice of a meeting at which the holders of the preferred stock are entitled to vote, the depositary will mail the particulars of the meeting to the record holders of the depositary shares. Each record holder of depositary shares on the record date may instruct the depositary on how to vote the shares of preferred stock underlying the holder’s depositary shares. The depositary will try, if practical, to vote the number of shares of preferred stock underlying the depositary shares according to the instructions. The depositary will abstain from voting shares of the preferred stock to the extent it does not receive specific instructions from the holders of depositary shares representing such preferred stock. We will agree to take all reasonable action requested by the depositary to enable it to vote as instructed.

Record Date

Whenever (1) any cash dividend or other cash distribution shall become payable, any distribution other than cash shall be made, or any rights, preferences or privileges shall be offered with respect to the preferred stock, or (2) the depositary shall receive notice of any meeting at which holders of preferred stock are entitled to vote or of which holders of preferred stock are entitled to notice, or of the mandatory conversion of or any election on our part to call for the redemption of any preferred stock, the depositary shall in each such instance fix a record date (which shall be the same as the record date for the preferred stock) for the determination of the holders of depositary receipts (x) who shall be entitled to receive such dividend, distribution, rights, preferences or privileges or the net proceeds of the sale thereof or (y) who shall be entitled to give instructions for the exercise of voting rights at any such meeting or to receive notice of such meeting or of such redemption or conversion, subject to the provisions of the Deposit Agreement.

Amendments

We and the depositary may agree to amend the Deposit Agreement and the depositary receipt evidencing the depositary shares. Any amendment that (a) imposes or increases certain fees, taxes or other charges payable by the holders of the depositary shares as described in the Deposit Agreement or (b) otherwise prejudices any substantial existing right of holders of depositary shares, will not take effect until 30 days after the depositary has mailed notice of the amendment to the record holders of depositary shares. Any holder of depositary shares that continues to hold its shares at the end of the30-day period will be deemed to have agreed to the amendment.

Termination

We may direct the depositary to terminate the Deposit Agreement by mailing a notice of termination to holders of depositary shares at least 30 days prior to termination. In addition, a Deposit Agreement will automatically terminate if:

The depositary may likewise terminate the Deposit Agreement if at any time 60 days shall have expired after the depositary shall have delivered to us a written notice of its election to resign and a successor depositary

shall not have been appointed and accepted its appointment. If any depositary receipts remain outstanding after the date of termination, the depositary thereafter will discontinue the transfer of depositary receipts, will suspend the distribution of dividends to the holders thereof, and will not give any further notices (other than notice of such termination) or perform any further acts under the Deposit Agreement except as provided below and except that the depositary will continue (1) to collect dividends on the preferred stock and any other distributions with respect thereto and (2) to deliver the preferred stock together with such dividends and distributions and the net proceeds of any sales of rights, preferences, privileges or other property, without liability for interest thereon, in exchange for depositary receipts surrendered. At any time after the expiration of two years from the date of termination, the depositary may sell the preferred stock then held by it at public or private sales, at such place or places and upon such terms as it deems proper and may thereafter hold the net proceeds of any such sale, together with any money and other property then held by it, without liability for interest thereon, for the pro rata benefit of the holders of depositary receipts which have not been surrendered.

Payment of Fees and Expenses

We will pay all fees, charges and expenses of the depositary, including the initial deposit of the preferred stock and any redemption of the preferred stock. Holders of depositary shares will pay transfer and other taxes and governmental charges and any other charges as are stated in the Deposit Agreement for their accounts.

Resignation and Removal of Depositary

At any time, the depositary may resign by delivering notice to us, and we may remove the depositary. Resignations or removals will take effect upon the appointment of a successor depositary and its acceptance of the appointment. The successor depositary must be appointed within 60 days after delivery of the notice of resignation or removal and must be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50 million.

Reports

The depositary will forward to the holders of depositary shares all reports and communications from us that are delivered to the depositary and that we are required by law, the rules of an applicable securities exchange or our amended and restated certificate of incorporation to furnish to the holders of the preferred stock. Neither we nor the depositary will be liable if the depositary is prevented or delayed by law or any circumstances beyond its control in performing its obligations under the Deposit Agreement. The Deposit Agreement limits our obligations and the depositary’s obligations to performance in good faith of the duties stated in the Deposit Agreement. Neither we nor the depositary will be obligated to prosecute or defend any legal proceeding connected with any depositary shares or preferred stock unless the holders of depositary shares requesting us to do so furnish us with satisfactory indemnity. In performing our obligations, we and the depositary may rely upon the written advice of our counsel or accountants, on any information that competent people provide to us and on documents that we believe are genuine.

