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                                                                    EXHIBIT 4.20

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER THE SECURITIES LAWS OF ANY STATE.
THESE SECURITIES ARE SUBJECT TO RESTRICTIONS ON TRANSFERABILITY AND RESALE AND
MAY NOT BE TRANSFERRED OR RESOLD EXCEPT AS PERMITTED UNDER THE ACT AND THE
APPLICABLE STATE SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION
THEREFROM. THE ISSUER OF THESE SECURITIES MAY REQUIRE AN OPINION OF COUNSEL IN
FORM AND SUBSTANCE SATISFACTORY TO THE ISSUER TO THE EFFECT THAT ANY PROPOSED
TRANSFER OR RESALE IS IN COMPLIANCE WITH THE ACT AND ANY APPLICABLE STATE
SECURITIES LAWS.


                                     AVIRON

               WARRANT FOR THE PURCHASE OF SHARES OF COMMON STOCK


No. CSW- 6                                                         10,527 Shares


        FOR VALUE RECEIVED, AVIRON, a Delaware corporation (the "Company"), with
its principal office at 297 North Bernardo Avenue, Mountain View, CA 94043,
hereby certifies that Evans Vaccines Limited, or its assigns (the "Holder"), is
entitled, subject to the provisions of this Warrant, to purchase from the
Company, at any time after the Exercise Date (as defined below) and prior to the
Expiration Date (as defined below), Ten Thousand Five Hundred Twenty-Seven
(10,527) fully paid and nonassessable shares of Common Stock of the Company, at
an exercise price per share equal to forty-seven dollars and fifty cents
($47.50) (the "Exercise Price"). The term "Common Stock" shall mean the
aforementioned Common Stock of the Company, together with any other equity
securities that may be issued by the Company in addition thereto or in
substitution therefor as provided herein.

        The number of shares of Common Stock to be received upon the exercise of
this Warrant and the price to be paid for a share of Common Stock are subject to
adjustment from time to time as hereinafter set forth. The shares of Common
Stock deliverable upon such exercise, as adjusted from time to time, are
hereinafter sometimes referred to as "Warrant Shares."

        SECTION 1. EXERCISE OF WARRANT. This Warrant may be exercised in whole
or in part on any business day after the date hereof (the "Exercise Date") and
prior to the Expiration Date by presentation and surrender to the Company at its
principal office at the address set forth in the initial paragraph hereof (or at
such other address as the Company may hereafter notify the Holder in writing)
with the Purchase Form annexed hereto duly executed and accompanied by proper
payment of the Exercise Price in lawful money of the United States of America in
the form of a check, subject to collection, for the number of Warrant Shares
specified in the Purchase Form. If this Warrant should be exercised in part
only, the Company shall, upon surrender of this Warrant, execute and deliver a
new Warrant evidencing the rights of the Holder thereof to purchase the balance
of the Warrant Shares purchasable hereunder. Upon receipt by



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the Company of this Warrant and such Purchase Form, together with proper payment
of the Exercise Price, at such office, the Holder shall be deemed to be the
holder of record of the Warrant Shares, notwithstanding that the stock transfer
books of the Company shall then be closed or that certificates representing such
Warrant Shares shall not then be actually delivered to the Holder.

        SECTION 2. ISSUANCE OF NEW WARRANT. In the event of any exercise of the
rights represented by this Warrant, certificates for the Warrant Shares so
purchased shall be delivered to the Holder hereof as soon as practicable and,
unless this Warrant has been fully exercised or has expired, a new Warrant
representing the portion of the Warrant Shares, if any, with respect to which
this Warrant shall not then have been exercised shall also be issued to the
Holder hereof within a reasonable time. Such exercise shall be deemed to have
been made immediately prior to the close of business on the date of surrender of
this Warrant.

