Altimmune Inc — 8-K exhibit

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Exhibit 5.1

Altimmune, Inc.

910 Clopper Road

Suite 201S, Gaithersburg, MD 20878

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-3 (File No. 333-291329) (the “Registration Statement”), as amended, initially filed on November 6, 2025 and deemed effective on December 5, 2025 with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration of the offer by Altimmune, Inc., a Delaware corporation (the “Company”), of any combination of securities of the types specified therein. Reference is made to our opinion letter dated November 6, 2025 and included as Exhibit 5.1 to the Registration Statement. We are delivering this supplemental opinion letter in connection with the prospectus supplement (the “Prospectus Supplement”) filed on January 28, 2026 by the Company with the Commission pursuant to Rule 424 under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to (i) 12,397,920 shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share (the “Common Stock”), (ii) pre-funded warrants (the “Pre-Funded Warrants”) to purchase up to 4,647,534 shares of Common Stock and (iii) the shares of Common Stock issuable from time to time upon exercise of the Pre-Funded Warrants (the “Pre-Funded Warrant Shares”), covered by the Registration Statement. The Shares and the Pre-Funded Warrants are being sold to the several purchasers named in, and pursuant to, a securities purchase agreement among the Company and the purchaser identified on the signature page thereto (the “Securities Purchase Agreement”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinions set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinions set forth below, on certificates of officers of the Company. For purposes of the opinion set forth in paragraph 3 below, we have assumed that before the Pre-Funded Warrant Shares are issued, the Company does not issue shares of Common Stock or reduce the total number of shares of Common Stock the Company is authorized to issue under its certificate of incorporation such that the number of unissued shares of Common Stock authorized under the Company’s certificate of incorporation is less than the number of Pre-Funded Warrant Shares.

The opinions set forth below are limited to the Delaware General Corporation Law, and with respect to the opinion in paragraph 2, the law of the State of New York.

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Based on the foregoing, we are of the opinion that

The opinions expressed above are subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and other similar laws of general application affecting the rights and remedies of creditors and to general principles of equity.

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This opinion letter and the opinions it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).

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We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Current Report on Form 8-K and to the references to our firm under the caption “Legal Matters” in the Registration Statement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

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