DESCRIPTION OF DEBT SECURITIES

We may issue debt securities under one or more trust indentures to be executed by us and a specified trustee. The terms of the debt securities will include those stated in the indenture and those made a part of the indenture by reference to the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”). The indentures will be qualified under the Trust Indenture Act.

The following description sets forth certain anticipated general terms and provisions of the debt securities to which an accompanying prospectus supplement may relate. The particular terms of the debt securities offered by an accompanying prospectus supplement (which terms may be different than those stated below) and the extent, if any, to which such general provisions may apply to the debt securities so offered will be described in the prospectus supplement relating to such debt securities. Accordingly, for a description of the terms of a particular issue of debt securities, investors should review both the accompanying prospectus supplement relating thereto and the following description. A form of the indenture has been filed as an exhibit to the registration statement of which this prospectus is a part.

The debt securities will be our direct obligations and may be either senior debt securities or subordinated debt securities. The indebtedness represented by subordinated securities will be subordinated in right of payment to the prior payment in full of our senior debt (as defined in the applicable indenture).

Except as set forth in the applicable indenture and described in an accompanying prospectus supplement relating thereto, the debt securities may be issued without limit as to aggregate principal amount, in one or more series, secured or unsecured, in each case as established from time to time in or pursuant to authority granted by a resolution of the board of trustees or as established in the applicable indenture. All debt securities of one series need not be issued at the same time and, unless otherwise provided, a series may be reopened, without the consent of the holders of the debt securities of such series, for issuance of additional debt securities of such series.

The accompanying prospectus supplement relating to any series of debt securities being offered will contain their specific terms, including, without limitation:

The debt securities may provide for less than the entire principal amount thereof to be payable upon declaration of acceleration of the maturity thereof. Special federal income tax, accounting and other considerations applicable to debt securities will be described in the accompanying prospectus supplement.

The applicable indenture may contain provisions that would limit our ability to incur indebtedness or that would afford holders of debt securities protection in the event of a highly leveraged or similar transaction involving us or in the event of a change of control.

Investors should review the accompanying prospectus supplement for information with respect to any deletions from, modifications of or additions to the events of default or covenants that are described below, including any addition of a covenant or other provision providing event risk or similar protection.

Merger, Consolidation or Sale

The applicable indenture will provide that we may consolidate with or merge into, or convey, transfer or lease all or substantially all of our properties and assets to any other person (as defined therein), provided that:

Covenants

The applicable indenture will contain covenants requiring us to take certain actions and prohibiting us from taking certain actions. The covenants with respect to any series of debt securities will be described in the accompanying prospectus supplement.

Events of Default, Notice and Waiver

Each indenture will describe specific “events of default” with respect to a series of debt securities issued under the indenture. These “events of default” are likely to include (with grace and cure periods):

If an event of default resulting from certain events of bankruptcy described in the indenture occurs, all outstanding debt securities of that series will become due and payable immediately. If any other event of default under any indenture with respect to debt securities of any series at the time outstanding occurs and is continuing, then the applicable trustee or the holders of not less than 25% of the principal amount of the outstanding debt securities of that series may declare the principal amount (or, if the debt securities of that series are original issue discount securities or indexed securities, such portion of the principal amount as may be specified in the terms thereof) of all the debt securities of that series to be due and payable immediately by written notice thereof to us (and to the applicable trustee if given by the holders). However, at any time after such a declaration of acceleration with respect to debt securities of such series (or of all debt securities then outstanding under any indenture, as the case may be) has been made, the holders of not less than a majority in principal amount of outstanding debt securities of such series (or of all debt securities then outstanding under the applicable indenture, as the case may be) may rescind and annul such declaration and its consequences if:

Each indenture also will provide that the holders of not less than a majority in principal amount of the outstanding debt securities of any series (or of all debt securities then outstanding under the applicable indenture, as the case may be) may waive any past default with respect to the series and its consequences, except a:

Each trustee will be required to give notice to the holders of debt securities within a certain number of days of a default under the applicable indenture unless the default has been cured or waived; provided, however, that the trustee may withhold notice to the holders of any series of debt securities of any default with respect to the series (except a default in the payment of the principal of (or premium, if any) or interest on any debt security of the series or in the payment of any sinking fund installment in respect of any debt security of the series) if specified responsible officers of the trustee consider withholding the notice to be in the interest of the holders.