        SECTION 3. RESERVATION OF SHARES. The Company hereby agrees that at all
times there shall be reserved for issuance and delivery upon exercise of this
Warrant all shares of its Common Stock or other shares of capital stock of the
Company from time to time issuable upon exercise of this Warrant. All such
shares shall be duly authorized and, when issued upon such exercise in
accordance with the terms of this Warrant, shall be validly issued, fully paid
and nonassessable.

        SECTION 4. FRACTIONAL INTEREST. The Company will not issue a fractional
share of Common Stock upon exercise of this Warrant. Instead, the Company will
deliver its check for the current fair market value of the fractional share, as
determined in good faith by the Board of Directors of the Company.

        SECTION 5. ASSIGNMENT OR LOSS OF WARRANT.

                (a) Except as provided in Section 9, the Holder of this Warrant
shall be entitled, without obtaining the consent of the Company, to assign its
interest in this Warrant in whole or in part to any person or persons. Subject
to the provisions of Section 9, upon surrender of this Warrant to the Company or
at the office of its stock transfer agent or warrant agent, with the Assignment
Form annexed hereto duly executed and funds sufficient to pay any transfer tax,
the Company shall, without charge, execute and deliver a new Warrant or Warrants
in the name of the assignee or assignees named in such instrument of assignment
and, if the Holders entire interest is not being assigned, in the name of the
Holder, and this Warrant shall promptly be canceled.

                (b) Upon receipt of evidence satisfactory to the Company of the
loss, theft, destruction or mutilation of this Warrant, and (in the case of
loss, theft or destruction) of indemnification satisfactory to the Company, and
upon surrender and cancellation of this Warrant, if mutilated, the Company shall
execute and deliver a new Warrant of like tenor and date.

        SECTION 6. RIGHTS OF THE HOLDER. The Holder shall not, by virtue hereof,
be entitled to any rights of a shareholder in the Company, either at law or
equity, and the rights of the Holder are limited to those expressed in this
Warrant. Nothing contained in this Warrant shall be



                                       2.
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construed as conferring upon the Holder hereof the right to vote or to consent
or to receive notice as a shareholder of the Company on any matters or with
respect to any rights whatsoever as a shareholder of the Company. No dividends
or interest shall be payable or accrued in respect of this Warrant or the
interest represented hereby or the Warrant Shares purchasable hereunder until,
and only to the extent that, this Warrant shall have been exercised in
accordance with its terms.

        SECTION 7. ADJUSTMENT OF EXERCISE PRICE AND NUMBER OF SHARES. The number
and kind of securities purchasable upon the exercise of the Warrant and the
Exercise Price shall be subject to adjustment from time to time upon the
occurrence of certain events, as follows:

                (a) RECLASSIFICATION OF OUTSTANDING SECURITIES; IN-KIND
DISTRIBUTION. In case of any reclassification, change or conversion of
securities of the class issuable upon exercise of this Warrant (other than a
change in par value, or from par value to no par value, or from no par value to
par value, or as a result of a subdivision or combination) or upon the payment
of a dividend in securities or property other than shares of Common Stock, the
Company shall execute a new Warrant (in form and substance reasonably
satisfactory to the Holder of this Warrant) providing that the Holder of this
Warrant shall have the right to exercise such new Warrant and upon such exercise
to receive, in lieu of each share of Common Stock theretofore issuable upon
exercise of this Warrant, the kind and amount of shares of stock, other
securities, money and property receivable upon such reclassification or change
by a holder of one share of Common Stock. Such new Warrant shall provide for
adjustments that shall be as nearly equivalent as may be practicable to the
adjustments provided for in this Section 7. The provisions of this subsection
(a) shall similarly apply to successive reclassification or changes.

                (b) SUBDIVISIONS OR COMBINATION OF SHARES. If the Company at any
time while this Warrant remains outstanding and unexpired shall subdivide or
combine its Common Stock, the Exercise Price and the number of Warrant Shares
issuable upon exercise hereof shall be proportionately adjusted.

