Exhibit 5.1

	Goodwin Procter LLP
	100 Northern Avenue Boston, MA 02210
	goodwinlaw.com
	+1 617 570 1000

April 22, 2026

Altimmune, Inc.

910 Clopper Road, Suite 201S

Gaithersburg, MD 20878

We have acted as counsel to you in connection with your filing of (i) a Registration Statement on Form S-3 (File No. 333-291329) (as amended or supplemented, the “Initial Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), and (ii) a Registration Statement on Form S-3 filed pursuant to Rule 462(b) promulgated under the Securities Act relating to the Initial Registration Statement (the “462(b) Registration Statement” and, together with the Initial Registration Statement, the “Registration Statement”), relating to the offering by Altimmune, Inc., a Delaware corporation (the “Company”), of up to $65,000,000 of any combination of (i) the Company’s common stock, par value $0.0001 per share (the “Common Stock”), (ii) warrants to purchase Common Stock or Units (as defined below ) (“Warrants”), and (iii) units, comprised of Common Stock, Warrants and other securities in any combination (“Units”). The Common Stock, Warrants and Units are sometimes referred to collectively herein as the “Securities.”.

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.

The opinions set forth below are limited to the Delaware General Corporation Law and the law of New York.

For purposes of the opinions set forth below, without limiting any other exceptions or qualifications set forth herein, we have assumed that (i) each of the Warrants and Units, and the warrant agreements, unit agreements and other agreements governing Securities offered pursuant to the Registration Statement will be governed by the internal law of New York and (ii) after the issuance of any Securities offered pursuant to the Registration Statement, the total number of issued shares of Common Stock, together with the total number of shares of such stock issuable upon the exercise, exchange, conversion or settlement, as the case may be, of any exercisable, exchangeable or convertible security (including without limitation any Unit), as the case may be, then outstanding, will not exceed the total number of authorized shares of Common Stock then available for issuance under the Company’s certificate of incorporation as then in effect (the “Charter”).

For purposes of the opinions set forth below, we refer to the following as the “Future Approval and Issuance” of Securities:

Based upon the foregoing, and subject to the additional qualifications set forth below, 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.

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).

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the 462(b) Registration Statement and to the references to our firm under the caption “Legal Matters” in the Initial 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.