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                                                                     EXHIBIT 4.3

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
                                 MEDIMMUNE, INC.
                                  COMMON STOCK
                           (PAR VALUE $.01 PER SHARE)


COMPANY:                   Aviron (the "Company"), a Delaware corporation and a
                           wholly owned subsidiary of MedImmune, Inc.
                           ("MedImmune"), a Delaware corporation, and any
                           corporation that shall succeed to the obligations of
                           the Company under this Warrant.
NUMBER OF SHARES:          53,750
CLASS OF STOCK:            MedImmune Common Stock (par value $.01 per share)
EXERCISE PRICE:            $9.30
DATE OF GRANT:             January 15, 2002

        THIS CERTIFIES THAT, for value received, The Regents of the University
of 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 MedImmune
at the 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 March 28, 2008.

        This Warrant is issued in accordance with the terms of, and replaces,
the warrant to purchase 50,000 shares of common stock, par value $.001, of
Aviron. The Aviron warrant is now null and void and of no further affect.

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 MedImmune 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 Exercise Price at which this
Warrant may be exercised, as adjusted pursuant to Section 5 hereof.


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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 within 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
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 of
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 the 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 MedImmune with or into another corporation (other
than a merger or reorganization with respect to which MedImmune 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, reorganization,
consolidation or merger by a holder of shares of the same Class of Stock of
MedImmune. 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


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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 MedImmune 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 MedImmune 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 of the stockholders entitled
to receive such dividend or distributions, to that price determined by
multiplying the Warrant Price in effect immediately prior to such 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.

6. NOTICE OF ADJUSTMENTS.

        So long as this 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


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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;

               (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.


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9. RIGHTS OF THE HOLDER.

        The Warrantholder shall not, by virtue hereof, be entitled to any rights
of a shareholder in MedImmune, 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
MedImmune on any matters or with respect to any rights whatsoever as a
shareholder of MedImmune. 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 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.






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                                       AVIRON


                                       By:
                                          --------------------------------------
                                          Fred Kurland
                                          Senior Vice President and
                                          Chief Financial Officer





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                                   APPENDIX A

                               NOTICE OF EXERCISE

TO:     Aviron

        1. The undersigned hereby elects to purchase shares of the stock of
MedImmune, Inc., 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 is
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:
     ------------------------------------------







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