                (c) STOCK DIVIDENDS. If the Company at any time while this
Warrant is outstanding and unexpired shall pay a dividend payable in shares of
Common Stock (except any distribution specifically provided for in the foregoing
subsections (a) and (b)), then the Exercise Price shall be adjusted, from and
after the date of determination of shareholders entitled to receive such
dividend or distribution, to that price determined by multiplying the Exercise
Price in effect immediately prior to such date of determination by a fraction
(a) the numerator of which shall be the total number of shares of Common Stock
outstanding immediately prior to such dividend or distribution, and (b) the
denominator of which shall be the total number of shares of Common Stock
outstanding immediately after such dividend or distribution and the number of
Warrant Shares subject to this Warrant shall be proportionately adjusted.

                (d) NOTICE OF RECORD DATE. In the event of any taking by the
Company of a record of its shareholders for the purpose of determining
shareholders who are entitled to receive payment of any dividend (other than a
cash dividend) or other distribution, any right to subscribe for, purchase or
otherwise acquire any share of any class or any other securities or property, or
to receive any other right, or for the purpose of determining shareholders who
are entitled to vote in connection with any proposed merger or consolidation of
the Company with or into any other



                                       3.
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corporation, or any proposed sale, lease or conveyance of all or substantially
all of the assets of the Company, or any proposed liquidation, dissolution or
winding up of the Company, the Company shall mail to the Holder of this Warrant,
at least ten days prior to the date specified therein, a notice specifying the
date on which any such record is to be taken for the purpose of such dividend,
distribution or right, and the amount and character of such dividend,
distribution or right.

                (e) NO ADJUSTMENT UPON EXERCISE OF WARRANTS. No adjustments
shall be made under any Section herein in connection with the issuance of
Warrant Shares upon exercise of the Warrants.

        SECTION 8. OFFICER'S CERTIFICATE. Whenever the Exercise Price shall be
adjusted as required by the provisions of Section 7, the Company shall deliver
an officer's certificate showing the adjusted Exercise Price determined as
herein provided, setting forth in reasonable detail the facts requiring such
adjustment and the manner of computing such adjustment. Each such officer's
certificate shall be signed by the chairman, president or chief financial
officer of the Company.

        SECTION 9. TRANSFER TO COMPLY WITH THE SECURITIES ACT OF 1933. This
Warrant may not be exercised and neither this Warrant nor any of the Warrant
Shares, nor any interest in either, may be sold, assigned, pledged,
hypothecated, encumbered or in any other manner transferred or disposed of, in
whole or in part, except in compliance with applicable United States federal and
state securities or Blue Sky laws and the terms and conditions hereof. Each
Warrant shall bear a legend in substantially the same form as the legend set
forth on the first page of this Warrant. Each certificate for Warrant Shares
issued upon exercise of this Warrant, unless at the time of exercise such
Warrant Shares are acquired pursuant to a registration statement that has been
declared effective under the Act, shall bear a legend substantially in the
following form:

        THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
        SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER THE SECURITIES
        LAWS OF ANY STATE. THESE SECURITIES ARE SUBJECT TO RESTRICTIONS ON
        TRANSFERABILITY AND RESALE AND MAY NOT BE TRANSFERRED OR RESOLD EXCEPT
        AS PERMITTED UNDER THE ACT AND THE APPLICABLE STATE SECURITIES LAWS,
        PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM. THE ISSUER OF THESE
        SECURITIES MAY REQUIRE AN OPINION OF COUNSEL IN FORM AND SUBSTANCE
        SATISFACTORY TO THE ISSUER TO THE EFFECT THAT ANY PROPOSED TRANSFER OR
        RESALE IS IN COMPLIANCE WITH THE ACT AND ANY APPLICABLE STATE SECURITIES
        LAWS.

