Altimmune, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

PROXY STATEMENT PURSUANT TO SECTION 14(a) OF THE

SECURITIES EXCHANGE ACT OF 1934

(Amendment No. )

(Name of Registrant as Specified In Its Charter)

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

March 17, 2026

To the Stockholders of Altimmune, Inc.:

You are invited to attend the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of Altimmune, Inc. (the “Company”) scheduled for Thursday, April 16, 2026, at 8:30 a.m., Eastern Time. The Annual Meeting will be held virtually, conducted via live webcast. You may attend the meeting virtually via the Internet at https://www.cstproxy.com/altimmune/2026, where you will be able to vote electronically and submit questions. You will need the 12-digit control number included with these proxy materials to attend the Annual Meeting. The Company’s Board of Directors and management look forward to speaking with you.

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

At this Annual Meeting, the agenda includes the election of nine directors, each for a one-year term, the ratification of the appointment of Ernst & Young LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2026, an advisory vote on the compensation of the Company’s named executive officers, approval of an amendment to our amended and restated certificate of incorporation to increase the authorized shares of common stock and approval of an amendment to the Altimmune, Inc. 2019 Employee Stock Purchase Plan (the “2019 ESPP”).

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

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

NOTICE OF

2026 ANNUAL MEETING OF STOCKHOLDERS

To the Stockholders of Altimmune, Inc.:

NOTICE IS HEREBY GIVEN that the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of Altimmune, Inc., a Delaware corporation (the “Company”), will be held on Thursday, April 16, 2026, at 8:30 a.m., Eastern Time. The Annual Meeting will be held virtually, conducted via live webcast. You may attend the meeting virtually via the Internet at https://www.cstproxy.com/altimmune/2026.

At the Annual Meeting we will:

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

The record date for the determination of the stockholders entitled to notice of, and to vote at, the Annual Meeting, or any adjournment or postponement thereof, was the close of business on March 13, 2026. You have the right to receive this Notice and vote at the Annual Meeting if you were a stockholder of record at the close of business on March 13, 2026. All stockholders as of the record date, or their duly appointed proxies, may attend the meeting virtually. In order to be able to attend the meeting, you will need the 12-digit control number, which is located on your proxy card, or in the instructions accompanying your proxy materials. Instructions on how to participate in the Annual Meeting are also posted online at https://www.cstproxy.com/altimmune/2026. Please remember that your shares cannot be voted unless you cast your vote by one of the following methods: (1) vote via the Internet or call the toll-free number as indicated on the proxy card; (2) sign and return a paper proxy card; or (3) vote at the Annual Meeting via the Internet.

YOUR VOTE IS VERY IMPORTANT,REGARDLESS OF THE NUMBER OF SHARES YOU OWN.PLEASE READ THE ATTACHED PROXY STATEMENT CAREFULLY AND COMPLETE AND SUBMIT YOUR PROXY CARD VIA THE INTERNET OR SIGN AND DATE YOUR PAPER PROXY CARD AS PROMPTLY AS POSSIBLE AND RETURN IT IN THE ENCLOSED ENVELOPE.ALTERNATIVELY,YOU MAY BE ABLE TO SUBMIT YOUR PROXY BY TOUCH-TONE PHONE AS INDICATED ON THE PROXY CARD.