1
                                                                     EXHIBIT 5.1


                         [LATHAM & WATKINS LETTERHEAD]


                                  April 9, 2001



Aviron
297 N. Bernardo Avenue
Mountain View, CA  94043

          Re:  Offering of 450,000 Shares of Aviron's Common Stock

Ladies and Gentlemen:

          In connection with the registration under the Securities Act of 1933,
as amended (the "Act"), of 450,000 shares of common stock, par value $0.001 per
share (the "Shares"), of Aviron, a Delaware corporation (the "Company"), you
have requested our opinion with respect to the matters set forth below.

          In our capacity as your special counsel in connection with such
registration, 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 have been furnished with, and with your consent have exclusively
relied upon, certificates of officers of the Company with respect to certain
factual matters. In addition, we have obtained and relied upon such certificates
and assurances from public officials as we have deemed necessary.

          Our opinion is limited to the Federal Laws of the United States and
the General Corporation Law of the State of Delaware, and we express no opinion
with respect to the applicability or effect of any other laws.

          Subject to the foregoing, we advise you that, in our opinion the
Shares have been duly authorized and validly issued and are fully paid and
nonassessable.
   2
Aviron
April 9, 2001
Page 2


          We consent to your filing this opinion as an exhibit to the
registration statement relating to the Shares and to the reference to our firm
contained under the heading "Legal Matters."

          This opinion is rendered only to you and is solely for your benefit in
connection with the transactions covered hereby. This opinion may not be relied
upon by you for any other purpose, or furnished to, quoted to or relied upon by
any other person, firm or corporation for any purpose, without our prior written
consent.

                                         Very truly yours,

                                         /s/ LATHAM & WATKINS