Any certificate for any Warrant Shares issued at any time in exchange or
substitution for any certificate for any Warrant Shares bearing such legend
(except a new certificate for any Warrant Shares issued after the acquisition of
such Warrant Shares pursuant to a registration statement that has been declared
effective under the Act) shall also bear such legend unless, in the opinion of
counsel for the Company, the Warrant Shares represented thereby need no longer
be subject to



                                       4.
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the restriction contained herein. The provisions of this Section 9 shall be
binding upon all subsequent Holders of certificates for Warrant Shares bearing
the above legend and all subsequent Holders of this Warrant, if any. In addition
in connection with the issuance of this Warrant, the Holder specifically
represents to the Company by acceptance of this Warrant as follows:

                (a) The Holder is aware of the Company's business affairs and
financial condition, and has acquired information about the Company sufficient
to reach an informed and knowledgeable decision to acquire this Warrant. The
Holder is acquiring this Warrant for its own account for investment purposes
only and not with a view to, or for the resale in connection with, any
"distribution" thereof in violation of the Act.

                (b) The Holder understands that this Warrant has not been
registered under the Act in reliance upon a specific exemption therefrom, which
exemption depends upon, among other things, the bona fide nature of the Holder's
investment intent as expressed herein.

                (c) The Holder further understands that this Warrant must be
held indefinitely unless subsequently registered under the Act and qualified
under any applicable state securities laws, or unless exemptions from
registration and qualification are otherwise available.

                (d) The Holder is aware of the provisions of Rule 144
promulgated under the Act, which, in substance, permit limited public resale of
"restricted securities" acquired, directly or indirectly, from the issuer
thereof (or from an affiliate of such issuer), in a non-public offering subject
to the satisfaction of certain conditions, if applicable, including, among other
things, the availability of certain public information about the Company, the
resale occurring not less than two years after the party has purchased and paid
for the securities to be sold; the sale being made through a broker in an
unsolicited "broker's transaction" or in transactions directly with a market
maker (as said term is defined under the Securities Exchange Act of 1934, as
amended (the "Exchange Act")) and the amount of securities being sold during any
three month period not exceeding the specified limitations stated therein.

                (e) The Holder further understands that in the event all of the
requirements of Rule 144 are not satisfied, registration under the Act,
compliance with Regulation A, or some other registration exemption will be
required; and that, notwithstanding the fact that Rule 144 is not exclusive, the
staff of the Securities and Exchange Commission (the "SEC") has expressed its
opinion that persons proposing to sell private placement securities other than
in a registered offering and otherwise than pursuant to Rule 144 will have a
substantial burden of proof in establishing that an exemption from registration
is available for such offers or sales, and that such persons and their
respective brokers who participate in such transactions do so at their own risk.

        SECTION 10. EXPIRATION DATE. This Warrant shall expire and shall be
wholly void and have no effect after 5:00 p.m. (San Francisco time) on the date
(the "Expiration Date") which is the earlier of (a) two years following the date
hereof, or (b) the closing date of a merger or consolidation of the Company with
or into any other entity, including a reverse triangular merger involving the
Company (other than a merger or consolidation in which the Holders of the voting
power of the Company immediately prior to such consolidation or merger hold a
majority of the



                                       5.
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surviving or resulting entity immediately following such consolidation or
merger) (a "Change of Control"), provided that, if the last day on which this
Warrant may be exercised, or on which it may be exercised at a particular
Exercise Price, is a Sunday or a legal holiday or a day on which banking
institutions doing business in the city of San Francisco are authorized by law
to close, this Warrant may be exercised prior to 5:00 p.m. (San Francisco time)
on the next succeeding full business day with the same force and effect and at
the same Exercise Price as if exercised on such last day specified herein.

