LIMITED POWER OF ATTORNEY FOR
                          SECTION 16 REPORTING PURPOSES

     Know all by these presents, that the undersigned hereby makes, constitutes
and appoints William Enright, Elizabeth Czerepak and Ori Solomon, each acting
individually, as the undersigned's true and lawful attorneys-in-fact, with full
power and authority as hereinafter described on behalf of and in the name, place
and stead of the undersigned to:

     (1)  execute for, and on behalf of, the undersigned, in the undersigned's
capacity as an officer, director and/or 10% beneficial owner of Altimmune, Inc.
(f/k/a PharmAthene, Inc.) (the "Company"), Forms 3, 4, and 5 in accordance with
Section 16(a) of the Securities Exchange Act of 1934 and the rules and
regulations promulgated thereunder (the "Exchange Act");

     (2)  seek or obtain, as the undersigned's representative and on the
undersigned's behalf, information on transactions in the Company's securities
from any third party, including brokers, employee benefit plan administrators
and trustees, and the undersigned hereby authorizes any such person to release
any such information to any attorney-in-fact and further approves and ratifies
any such release of information;

     (3)  do and perform any and all acts for and on behalf of the undersigned
that may be necessary or desirable to prepare, complete and execute any such
Form 3, 4 or 5, and any amendments thereto, or other required report and timely
file such Forms or reports with the United States Securities and Exchange
Commission (the "Commission"), the NASDAQ and any stock exchange or similar
authority as considered necessary or advisable under Section 16(a) of the
Exchange Act (including, without limitation, executing and delivering a Form ID
to the Commission to obtain EDGAR filing codes); and

     (4)  take any other action of any type whatsoever in connection with the
foregoing which, in the opinion of such attorney-in-fact, may be of benefit to,
in the best interest of, or legally required by, the undersigned, it being
understood that the documents executed by such attorney-in-fact on behalf of the
undersigned pursuant to this Limited Power of Attorney shall be in such form and
shall contain such terms and conditions as such attorney-in-fact may approve in
such attorney-in-fact's sole discretion.

     The undersigned hereby gives and grants to each such attorney-in-fact full
power and authority to do and perform any and every act and thing whatsoever
requisite, necessary, or proper to be done in the exercise of any of the rights
and powers herein granted, as fully to all intents and purposes as the
undersigned might or could do if personally present, with full power of
substitution or revocation, hereby ratifying and confirming all that such
attorney-in-fact, or such attorney-in-fact's substitute or substitutes, shall
lawfully do or cause to be done by virtue of this power of attorney and the
rights and powers herein granted.

     The undersigned hereby acknowledges that (a) the foregoing
attorneys-in-fact are serving in such capacity at the request of the
undersigned; (b) this Limited Power of Attorney authorizes, but does not
require, each such attorney-in-fact to act in his or her discretion on
information provided to such attorney-in-fact without independent verification
of such information; (c) any documents prepared and/or executed by any
attorney-in-fact on behalf of the undersigned pursuant to this Limited Power of
Attorney shall be in such form and shall contain such information and disclosure
as such attorney-in-fact, in his or her sole discretion, deems necessary or
advisable; (d) neither the Company nor any attorney-in-fact assumes (i) any
liability for the undersigned's responsibility to comply with the requirements
of the Exchange Act, (ii) any liability of the undersigned for any failure to
comply with such requirements or (iii) any obligation or liability of the
undersigned for profit disgorgement under Section 16(b) of the Exchange Act; and
(e) this Limited Power of Attorney does not relieve the undersigned from
responsibility for compliance with the undersigned's obligations under the
Exchange Act, including, without limitation, the reporting requirements under
Section 16 of the Exchange Act.

     This Limited Power of Attorney shall remain in full force and effect until
the undersigned is no longer required to file Forms 3, 4, and 5 with respect to
the undersigned's holdings of, and transactions in, securities issued by the
Company, unless earlier revoked as to any attorney-in-fact by the undersigned in
a signed writing delivered to such attorney-in-fact.

     IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to be
executed as of this 8th day of May, 2017.

                                        /s/ Klaus O. Shafer
                                        -------------------------
                                        Klaus O. Schafer