Each indenture will prohibit the holders of debt securities of any series from instituting any proceedings, judicial or otherwise, with respect to the indenture or for any remedy thereunder, except in the case of failure of the applicable trustee, for a certain period of time after the trustee has received a written request to institute proceedings in respect of an event of default from the holders of not less than a majority in principal amount of the outstanding debt securities of such series, as well as the furnishing of indemnity reasonably satisfactory to it. This provision will not prevent any holder of debt securities from instituting a suit to enforce the payment of the principal of (and premium, if any) and interest on the debt securities at the respective due dates thereof.

Subject to the indenture, no trustee will be under any obligation to exercise any of its rights or powers under an indenture at the request or direction of any holders of any series of debt securities then outstanding, unless the holders furnish the trustee thereunder reasonable security or indemnity. The holders of not less than a majority in principal amount of the outstanding debt securities of any series (or of all debt securities then outstanding under an indenture, as the case may be) will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the applicable trustee, or of exercising any trust or power conferred upon the trustee. However, a trustee may refuse to follow any direction, which is in conflict with any law or the applicable indenture, which may involve the trustee in personal liability or which may be unduly prejudicial to the holders of debt securities of such series not joining therein.

Within a certain period of time of the close of each fiscal year, we will be required to deliver to each trustee, a certificate, signed by one of several specified officers, stating whether or not the officer has knowledge of any default under the applicable indenture and, if so, specifying each default and the nature and status thereof.

Modification of the Indenture

The indenture will likely provide that it may be modified or amended, with the consent of the holders of not less than a majority in principal amount of each series of the outstanding debt securities issued under the indenture affected by the modification or amendment, provided that no modification or amendment may, without the consent of each affected holder of the debt securities:

A record date may be set for any act of the holders with respect to consenting to any amendment.

The holders of not less than a majority in principal amount of the outstanding debt securities of each series affected thereby will have the right to waive our compliance with certain covenants in the indenture. Each indenture will contain provisions for convening meetings of the holders of debt securities of a series to take permitted action. Under certain circumstances, we and the trustee may make modifications and amendments to an indenture without the consent of any holders of outstanding debt securities.

Redemption of Debt Securities

The debt securities may be subject to optional or mandatory redemption on terms and conditions described in the applicable accompanying prospectus supplement.

Conversion of Debt Securities

The terms and conditions, if any, upon which any debt securities are convertible or into our common stock or preferred stock will be set forth in the applicable accompanying prospectus supplement. The terms will include:

Subordination

Upon any distribution to our creditors in a liquidation, dissolution or reorganization, the payment of the principal of and interest on any subordinated securities will be subordinated to the extent provided in the applicable indenture to the prior payment in full of all senior securities. No payment of principal or interest will be permitted to be made on subordinated securities at any time if any payment default or any other default which permits accelerations exists. After all senior securities are paid in full and until the subordinated securities are paid in full, holders of subordinated securities will be subrogated to the right of holders of senior securities to the extent that distributions otherwise payable to holders of subordinated securities have been applied to the payment of senior securities. By reason of any subordination, in the event of a distribution of assets upon our insolvency, some of our general creditors may recover more, ratably, than holders of subordinated securities. The accompanying prospectus supplement or the information incorporated herein by reference will contain the approximate amount of senior securities outstanding as of the end of our most recent fiscal quarter.

Global Debt Securities

The debt securities of a series may be issued in whole or in part in global form. The global securities will be deposited with a depositary, or with a nominee for a depositary, identified in the accompanying prospectus supplement. In this case, one or more global securities will be issued in a denomination or aggregate denominations equal to the portion of the aggregate principal amount of outstanding debt securities of the series to be represented by the global security or securities. Unless and until it is exchanged in whole or in part for debt securities in definitive form, a global security may not be transferred except as a whole by the depositary for the global security to a nominee of the depositary or by a nominee of the depositary to the depositary or another nominee of the depositary or by the depositary or any nominee to a successor of the depositary or a nominee of the successor.

The specific material terms of the depositary arrangement with respect to any portion of a series of debt securities to be represented by a global security will be described in the applicable accompanying prospectus supplement. We anticipate that the following provisions will apply to all depositary arrangements.

