Document:

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                                                                     EXHIBIT 4.2

                                                                    COMMON STOCK

                                                               CUSIP 053762 10 0

AVIRON

INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE
THIS CERTIFICATE IS TRANSFERABLE IN CANTON, MA OR NEW YORK, NY

                                             SEE REVERSE FOR CERTAIN DEFINITIONS

THIS CERTIFIES THAT

IS THE RECORD HOLDER OF

 FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, $0.001 PAR VALUE, OF

AVIRON transferable on the books of the Corporation in person or by duly
authorized attorney on surrender of this Certificate properly endorsed. This
Certificate is not valid until countersigned and registered by the Transfer
Agent and Registrar. Witness the signatures of the Corporation's duly
authorized officers.

DATED:

COUNTERSIGNED AND REGISTERED:

FLEET NATIONAL BANK                                 /s/ BOYD CLARKE
Transfer Agent and Registrar               President and Chief Executive Officer

/s/ TIMOTHY D. RYAN                              /s/ CHARLENE FRIEDMAN
                                             Vice President, General Counsel
AUTHORIZED OFFICER                                    and Secretary

                                     AVIRON

     A statement of the powers, designations, preferences and relative,
participating, optional or other special rights of each class of stock or series
thereof and the qualifications, limitations or restrictions of such preferences
and/or rights as established, from time to time, by the Certificate of
Incorporation of the Corporation and by any certificate of determination, the
number of shares constituting each class and series, and the designations
thereof, may be obtained by the holder hereof upon request and without charge at
the principal office of the Corporation.

     The following abbreviations, when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations.

TEN COM -- as tenants in common
TEN ENT -- as tenants by the entireties
JT TEN  -- as joint tenants with right of
           survivorship and not as tenants
           in common

UNIF GIFT MIN ACT --             Custodian
                     -------------------------------
                         (Cust)            (Minor)
                     under Uniform Gifts to Minors

                     Act
                        ----------------------------
                                   (State)

UNIF TRF MIN ACT --        Custodian (until age      )
                     ------                    ------
                     (Cust)

                              under Uniform Transfers
                     ---------
                      (Minor)
                     to Minors Act
                                  -------------------
                                        (State)

Additional abbreviations may also be used though not in the above list.

FOR VALUE RECEIVED,                                      HEREBY SELL, ASSIGN AND
                   -------------------------------------
TRANSFER UNTO

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE

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

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

--------------------------------------------------------------------------------
 (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OR ASSIGNEE)

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                                                                          SHARES
--------------------------------------------------------------------------
OF THE CAPITAL STOCK REPRESENTED BY THE WITHIN CERTIFICATE, AND DO HEREBY
IRREVOCABLY CONSTITUTE AND APPOINT

                                                                        ATTORNEY
------------------------------------------------------------------------
TO TRANSFER THE SAID STOCK ON THE BOOKS OF THE WITHIN NAMED CORPORATION WITH
FULL POWER OF SUBSTITUTION IN THE PREMISES.

DATED
     -----------------------------------

                                        X
                                         ---------------------------------------

                                        X
                                         ---------------------------------------

                     NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND
                             WITH THE NAME(S) AS WRITTEN UPON THE FACE OF THE
                             CERTIFICATE IN EVERY PARTICULAR, WITHOUT ALTERATION
                             OR ENLARGEMENT OR ANY CHANGE WHATEVER.

Signature(S) Guaranteed

BY
  --------------------------------------
  THE SIGNATURE(S) SHOULD BE GUARANTEED
  BY AN ELIGIBLE GUARANTOR INSTITUTION
  (BANKS, STOCKBROKERS, SAVINGS AND LOAN
  ASSOCIATIONS AND CREDIT UNIONS WITH
  MEMBERSHIP IN AN APPROVED SIGNATURE
  GUARANTEE MEDALLION PROGRAM), PURSUANT
  TO S.E.C. RULE 17Ad-15.<PAGE>   1

                                                                    EXHIBIT 4.17

                                     AVIRON

                         COMMON STOCK PURCHASE AGREEMENT

     This COMMON STOCK PURCHASE AGREEMENT (the "Agreement") is made as of
October 10, 2000 (the "Effective Date"), by and between AVIRON, a Delaware
corporation (the "Company") and BIOTECH INVEST, S.A., a corporation organized
under the laws of Panama (the "Purchaser").

SECTION 1. PURCHASE AND SALE OF COMMON STOCK

     Subject to the terms and conditions of this Agreement, the Company agrees
to issue and sell to Purchaser and Purchaser agrees to purchase from the Company
four hundred fifty thousand (450,000) shares of the Company's Common Stock,
$0.001 par value (the "Shares"), for a purchase price of $48.00 per Share.

SECTION 2. CLOSING DATE; DELIVERY

     2.1  CLOSING DATE. The closing of the purchase and sale of the Shares
hereunder (the "Closing") shall be held at the offices of Cooley Godward LLP,
Five Palo Alto Square, 3000 El Camino Real, Palo Alto, California, 94306 at
10:00 a.m., on October 12, 2000 or at such other time and place upon which the
Company and Purchaser shall agree. The date of the Closing is hereinafter
referred to as the "Closing Date."

