================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                          ----------------------------

                                   SCHEDULE TO

            TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR 13(e)(1)
                     OF THE SECURITIES EXCHANGE ACT OF 1934
                                (Amendment No. 1)

                              ---------------------

                                     AVIRON
                       (Name of Subject Company (Issuer))

                          ----------------------------

                               APPLE MERGER CORP.
                                 MEDIMMUNE, INC.
                                    (Offeror)

       (Names of Filing Persons (identifying status as offeror, issuer or
                                 other person))

                    Common Stock, Par Value $0.001 Per Share
                           Including Associated Rights
                         (Title of Class of Securities)

                                   0537 62100
                      (CUSIP Number of Class of Securities)

                                  DAVID M. MOTT
                   CHIEF EXECUTIVE OFFICER, AND VICE CHAIRMAN
                            35 WEST WATKINS MILL RD.
                             GAITHERSBURG, MD 20878
                                 (301) 417-0770
  (Name, Address and Telephone Number of Persons Authorized to Receive Notices
                 and Communications on Behalf of filing persons)
                          ----------------------------

                                   Copies to:

                   Frederick W. Kanner              Peter Kerman
                    Richard D. Pritz              Latham & Watkins
                  Dewey Ballantine LLP         135 Commonwealth Drive
               1301 Avenue of the Americas      Menlo Park, CA 94205
                   New York, NY 10019              (650) 328-4600



 Check the box if the filing relates solely to preliminary communications made
before the commencement of a tender offer. / /



Check the appropriate boxes to designate any transactions to which the statement
relates:

/X/  third-party tender offer subject to Rule 14d-1.

/ /  issuer tender offer subject to Rule 13e-4.

/ /  going-private transaction subject to Rule 13e-3.

/ /  amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results
of the tender offer: / /

================================================================================


                                       2






      This Amendment amends and supplements the Tender Offer Statement on
Schedule TO (as amended, the "Schedule TO") initially filed on December 10, 2001
by MedImmune Inc., a Delaware corporation ("Parent"), and Apple Merger Corp.,
Delaware corporation ("Sub") and a wholly owned subsidiary of Parent. This
Schedule TO relates to the offer by Sub to exchange 1.075 shares of common
stock, par value $0.01 per share, of Parent for each outstanding share of common
stock, par value $0.001 per share, of Aviron, a Delaware corporation (the
"Company"), including the associated rights to purchase common stock, upon the
terms and subject to the conditions set forth in the Prospectus dated December
10, 2001 (the "Prospectus") and in the related Letter of Transmittal, copies of
which are attached hereto as Exhibits (a)(1) and (a)(2) (which, together with
any amendments or supplements thereto, collectively constitute the "Offer").
Capitalized terms used herein and not otherwise defined shall have the meanings
set forth in the Schedule TO.

ITEM 12. MATERIAL TO BE FILED AS EXHIBITS.

      Item 12 of the Schedule TO is hereby amended and supplemented to include
the following exhibit:

      (a)(8)      Summary Advertisement as published in The Wall Street Journal
                  on December 11, 2001 (incorporated by reference to the filing
                  by Parent under Rule 425 on December 11, 2001).




                                       3




      After due inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.

Dated: December 11, 2001


                                          MEDIMMUNE, INC.

                                          By: /s/ David M. Mott
                                              -----------------
                                              Name: David M. Mott
                                              Title: Chief Executive Officer

                                          APPLE MERGER CORP.

                                          By: /s/ David M. Mott
                                              -----------------
                                              Name: David M. Mott
                                              Title: Chief Executive Officer






                                  EXHIBIT INDEX

EXHIBIT NO.

(a)(1)            Prospectus dated December 10, 2001 (incorporated by reference
                  from Parent's Registration Statement on Form S-4 filed on
                  December 10, 2001).

(a)(2)            Form of Letter of Transmittal (incorporated by reference to
                  exhibit 99.1 to Parent's Registration Statement on Form S-4
                  filed on December 10, 2001).

(a)(3)            Form of Notice of Guaranteed Delivery (incorporated by
                  reference to exhibit 99.2 to Parent's Registration Statement
                  on Form S-4 filed on December 10, 2001).

(a)(4)            Form of Letter from the Dealer Manager to Brokers, Dealers,
                  Commercial Banks, Trust Companies and Other Nominees
                  (incorporated by reference to exhibit 99.3 to Parent's
                  Registration Statement on Form S-4 filed on December 10,
                  2001).

(a)(5)            Form of Letter from Brokers, Dealers, Commercial Banks, Trust
                  Companies and Other Nominees to Clients (incorporated by
                  reference to exhibit 99.4 to Parent's Registration Statement
                  on Form S-4 filed on December 10, 2001).

(a)(6)            Form of Guidelines for Certification of Taxpayer
                  Identification Number on Substitute Form W-9 (incorporated by
                  reference to exhibit 99.5 to Parent's Registration Statement
                  on Form S-4 filed on December 10, 2001).

(a)(7)            Press Release issued by Parent on December 3, 2001
                  (incorporated by reference to the filing by Parent under Rule
                  425 on December 3, 2001).

(a)(8)*           Summary Advertisement as published in The Wall Street Journal
                  on December 11, 2001 (incorporated by reference to the filing
                  by Parent under Rule 425 on December 11, 2001).

(b)               None.

(d)(1)            Agreement and Plan of Merger, dated as of December 2, 2001,
                  among Parent, Sub and the Company (incorporated by reference
                  to exhibit 2.1 to Parent's Registration Statement on Form S-4
                  filed on December 10, 2001).

(d)(2)            Confidentiality Agreement, dated October 10, 2001, between
                  Parent and the Company.



(g)               None.

(h)(1)            Tax opinion of Dewey Ballantine LLP (incorporated by reference
                  to exhibit 8.1 to Parent's Registration Statement on Form S-4
                  filed on December 10, 2001).

(h)(2)            Tax opinion of Latham & Watkins (incorporated by reference to
                  exhibit 8.2 to Parent's Registration Statement on Form S-4
                  filed on December 10, 2001).

-------------
* Other than (a)(8), all exhibits previously filed.