Upon the issuance of a global security, the depositary for the global security will credit, on its book-entry registration and transfer system, the respective principal amounts of the debt securities represented by the global security to the accounts of persons or participants that have accounts with the depositary. The accounts to be credited will be designated by any underwriters or agents participating in the distribution of the debt securities. Ownership of beneficial interests in a global security will be limited to participants or persons that may hold interests through participants. Ownership of beneficial interests in the global security will be shown on, and the transfer of that ownership will be effected only through, records maintained by the depositary for the global security, with respect to interests of participants, or by participants or persons that hold through participants, with respect to interests of persons other than participants. So long as the depositary for a global security, or its nominee, is the registered owner of the global security, the depositary or the nominee, as the case may be, will be considered the sole owner or holder of the debt securities represented by the global security for all purposes under the indenture; provided, however, that for purposes of obtaining any consents or directions required to be given by the holders of the debt securities, we, the trustee and our agents will treat a person as the holder of the principal amount of debt securities as specified in a written statement of the depositary. Except as set forth herein or otherwise provided in the accompanying prospectus supplement, owners of beneficial interests in a global security will not be entitled to have the debt securities represented by the global security registered in their names, will not receive physical delivery of the debt securities in definitive form and will not be considered the owners or holders thereof under the indenture.

Principal, premium, if any, and interest payments on debt securities represented by a global security registered in the name of a depositary or its nominee will be made to the depositary or its nominee, as the case may be, as the registered owner of the global security. Neither we, the trustee nor any paying agent for the debt securities will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in the global security or for maintaining, supervising or reviewing any records relating to the beneficial ownership interests.

We expect that the depositary for any debt securities represented by a global security, upon receipt of any payment of principal, premium, if any, or interest will immediately credit participants’ accounts with payments in amounts proportionate to their respective beneficial interests in the principal amount of the global security as shown on the records of the depositary. We also expect that payments by participants will be governed by standing instructions and customary practices, as is now the case with the securities held for the accounts of customers registered in “street names” and will be the responsibility of the participants.

If the depositary for any debt securities represented by a global security is at any time unwilling or unable to continue as depositary and a successor depositary is not appointed by us within the period of time set forth in the indenture, we will issue the debt securities in definitive form in exchange for the global security. In addition, we

may at any time, and in our sole discretion, determine not to have any of the debt securities of a series represented by one or more global securities and, in that event, will issue debt securities of the series in definitive form in exchange for all of the global security or securities representing the debt securities.

Governing Law

The indenture and the debt securities will be governed by and construed in accordance with the internal laws of the State of New York.

DESCRIPTION OF WARRANTS

The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the warrants that we may offer under this prospectus. While the terms we have summarized below will apply generally to any warrants that we may offer under this prospectus, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. The terms of any warrants offered under a prospectus supplement may differ from the terms described below.

We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from another report that we file with the SEC, the form of warrant agreement, which may include a form of warrant certificate, that describes the terms of the particular series of warrants we are offering. The following summary of material provisions of the warrants and the warrant agreements are subject to all the provisions of the warrant agreement and warrant certificate applicable to a particular series of warrants. We urge you to read the applicable prospectus supplements related to the particular series of warrants that we sell under this prospectus, as well as the complete warrant agreements and/or warrant certificates that contain the terms of the warrants.

General

We will describe in the applicable prospectus supplement the terms relating to warrants being offered, which may include:

Before exercising their warrants, holders of warrants will likely not have any of the rights of holders of the securities purchasable upon such exercise, including, in the case of warrants to purchase common stock or preferred stock, the right to receive dividends, if any, or payments upon our liquidation, dissolution or winding up of our affairs or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. Unless we otherwise

specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to the specified time on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

Holders of the warrants may exercise the warrants by delivering the warrant certificate representing the warrants to be exercised together with specified information, and paying the required amount to the warrant agent in immediately available funds, as provided in the applicable prospectus supplement. We intend to set forth in any warrant agreement and in the applicable prospectus supplement the information that the holder of the warrant will be required to deliver to the warrant agent.

Upon receipt of the required payment and any warrant certificate or other form required for exercise properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will issue and deliver the securities purchasable upon such exercise. If fewer than all of the warrants represented by the warrant or warrant certificate are exercised, then we will issue a new warrant or warrant certificate for the remaining amount of warrants. If we so indicate in the applicable prospectus supplement, holders of the warrants may surrender securities as all or part of the exercise price for warrants.

DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS

We may issue stock purchase contracts, representing contracts obligating holders to purchase from us, and requiring us to sell to the holders, a specified number of shares of common stock at a future date or dates. The price per share of common stock may be fixed at the time the stock purchase contracts are issued or may be determined by reference to a specific formula set forth in the stock purchase contracts. The stock purchase contracts may be issued separately or as a part of units, or stock purchase units, consisting of a stock purchase contract and either (x) senior debt securities, senior subordinated debt securities, subordinated debt securities or junior subordinated debt securities, or (y) debt obligations of third parties, including U.S. Treasury securities, in each case, securing the holder’s obligations to purchase the common stock under the stock purchase contracts. The stock purchase contracts may require us to make periodic payments to the holders of the stock purchase contracts or vice versa, and such payments may be unsecured or prefunded on some basis. The stock purchase contracts may require holders to secure their obligations thereunder in a specified manner and in certain circumstances we may deliver newly issued prepaid stock purchase contracts, or prepaid securities, upon release to a holder of any collateral securing such holder’s obligations under the original stock purchase contract.

The applicable prospectus supplement will describe the terms of any stock purchase contracts or stock purchase units and, if applicable, prepaid securities. The description in the prospectus supplement will not purport to be complete and will be qualified in its entirety by reference to the stock purchase contracts, the collateral arrangements and depositary arrangements, if applicable, relating to such stock purchase contracts or stock purchase units and, if applicable, the prepaid securities and the document pursuant to which such prepaid securities will be issued.

PLAN OF DISTRIBUTION

TheseAltimmune may sell common stock, preferred stock, depositary shares, debt securities, may be offered and sold directly by us, through dealerswarrants, stock purchase contracts or agents designatedstock purchase units in one or more of the following ways from time to time, or time:

Altimmune may also sell it securities, including shares of its common stock, in one or more of the following transactions: (i) block transactions (which may involve crosses) in which a broker-dealer may sell all or a portion of such shares as agent, but may position and resell all or a portion of the block as principal to facilitate the transaction; (ii) purchases by any such broker-dealer as principal, and resale by such broker-dealer for its own account pursuant to an accompanying prospectus supplement; (iii) a special offering, an exchange distribution or a secondary distribution in accordance with applicable Nasdaq Global Market or other stock exchange, quotation system orover-the-counter market rules; (iv) ordinary brokerage transactions and transactions in which any such broker-dealer solicits purchasers; (v) sales “at the market” to or through a market maker or into an existing trading market, on an exchange or otherwise, for such shares; and (vi) sales in other ways not involving market makers or established trading markets.

For each offering of securities, the applicable prospectus supplement with respector other offering materials relating to the securities being offeredoffering will set forth the terms of such offering, including:

Any initial public offering prices, discounts or concessions allowed or reallowed or paid to our shareholders in lieu of dividends.

The distribution of securitiesdealers may be effected,changed from time to time, in one or more transactions, including:time.

The securities may be sold at a fixed price or prices, which may be changed, or at market prices prevailing at the time of sale, at prices relating to the prevailing market prices or at negotiated prices. The consideration may be cash or another form negotiated by the parties. Agents, underwriters or broker-dealers may be paid compensation for offering and selling the securities. That compensation may be in the form of discounts, concessions or commissions to be received from us or from the purchasers of the securities. Dealers and agents participating in the distribution of the securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts and commissions under the Securities Act. If such dealers or agents were deemed to be underwriters, they may be subject to statutory liabilities under the Securities Act.

We may also make direct sales through subscription rights distributed to our existing shareholders on a pro rata basis, which may or may not be transferable. In any distribution of subscription rights to our shareholders, if all of the underlying securities are not subscribed for, we may then sell the unsubscribed securities directly to third parties or may engage the services of one or more underwriters, dealers or agents, including standby underwriters, to sell the unsubscribed securities to third parties.

If underwriters are used in an offering, we may execute an underwriting agreement with such underwriters and will specify the name of each underwriter and the terms of the transaction (including any underwriting discounts and other terms constituting compensation of the underwriters and any dealers) in a prospectus supplement. If an underwriting syndicate is used, the managing underwriter(s) will be specified on the cover of the prospectus supplement. If underwriters are used in the sale, the underwriters will acquire the offered securities will be acquired by the underwriters for their own accountsaccount and may be resoldresell them from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealersThe offered securities may be changed from timeoffered either to time.the public through underwriting syndicates represented by one or more managing underwriters or by one or more underwriters without a syndicate. Unless otherwise set forth in thea prospectus supplement, the obligations of the underwriters to purchase the offeredany series of securities will be subject to certain conditions precedent, and the underwriters will be obligated to purchase all of the offeredsuch series of securities, if any are purchased.

