<PAGE>   1
                                                                    Exhibit 4.25

THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, AND MAY NOT BE OFFERED FOR SALE, SOLD, PLEDGED OR OTHERWISE TRANSFERRED
WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR PURSUANT TO RULE 144
OR AN OPINION OF COUNSEL, REASONABLY SATISFACTORY TO THE COMPANY AND ITS
COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED.

                           WARRANT TO PURCHASE SHARES
                                       OF
                                     AVIRON

<TABLE>
<S>                                        <C>
COMPANY:                                   AVIRON, a Delaware corporation (the "Company"), and any corporation that
                                           shall succeed to the obligations of the Company under this Warrant.
NUMBER OF SHARES:                          50,000
CLASS OF STOCK:                            Common Stock
INITIAL EXERCISE PRICE:                    $10.00
DATE OF GRANT:                             March 29, 2001
</TABLE>

     THIS CERTIFIES THAT, for value received, The Regents of the University
Michigan ("Michigan") or any permitted transferee of its rights hereunder is
entitled to purchase the above number (as adjusted pursuant to Section 5 hereof)
of fully paid and nonassessable shares of the above Class of Stock of the
Company at the Initial Exercise Price above (as adjusted pursuant to Section 5
hereof), subject to the provisions and upon the terms and conditions set forth
herein. THE EXPIRATION DATE OF THIS WARRANT SHALL BE SEVEN (7) YEARS FROM THE
DATE OF GRANT.

1.   DEFINITIONS.

     In addition to the terms defined above, the following capitalized terms
shall have the following meanings, unless the context otherwise requires:

     (a)  "Act" shall mean the Securities Act of 1933, as amended, or any
          similar federal statute, and the rules and regulations thereunder, as
          shall be in effect at the time.

     (b)  "Common Stock" shall mean shares of the authorized common stock of the
          Company and any stock into which such common stock may hereafter be
          exchanged.

     (c)  "Warrantholder" shall mean any person who shall at the time be the
          holder of this Warrant.

     (d)  "Shares" shall mean the shares of the Class of Stock that the
          Warrantholder is entitled to purchase upon exercise of this Warrant,
          as adjusted pursuant to Section 5 hereof.

     (e)  "Warrant Price" shall mean the Initial Exercise Price at which this
          Warrant may be exercised, as adjusted pursuant to Section 5 hereof.


<PAGE>   2

2.   TERM.

     The purchase right and Conversion Right (as defined in Section 7.1),
represented by this Warrant is exercisable, in whole or in part, at any time on
or before the Expiration Date.

3.   METHOD OF EXERCISE; PAYMENT; ISSUANCE OF NEW WARRANT.

     Subject to Section 2 hereof, the purchase right represented by this Warrant
may be exercised by the Warrantholder, in whole or in part, by the surrender of
this Warrant (with the notice of exercise form attached hereto as Appendix A
duly executed) at the principal office of the Company and by the payment to the
Company, by check made payable to the Company drawn on a United States bank and
for United States funds of an amount equal to the then applicable Warrant Price
per share multiplied by the number of Shares then being purchased. In the event
of any exercise of the purchase right represented by this Section 3,
certificates for the Shares so purchased shall be delivered to the Warrantholder
with thirty (30) days of receipt of such payment and, unless this Warrant has
been fully exercised or expired, a new Warrant representing the portion of the
Shares, if any, with respect to which this Warrant shall not then have been
exercised shall also be issued to the Warrantholder within such thirty (30) day
period.

4.   EXERCISE PRICE.

     The Warrant Price at which this Warrant may be exercised shall be the
Initial Exercise Price, as adjusted from time to time pursuant to Section 5
hereof.

5.   ADJUSTMENT OF NUMBER AND KIND OF SHARES AND ADJUSTMENT OF WARRANT PRICE.

     5.1  CERTAIN DEFINITIONS. As used in this Section 5 the following terms
          shall have the following respective meanings:

          (a)  "Options" shall mean rights, options or warrants to subscribe
               for, purchase or otherwise acquire either shares of Common Stock
               or Convertible Securities;

          (b)  "Convertible Securities" shall mean any evidences or
               indebtedness, shares of stock or other securities directly or
               indirectly convertible into or exchangeable for Common Stock.

     5.2  ADJUSTMENTS. The number and kind of securities purchasable upon the
          exercise of this Warrant and Warrant Price shall be subject to
          adjustment from time to time upon the occurrence of certain events, as
          follows:

          (a)  Reclassification, Reorganization, Consolidation or Merger. In the
               case of any reclassification of the Class of Stock that the
               Warrantholder is entitled to purchase upon exercise of this
               Warrant, or any reorganization, consolidation or merger of the
               Company with or into another corporation (other than a merger or
               reorganization with respect to which the Company is the surviving
               corporation and which does not result in any reclassification of
               such Class of Stock), the Company, or such successor corporation,
               as the case may be, shall execute a new warrant, providing that
               the Warrantholder shall have the right to exercise such new
               warrant and upon such exercise to receive, in lieu of each share
               of the Class of Stock theretofore issuable upon exercise of this
               Warrant, the kind of securities receivable upon such
               reclassification,

