PIP Pharmathene, (delisted)

Supplemental Disclosure

As previously disclosed in the Current Report on Form 8-K filed by Altimmune, Inc. (“Altimmune” or the “Company”) on May 8, 2017, on May 4, 2017, the Company, formerly known as PharmAthene, Inc., completed its business combination (the “Merger”) with what was then known as Altimmune, Inc. in accordance with the terms of the Agreement and Plan of Merger and Reorganization, dated as of January 18, 2017 (as amended on March 29, 2017 and as further amended from time to time, the “Merger Agreement”).

Exhibit 99.1, Exhibit 99.2, and Exhibit 99.3, each of which are filed herewith and incorporated by reference herein, contain certain supplemental disclosure regarding Altimmune.

Annual Meeting of the Company’s Stockholders

The Board of Directors of the Company has determined that the Company’s 2017 Annual Meeting of Stockholders (the “2017 Annual Meeting”), will occur on or about October 13, 2017, instead of September 7, 2017 as previously disclosed in the Company’s Current Report on Form 8-K filed on June 9, 2017. The Board has set August 18, 2017 as the new record date for the 2017 Annual Meeting. Additional information about the 2017 Annual Meeting will be included in the Company’s proxy materials. Because the anticipated date of the 2017 Annual Meeting has been changed to a date that is more than 30 days later than the one-year anniversary date of the Company’s 2016 Annual Meeting of Stockholders, in accordance with Rule 14a-8 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and the Company’s Amended and Restated Bylaws (the “Bylaws”), the deadlines applicable to stockholder proposals have changed.

Stockholders who wish to have a proposal considered for inclusion in the Company’s proxy materials for the 2017 Annual Meeting pursuant to Rule 14a-8 under the Exchange Act must ensure that such proposal is received by the Company not later than the close of business on August 27, 2017. Any such proposal should be delivered to the Company at 19 Firstfield Road, Suite 200 Gaithersburg, Maryland, Attention: Corporate Secretary and must comply with the rules and regulations of the Securities and Exchange Commission under Rule 14a-8 in order to be eligible for inclusion in the proxy materials for the 2017 Annual Meeting.

In accordance with the Company’s Bylaws, for director nominations or other stockholder proposals (other than proposals pursuant to Rule 14a-8 under the Exchange Act) to be brought before the 2017 Annual Meeting, written notice must be received by the Company not later than August 27, 2017 by delivering such nominations or proposals in writing to the Company at 19 Firstfield Road, Suite 200 Gaithersburg, Maryland, Attention: Corporate Secretary. Such notices must comply with the requirements of the Company’s Bylaws and applicable law, and no director nomination or stockholder proposal may be presented at the 2017 Annual Meeting otherwise.