In connection with underwritten offerings of the offered securities and in accordance with applicable law and industry practice, underwriters may over-allot or effect transactions that stabilize, maintain or otherwise affect the market price of the offered securities at levels above those that might otherwise prevail in the open market, including by entering stabilizing bids, effecting syndicate covering transactions or imposing penalty bids, each of which is described below:

These transactions may be effected on the New York Stock Exchange, in theover-the-counter market, or otherwise. Underwriters are not required to engage in any of these activities, or to continue such activities if commenced.

If dealers area dealer is used in an offering, wethe sale, Altimmune will sell thesuch offered securities to the dealersdealer, as principals.principal. The dealersdealer may then resell the offered securities to the public at varying prices which they determineto be determined by that dealer at the time offor resale. The names of the dealers and the terms of the transaction will be specifiedset forth in athe prospectus supplement.supplement relating to that transaction.

TheOffered securities may be sold directly by usAltimmune to one or more institutional purchasers, or through agents we designate. If agents are useddesignated by Altimmune from time to time, at a fixed price or prices, which may be changed, or at varying prices determined at the time of sale. Any agent involved in an offering, the namesoffer or sale of the agents and the termsoffered securities in respect of the agencywhich this prospectus is delivered will be specifiednamed, and any commissions payable by Altimmune to such agent will be set forth, in athe prospectus supplement.supplement relating to that offering. Unless otherwise indicated in asuch prospectus supplement, the agentsany such agent will actbe acting on a fortsbest efforts basis for the period of theirits appointment.

Dealers and agents named in a prospectus supplement may be deemed to be underwriters (within the meaning of the Securities Act) of the securities described therein. In addition, we may sell the securities directly to institutional investors or others who may be deemed to be underwriters within the meaning of the Securities Act with respect to any resales thereof.

In compliance with the guidelines of the Financial Industry Regulatory Authority (“FINRA”), the aggregate value of all compensation to be received by participating FINRA members in any offering will not exceed 8% of the offering proceeds.

Underwriters, dealers and agents may be entitled under agreements entered into with us to indemnification by us against specificcertain civil liabilities, including liabilities under the Securities Act, or to contribution with respect to payments whichthat the underwriters, dealers or agents may be required to make in respect thereof, under underwriting or other agreements. The termsthereof. Underwriters, dealers and agents may be customers of, any indemnification provisions will be set forth in a prospectus supplement. Certain underwriters, dealers or agents and their associates may engage in transactions with, andor perform services for us and our affiliates in the ordinary course of business.

If so indicated in a prospectus supplement, we will authorize underwriters or other persons acting asOther than our agents to solicit offers by institutional investors to purchasecommon stock, which is listed on the Nasdaq Global Market, each of the securities pursuant to contracts providing for payment and delivery on a future date. We may enter contracts with commercial and savings banks, insurance companies, pension funds, investment companies, educational and charitable institutions and other institutional investors. The obligations of any institutional investorissued hereunder will be subject to the condition that its purchasea new issue of the offered securities, will not be illegal at the time of delivery. The underwritershave no prior trading market, and other agents willmay or may not be responsible for the validity or performance of contracts.

Each prospectus supplement will indicate if the securities offered thereby will be listed on anya national securities exchange; however, we anticipate that anyexchange. Any common sharesstock sold pursuant to a prospectus supplement will be eligible for tradinglisted on the NYSE MKT,Nasdaq Global Market, subject to official notice of issuance. Any underwriters to whom Altimmune sells securities are sold by us for Publicpublic offering and sale may make a market in the securities, but such underwriters will not be obligated to do so and may discontinue any market making at any time without notice.

We cannot assure you that there will be a market for the offered securities.

LEGAL MATTERS

Certain legal matters relatingUnless otherwise stated in an accompanying prospectus supplement, Morrison & Foerster LLP, Boston, Massachusetts, will provide us with an opinion as to the issuance and salelegality of the securities hereby will be passed upon by Dentons US LLP, New York, New York. Additional legal matters may be passed upon for us oroffered under this prospectus. Counsel representing any underwriters, dealers or agents by counsel that we will namebe named in the applicable prospectus supplement.

