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                        [COOLEY GODWARD LLP LETERHEAD]




October 24, 2000

Aviron
297 North Bernardo Avenue
Mountain View, Ca 94043


RE:     AMENDMENT NO. 1 TO REGISTRATION STATEMENT ON FORM S-3

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Aviron (the "Company") of Amendment No. 1 to the Registration
Statement on Form S-3 (the "Registration Statement") with the Securities and
Exchange Commission, including a prospectus (the "Prospectus"), covering the
offering of up to (i) 15,316 shares of the Company's Common Stock, $0.001 par
value (the "Sinai Shares"), and 26,079 shares of the Company's Common Stock (the
"Sinai Warrant Shares") issuable upon the exercise of warrants (the "Sinai
Warrants") sold pursuant to the Product Technology Transfer Agreement, dated
February 9, 1993, between the Company and Mount Sinai; (ii) 996,840 shares of
the Company's Common Stock (together with the Sinai Shares, the "Shares") sold
pursuant to various common stock purchase agreements between the Company and
American Home Products Corporation; and (iii) 63,162 shares of the Company's
Common Stock (together with the Sinai Warrant Shares, the "Warrant Shares")
issuable upon the exercise of warrants (together with the Sinai Warrants, the
"Warrants") sold pursuant to the Amended and Restated Contract Manufacture
Agreement, dated October 11, 2000, between the Company and Evans Vaccines
Limited.

In connection with this opinion, we have examined and relied upon the
Registration Statement and related Prospectus, the Company's Amended and
Restated Certificate of Incorporation, the Company's Amended and Restated
Bylaws, and the originals or copies certified to our satisfaction of such
documents, records, certificates, memoranda and other instruments as in our
judgment are necessary or appropriate to enable us to render the opinion
expressed below. We have assumed the genuineness and authenticity of all
documents submitted to us as copies thereof, and the due execution and delivery
of all documents where due execution and delivery are a prerequisite to the
effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares are, and the Warrant Shares when paid for and issued upon
exercise of the Warrants will be, validly issued, fully paid, and nonassessable.

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Aviron
October 24, 2000
Page Two

We consent to the reference to our firm under the caption "Legal Matters" in the
Prospectus included in the Registration Statement and to the filing of this
opinion as an exhibit to the Registration Statement.

Very truly yours,


/s/ MATTHEW W. SONSINI
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Matthew W. Sonsini