EXHIBIT 5.1


                         [LATHAM & WATKINS LETTERHEAD]


October 23, 2001


Aviron
297 N. Bernardo Avenue
Mountain View, CA  94043

     Re: Registration Statement on Form S-8

Ladies and Gentlemen:

          In connection with the registration of 1,000,000 additional shares of
common stock, par value $.001 per share (the "Shares"), of Aviron, a Delaware
corporation (the "Company"), issuable under the Company's 1999 Non-Officer
Equity Incentive Plan, as amended, (the "Plan"), on a Registration Statement on
Form S-8 (the "Registration Statement"), to be filed with the Securities and
Exchange Commission under the Securities Act of 1933, as amended, you have
requested our opinion with respect to the matters set forth below.

          In our capacity as your counsel in connection with such registration,
we are familiar with the proceedings taken by the Company in connection with the
authorization, issuance and sale of the Shares. In addition, we have made such
legal and factual examinations and inquiries, including an examination of
originals or copies certified or otherwise identified to our satisfaction of
such documents, corporate records and instruments, as we have deemed necessary
or appropriate for purposes of this opinion.

          In our examination, we have assumed the genuineness of all signatures,
the authenticity of all documents submitted to us as originals, and the
conformity to authentic original documents of all documents submitted to us as
copies.

          We are opining herein as to the effect on the subject transaction of
only the General Corporation Law of the State of Delaware, and we assume no
responsibility as to the application to the subject transaction, or the effect
thereon, of any other laws, of the laws of any other jurisdiction or as to any
matters of municipal law or any other local agencies within any state.

          Subject to the foregoing it is our opinion that, as of the date
hereof, upon the issuance and sale of the Shares, each in the manner
contemplated by the Registration Statement and in accordance with the terms of
the Plan, and subject to the Company completing all action and proceedings
required on its part to be taken prior to the issuance of the Shares pursuant to
the terms of the Plan and the Registration Statement, including,
without limitation, collection of

October 23, 2001
Page 2


required payment for the Shares, the Shares will be validly issued, fully paid
and nonassessable securities of the Company.

          We consent to your filing this opinion as an exhibit to the
Registration Statement.



                                        Very truly yours,



                                        /s/ LATHAM & WATKINS