EXPERTS

The consolidated financial statements of PharmAtheneAltimmune, Inc. appearing in PharmAtheneAltimmune, Inc.’s Annual Report (Form10-K) for the year ended December 31, 2016, and the effectiveness of PharmAthene Inc.’s internal control over financial reporting as of December 31, 20162018, have been audited by ErnestErnst & Young LLP, independent registered public accounting firm, as set forth in their reportsreport thereon, incorporated by referenceincluded therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance uponon such reportsreport given on the authority of suchsaid firm as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We are subject to the informational reporting requirements of the Exchange Act and file annual, quarterly and current reports, proxy statements and other information with the SEC. You may read and copy any materials we file with the SEC at the SEC’s Public Reference Room located at 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC -0330 for further information on the Public Reference Room. You may also access filed documents at the SEC’s website at www.sec.gov.$250,000,000

Common Stock

Preferred Stock

Depositary Shares

Debt Securities

Warrants

Stock Purchase Contracts

Stock Purchase Units

PROSPECTUS

The date of filing of such information and automatically adds to, updates or supersedes the information listed below.

We incorporate by reference the following documents we have filed, or may file, with the SEC:

To the extent that any information contained in any Current Report on Form 8-K, or any exhibit thereto, is furnished to, rather than filed with, the SEC, such information or exhibit is specifically not incorporated by reference in this prospectus.

We make available free of charge through our website at www.pharmathene.com our press releases and all of the documents that we are required to file electronically with the SEC, including all amendments thereto, as soon as reasonably practical after they are electronically filed with, or furnished to, the SEC. Our website also contains our Code of Ethics. The information on our website is not part of nor incorporated by reference into this prospectus.

You may also read and copy any materials we file with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC -0330. The SEC also maintains an Internet site that contains reports, proxy and information statements, and other information regarding issuers, like PharmAthene, that file electronically with the SEC at http://www.sec.gov.

In addition, we will provide, without charge to each person, including any beneficial owner, to whom this prospectus is                 delivered, upon written or oral request of such person, a copy of any or all of the documents incorporated by reference in this prospectus other than exhibits, unless such exhibits specifically are incorporated by reference into such documents or this prospectus. Requests for such documents should be addressed in writing or by telephone to: PharmAthene, Inc., One Park Place, Suite 450, Annapolis, MD 21401, (410) 269-2600.2019.

PART II.

II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution.

Item 14.

Other Expenses of Issuance and Distribution

The following is a statement of the estimated costs and expenses other(other than underwriting compensation, incurred or expecteddiscounts and commissions) to be incurredborne by usAltimmune, Inc. in connection with the issuance and distribution of the securities being registered pursuant tounder this registration statement. All of the amounts shown are estimates exceptExcept for the SEC registration fee. Thefee, all amounts do not include the costs of preparing any prospectus supplements, NYSE MKT listing fees, FINRA filing fees, transfer agent fees or other expenses relating to the sale and distribution of particular securities registered pursuant to this registration statement, as those costs and expenses cannot be estimated at this time.are estimates.

Item 15. Indemnification of Officers and Directors.

+	The amount of securities and number of offerings are indeterminable and the expenses cannot be estimated at this time. An estimate of the aggregate expenses in connection with the sale and distribution of securities being offered will be included in any applicable prospectus supplement.

Our Amended and Restated Certificate of Incorporation, as amended, provides that the Company, to the full extent permitted by Section 145 of the Delaware General Corporation Law, as amended from time to time, shall indemnify all persons whom it may indemnify pursuant such law. It further provides that expenses (including attorneys’ fees) incurred by an officer or director in defending any civil, criminal, administrative, or investigative action, suit or proceeding for which such officer or director may be entitled to indemnification under our Amended and Restated Certificate of Incorporation, as amended, shall be paid by the Company in advance of the final disposition of such action, suit or proceeding upon receipt of an undertaking by or on behalf of such director or officer to repay such amount if it shall ultimately be determined that he is not entitled to be indemnified by the Company as authorized thereby.

Our Bylaws, as amended, provide the Company with the power to indemnify its officers, directors, employees and agents or any person serving at the Company’s request as a director, officer, employee or agent of another corporation, partnership, joint venture, trust or other enterprise to the fullest extent permitted by Delaware law.

All of our directors and officers are covered by insurance policies maintained by the Company against certain liabilities for actions taken in their capacities as such, including liabilities under the Securities Act.

On January 21, 2009, the Company’s board of directors approved a form of indemnification agreement (the “Indemnification Agreement”) and authorized the Company to enter into such Indemnification Agreement with each of the Company’s current directors and executive officers, as well as other key employees to be identified by the chief executive officer from time to time.

Pursuant to the Indemnification Agreements, the Company generally agreed, in exchange for each person’s continued service as a director, officer or other employee, as applicable, to indemnify and hold harmless each such person to the fullest extent permitted by law against certain expenses, judgments, penalties, fines and

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settlement payments incurred in connection with any proceeding other than a derivative suit (and against certain expenses in connection with a derivative suit) and resulting from his or her service to the Company.