        SECTION 11. MARKET STAND-OFF. In the event the Holder exercises this
Warrant prior to the first anniversary of the date hereof, then the Holder
agrees that until such first anniversary:

                (a) The Holder shall not offer, pledge, sell, contract to sell,
sell any option or contract to purchase, purchase any option or contract to
sell, grant any option, right or warrant to purchase, lend, or otherwise
transfer or dispose of, directly or indirectly, any Warrant Shares or any
securities convertible into or exercisable or exchangeable for the Warrant
Shares or enter into any swap or other arrangement that transfers to another, in
whole or in part, any of the economic consequences of ownership of the Warrant
Shares;

                (b) The Company, the Company's transfer agent or an escrow agent
of the Company's choosing and at the Company's discretion, shall hold the stock
certificate(s) evidencing the Warrant Shares until the first anniversary of the
date hereof;

                (c) The stock certificate(s) evidencing the Warrant Shares shall
bear a legend substantially in the following form:

        THE SECURITIES REPRESENTED HEREBY ARE SUBJECT TO A MARKET STAND-OFF
        AGREEMENT AND OTHER RESTRICTIONS ON TRANSFERABILITY AND RESALE, AND MAY
        NOT BE TRANSFERRED OR RESOLD EXCEPT IN ACCORDANCE WITH SUCH AGREEMENT, A
        COPY OF WHICH MAY BE OBTAINED FROM THE SECRETARY OF THE COMPANY.

    In the event of a public announcement by the Company of the execution of a
definitive agreement to effect a Change of Control of the Company, this Section
11 shall be of no further force or effect.

        SECTION 12. GOVERNING LAW. This Warrant is delivered in the State of
California and shall be construed and enforced in accordance with, and the
rights of the parties shall be governed by, the laws of that State.

        SECTION 13. MODIFICATION AND WAIVER. Neither this Warrant nor any term
hereof may be amended, waived, discharged or terminated other than by an
instrument in writing signed by the Company and by the Holder hereof.



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        SECTION 14. NOTICES. Any notice, request or other document required or
permitted to be given or delivered to the Holder hereof or the Company shall be
delivered or shall be sent by certified mail, postage prepaid, to each such
Holder at its address as shown on the books of the Company or to the Company at
the address indicated therefor in the first paragraph of this Warrant.

        SECTION 15. DESCRIPTIVE HEADINGS. The description headings of the
several sections and paragraphs of this Warrant are inserted for convenience
only and do not constitute a part of this Warrant.

        SECTION 16. ENTIRE AGREEMENT. This Warrant constitutes the entire
agreement between the parties pertaining to the subject matter herein and
supersedes all prior and contemporaneous agreements, representation and
undertakings of the parties.

        IN WITNESS WHEREOF, the Company has duly caused this Warrant to be
signed by its duly authorized officer and to be dated as of October 10, 2000.

                                        AVIRON


                                        By:
                                           -------------------------------------
                                           C. Boyd Clarke
                                           President and Chief Executive Officer



                                       7.
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                                SUBSCRIPTION FORM


        THE UNDERSIGNED, holder of this Warrant, (1) hereby irrevocably elects
to exercise the right of purchase represented by this Warrant for, and to
purchase thereunder, ________________ full Warrant Shares of the Common Stock of
Aviron provided for therein, (2) makes payment in full of the purchase price of
such Warrant Shares, (3) requests that certificates for such Warrant Shares be
issued in the name of


        ----------------------------------------------------------------
                         (Please print name and address)


        ----------------------------------------------------------------
           (Please insert social security or other identifying number)


and (4) if said number of shares shall not be all the shares purchasable
thereunder, requests that a new Warrant for the unexercised portion of this
Warrant be issued in the name of and delivered to:


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


        ----------------------------------------------------------------
                         (Please print name and address)



Dated:
      -----------------------------------

Signature:
          -------------------------------

By:
   --------------------------------------



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                                 ASSIGNMENT FORM


                                                   Dated _________, 20__


        FOR VALUE RECEIVED, ________________________________ hereby sells,

assigns and transfers unto _________________________________________________

(the "Assignee"),____________________________________________________________

                     (please type or print in block letters)

________________________________________________________________________________

                                (insert address)

its right to purchase up to ____ shares of Common Stock of AVIRON represented by

this Warrant and does hereby irrevocably constitute and appoint

____________________________ Attorney, to transfer the same on the books of the

Company, with full power of substitution in the premises.


        Signature ____________________________