                                       2

<PAGE>   3


               reorganization, consolidation or merger by a holder of shares of
               the same Class of Stock of the Company. The Warrant Price and the
               number of shares of such new securities to be received by the
               Warrantholder upon exercise of the Warrant shall be adjusted so
               that the Warrantholder shall receive upon exercise of the Warrant
               and payment of the same aggregate consideration the number of
               shares of new securities which the Warrantholder would have owned
               immediately following such reclassification, reorganization,
               consolidation or merger if the Warrantholder had exercised the
               Warrant immediately prior to such reclassifications,
               reorganization, consolidation or merger. The provisions of this
               subsection (a) shall similarly apply to successive
               reclassification, reorganizations, consolidations or mergers.

          (b)  Split, Subdivision or Combination of Shares. If the Company at
               any time while this Warrant remains outstanding and unexpired
               shall split, subdivide or combine the Class of Stock for which
               this Warrant is then exercisable, the Warrant Price shall be
               proportionately decreased in the case of a split or subdivision
               or proportionately increased in the case of a combination. Any
               adjustment under this subsection (b) shall become effective when
               the split, subdivision or combination becomes effective.

          (c)  Stock Dividends. If the Company at any time while this Warrant
               remains outstanding and unexpired shall pay a dividend with
               respect to the Class of Stock for which this Warrant is then
               exercisable, payable in shares of that Class of Stock, Options or
               Convertible Securities, the Warrant Price shall be adjusted, from
               and after the date of determination by a fraction (i) the
               numerator of which shall be the total number of shares of that
               Class of Stock outstanding immediately prior to such dividend or
               distribution, and (ii) the denominator of which shall be the
               total number of shares of the same Class of Stock outstanding
               immediately after such dividend or distribution (including shares
               of that Class of Stock issuable upon exercise, conversion or
               exchange of any Options or Convertible Securities issued as such
               dividend or distribution). If the Options or Convertible
               Securities issued as such dividend or distribution by their terms
               provide, with the passage of time or otherwise, for any decrease
               in the consideration payable to the Company, or any increase in
               the number of shares issuable upon exercise, conversion or
               exchange thereof (by change of rate or otherwise), the Warrant
               Price shall, upon any such decrease or increase becoming
               effective, be reduced to reflect such decrease or increase as if
               such decrease or increase became effective immediately prior to
               the issuance of the Options or Convertible Securities as the
               dividend or distribution. Any adjustment under this subsection
               (c) shall become effective on the record date set for such
               dividend or distribution.

          (d)  Adjustment of Number of Shares. Upon each adjustment in the
               Warrant Price pursuant to Section 5(b) or 5(c) above, the number
               of Shares issuable upon exercise of this Warrant shall be
               adjusted to the product obtained by multiplying the number of
               Shares issuable immediately prior to such adjustment in the
               Warrant Price by a fraction (i) the numerator of which shall be
               the Warrant Price immediately prior to such adjustment, and (ii)
               the denominator of which shall be the Warrant Price immediately
               after such adjustment.

                                       3

<PAGE>   4

6.   NOTICE OF ADJUSTMENTS.

     So long as the Warrant remains outstanding and unexpired, whenever the
Warrant Price shall be adjusted pursuant to Section 5 hereof, the Company shall
issue a certificate signed by its chief financial officer setting forth, in
reasonable detail, the event requiring the adjustment, the amount of the
adjustment, the method by which such adjustment was calculated and the Warrant
Price after giving effect to such adjustment, and shall cause a copy of such
certificate to be mailed (by first class mail, postage prepaid) to the
Warrantholder.

7.   RIGHT TO CONVERT WARRANT INTO STOCK.

     7.1  RIGHT TO CONVERT. In addition to the rights granted under Section 3 of
          this Warrant, the Warrantholder shall have the right to require the
          Company to convert this Warrant, in whole or in part, (the "Conversion
          Right"), into shares of the Class of Stock for which the Warrant is
          then exercisable, as provided in this Section 7. Upon exercise of the
          Conversion Right, the Company shall deliver to the Warrantholder
          (without payment by the Warrantholder of any Warrant Price) that
          number of shares of stock equal to the quotient obtained by dividing
          (x) the value of the portion of the Warrant specified for conversion
          at the time the Conversion Right is exercised (determined by
          subtracting the aggregate Warrant Price, immediately prior to the
          exercise of the Conversion Right, of the number of shares to be
          converted from the aggregate fair market value [as determined pursuant
          to Section 7.3 below], immediately prior to the exercise of the
          Conversion Right, of those Shares) by (y) the fair market value (as
          determined pursuant to Section 7.3 below) of one share of that Class
          of Stock immediately prior to the exercise of the Conversion Right.