The foregoing description of the Indemnification Agreements does not purport to be complete and is qualified in its entirety by reference to the actual agreement, a form of which is attached as an exhibit to the Company’s Current Report on Form8-K, filed on January 27, 2009.

Item 16. Exhibits.

Item 16.

Exhibits

See the indexExhibit Index attached to exhibits, which isthis Registration Statement and incorporated herein by reference.

Item 17. Undertakings.

Item 17.

Undertakings

(a) The undersigned registrant hereby undertakes:

(1) toTo file, during theany period in which offers or sales are being made, a post-effective amendment to this registration statement:

(i) toTo include any prospectus required by Section 10(a)(3) of the Securities Act of 1933, as amended (the “Securities Act” or “Securities Act of 1933”);

Act;

(ii) toTo reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the CommissionSEC pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than 20 percenta 20% change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement;

(iii) toTo include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;

provided,however, that paragraphs (a)(1)(i), (a)(1)(ii) and (a)(1)(iii) doshall not apply if the information required to be included in a post- effectivepost-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the CommissionSEC by the registrant pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) that are incorporated by reference in the registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement;

statement.

(2) that,That, for the purpose of determining any liability under the Securities Act, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof; and

thereof.

(3) toTo remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

(5) that,(4) That, for the purpose of determining liability under the Securities Act of 1933 to any purchaser:

(i) (A)(a) Each prospectus filed by the registrant pursuant to Rule 424(b)(3) shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and

(B)II-2

(b) Each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering made pursuant to Rule 415(a)(1)(i), (vii), or (x) for the purpose of providing the information required by Section 10(a) of the Securities Act of 1933 shall be deemed to be part of and included in the registration statement as of the earlier of the date such form of prospectus is first used after effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which thethat prospectus relates, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof.Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such effective date.

(6) that, for the purpose of determining liability of the registrant under the Securities Act of 1933 to any purchaser in the initial distribution of the securities, the undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

(i) Any preliminary prospectus or prospectus of the undersigned registrant relating to the offering required to be filed pursuant to Rule 424 (§230.424 of this chapter);

(ii) Any free writing prospectus relating to the offering prepared by or on behalf of the undersigned registrant or used or referred to by the undersigned registrant;

(iii) The portion of any other free writing prospectus relating to the offering containing material information about the undersigned registrant or its securities provided by or on behalf of the undersigned registrant; and

(iv) Any other communication that is an offer in the offering made by the undersigned registrant to the purchaser.

(b)(c) The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act, of 1933, each filing of the registrant’s annual report pursuant to Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934)Act) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initialbona fide offering thereof.

(h)(d) Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange CommissionSEC such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

(e) To file an application for the purpose of determining the eligibility of the trustee to act under subsection (a) of Section 310 of the Trust Indenture Act in accordance with the rules and regulations prescribed by the SEC under Section 305(b)(2) of the Trust Indenture Act.

EXHIBIT INDEX

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SIGNATURES

Pursuant to the requirements of the Securities Act, of 1933, the RegistrantAltimmune, Inc. certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on FormS-3 and has duly caused this Registration Statement on FormS-3 to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Annapolis,Gaithersburg, State of Maryland, on March 30, 2017.the 4^th day of April, 2019.

POWER OF ATTORNEY

KNOW ALL MENPERSONS BY THESE PRESENTS, that each of the undersignedperson whose signature appears below constitutes and appoints John M. Gill,Vipin K. Garg and Philip MacNeill,Will Brown, and each of them, as his or herattorneys-in-fact and agents, each with fullthe power of substitution, and resubstitution, for and in the name, place and stead of the undersigned,him or her in any and all capacities, to sign this Registration Statement, any and all amendments thereto (including post- effectivepost-effective amendments) to this Registration Statement or any Registration Statement for this offering that is to be effective upon the filing pursuant to Rule 462(b) under the Securities Act of 1933, as amended, and all post-effective amendments thereto,, and to file the same, with all exhibits thereto and all other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorneys-in-fact and agents full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as the undersigned might or could do in person,SEC, hereby ratifying and confirming all that each of said attorney-in-factattorneys-in fact, or his substitute or substitutes, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act, of 1933, this Registration Statement has been signed below by the following persons in the capacities indicated, on March 30, 2017.listed opposite their respective names.

Index to Exhibits

* Filed herewith.II-5