     7.2  METHOD OF EXERCISE. So long as the Warrant remains outstanding and
          unexpired, the Conversion Right may be exercised at any time by the
          Warrantholder by the surrender of this Warrant at the principal office
          of the Company together with a written statement specifying that the
          Warrantholder thereby intends to exercise the Conversion Right, and
          specifying the number of shares to be converted. Certificates of the
          shares of stock issuable upon exercise of the Conversion Right shall
          be delivered to the Warrantholder within thirty (30) days following
          the Company's receipt of this Warrant together with the aforesaid
          written statement, and unless this Warrant has been fully exercised or
          expired, a new Warrant representing the portion of the Shares, if any,
          with respect to which this Warrant shall not then have been exercised
          shall also be issued to the Warrantholder within such thirty (30) day
          period.

     7.3  VALUATION OF STOCK. For purposes of this Section 7, the fair market
          value of one share of the Class of Stock issuable upon exercise of
          this Warrant shall mean:

          (a)  The product of (i) the average of the closing price or, if no
               closing price is reported, the closing bid and asked prices of
               the Common Stock, quoted in the Over-The-Counter Market Summary
               or the closing price quoted on any exchange on which the Common
               Stock is listed, whichever is applicable, as published in the
               Western Edition of The Wall Street Journal for the ten (10)
               trading days prior to the date of determination of fair market
               value, and (ii) the number of shares of Common Stock into which
               each share of the Class of Stock is then convertible, if
               applicable;

                                       4

<PAGE>   5
\
          (b)  If the Common Stock is not traded Over-The-Counter or on an
               exchange, the fair market value of the Class of Stock per share
               shall be as determined in good faith by the Company's Board of
               Directors; provided, however, that if the Warrantholder disputes
               in writing the fair market value determined by the Board of
               Directors within thirty (30) days of being informed of such fair
               market value, the fair market value shall be determined by an
               independent appraiser, appointed in good faith by the Company's
               Board of Directors.

8.   COMPLIANCE WITH ACT; TRANSFERABILITY OF WARRANT; DISPOSITION OF SHARES.

     8.1  LEGENDS. This Warrant and the Shares issued upon exercise thereof
          shall be imprinted with a legend in substantially the following form:

     "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
     UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE OFFERED FOR
     SALE, SOLD, PLEDGED OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE
     REGISTRATION THEREOF UNDER SUCH ACT OR PURSUANT TO RULE 144 OR AN OPINION
     OF COUNSEL REASONABLY SATISFACTORY TO THE COMPANY AND ITS COUNSEL THAT SUCH
     REGISTRATION IS NOT REQUIRED.

     8.2  TRANSFERABILITY OF WARRANT AND SHARES. This Warrant and the Shares
          issued upon exercise thereof shall not be sold, transferred or
          assigned in whole or in part without compliance with applicable
          federal and state securities laws by the transferor and the transferee
          (including, without limitation, the delivery of investment
          representation letters and legal opinions reasonably satisfactory to
          the Company, if reasonably requested by the Company). Subject to the
          provisions of this Section 8.2, title to this Warrant may be
          transferred in the same manner as a negotiable instrument transferable
          by endorsement and delivery.

9.   RIGHTS OF THE HOLDER.

     The Warrantholder 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
Warrantholder are limited to those expressed in this Warrant. Nothing contained
in this Warrant shall be construed as conferring upon the Warrantholder 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 Shares purchasable
hereunder until, and only to the extent that this Warrant shall have been
exercised in accordance with its terms.

10.  MISCELLANEOUS.

     No fractional shares shall be issued in connection with any exercise
hereunder, but in lieu of such fractional shares the Company shall make a cash
payment therefor upon the basis of the Warrant Price then in effect. The terms
and provisions of this Warrant shall inure to the benefit of, and be binding
upon, the Company and the Warrantholder and their respective successors and
assigns. This Warrant shall be governed by and construed under the laws of the
State of California as applied to contracts entered into between residents of
the State of California to be wholly performed in the State of California. The
titles of the sections and subsections of this

                                       5

<PAGE>   6

Warrant are for convenience only and are not to be considered in construing this
Warrant. All pronouns used in the Warrant shall be deemed to include masculine,
feminine and neuter forms.

                                       AVIRON


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


                                       6

<PAGE>   7

APPENDIX A

NOTICE OF EXERCISE

TO:  AVIRON

     1.   The undersigned hereby elects to purchase shares of the stock of
          Aviron, a Delaware corporation, pursuant to terms of the attached
          Warrant, and tenders herewith payment of the purchase price of such
          shares in full, together with all applicable transfer taxes, if any.

     2.   Please issue a certificate or certificates representing said shares of
          the stock in the name of the undersigned or in such other name as
          specified below.

     3.   The undersigned represents it is acquiring the shares of stock solely
          for its own account for investment and not as a nominee for any other
          party and not with a view toward the resale or distribution thereof
          within the meaning of the Securities Act of 1933, as amended.


                 ------------------------------------------------
                                     (Name)


                 ------------------------------------------------
                                    (Address)


                 ------------------------------------------------
                        (Taxpayer Identification Number)



-----------------------------------
(print name of Warrantholder)


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

Title:
        ---------------------------

Date:
       ----------------------------


                                       7