     2.2  DELIVERY. At the Closing, the Company will deliver to Purchaser a
certificate, registered in Purchaser's name, representing the number of shares
of Common Stock to be purchased by Purchaser. Such delivery shall be against
payment of the purchase price therefor by wire transfer to the Company's bank
account.

SECTION 3. REPRESENTATIONS AND WARRANTIES OF THE COMPANY

     The Company represents and warrants to Purchaser as of the Closing Date as
follows:

     3.1  ORGANIZATION AND STANDING. The Company is a corporation duly
organized, validly existing and in good standing under the laws of the State of
Delaware, and has full power and authority to own and operate its properties and
assets and to carry on its business as presently conducted and as proposed to be
conducted. The Company is qualified as a foreign corporation to do business in
each jurisdiction in the United States in which the ownership of its property or
the conduct of its business requires such qualification, except where any
statutory fines or penalties or any corporate disability imposed for the failure
to qualify would not materially or adversely affect the Company, its assets,
financial condition or operations. The Company has no subsidiaries.

     3.2  CORPORATE POWER; AUTHORIZATION. The Company has all requisite
corporate power to, and has taken all requisite corporate action to, execute and
deliver this Agreement, to sell and issue the Shares and to carry out and
perform all of its obligations under this Agreement. This Agreement constitutes
the legal, valid and binding obligation of the Company, enforceable in
accordance with its terms, except (i) as limited by applicable bankruptcy,
insolvency,

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reorganization, moratorium or similar laws relating to or affecting the
enforcement of creditors' rights generally, (ii) as limited by equitable
principles generally and (iii) as to those provisions of Section 7.2 relating to
indemnity or contribution, as may be limited by applicable laws. The execution
and delivery of this Agreement does not, and the performance of this Agreement
and the compliance with the provisions hereof and the issuance, sale and
delivery of the Shares by the Company will not materially conflict with, or
result in a material breach or violation of the terms, conditions or provisions
of, or constitute a material default under, or result in the creation or
imposition of any material lien pursuant to the terms of, the Certificate of
Incorporation or Bylaws of the Company or (a) any governmental statute, law,
rule applicable to the Company or (b) order, writ, judgment, injunction, decree,
determination or award which has been entered against the Company and of which
we are aware, the violation of which would materially and adversely affect the
Company, its assets, financial condition or operations.

     3.3  ISSUANCE AND DELIVERY OF THE SHARES. The Shares, when issued and paid
for in compliance with the provisions of this Agreement, will be validly issued,
fully paid and nonassessable. The issuance and delivery of the Shares is not
subject to preemptive, co-sale, right of first refusal or any other similar
rights of the stockholders of the Company or any liens or encumbrances,
provided, however, that the Shares may be subject to restrictions on transfer
under state and/or federal securities laws as set forth herein or as otherwise
required by such laws at the time a transfer is proposed.

     3.4  FULL DISCLOSURE. The Company warrants that the information contained
in the following documents filed with the Securities and Exchange Commission
(collectively, the "SEC Documents"), as of their respective dates, did not
contain any untrue statement of a material fact, and did not omit to state any
material fact necessary to make any statement, in light of the circumstances
under which such statement was made, not misleading:

          (a)  The Company's Annual Report on Form 10-K for the year ended
December 31, 1999, as amended by Form 10-K/A on April 3, 2000 and April 28,
2000.

          (b)  The Company's Definitive Proxy Statement dated May 4, 2000
relating to its 2000 Annual Meeting of Stockholders.

          (c)  The Company's Current Report on Form 8-K, filed June 13, 2000.

          (d)  The Company's Quarterly Reports on Form 10-Q for the quarters
ended March 31, 2000 and June 30, 2000.

          (e)  The Company's Registration Statement on Form S-3, filed September
16, 1999, as supplemented on November 17, 1999, December 16, 1999, January 13,
2000, February 2, 2000, March 6, 2000, April 12, 2000, May 11, 2000, June 9,
2000, July 10, 2000, August 7, 2000, September 5, 2000 and October 2, 2000.

          (f)  The Company's Registration Statement on Form S-3 filed March 8,
2000, as amended.

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          (g)  The Company's Registration Statement on Form S-3, filed September
1, 2000.

     3.5  LITIGATION. Except as set forth in the SEC Documents, there is no
pending or, to the Company's knowledge, threatened action, suit or other
proceeding before any court, governmental body or authority, or arbitrator to
which the Company is a party or to which its property or assets are subject.

     3.6  GOVERNMENTAL CONSENTS. No consent, approval, order or authorization
of, or registration, qualification, designation, declaration or filing with, any
Federal, state, or local governmental authority on the part of the Company is
required in connection with the consummation of the transactions contemplated by
this Agreement except for (a) compliance with the Hart-Scott-Rodino Antitrust
Improvements Act of 1976, as amended (the "HSR Act") and securities and blue sky
laws in the states and other jurisdictions in which shares of Common Stock are
offered and/or sold, which compliance will be effected in accordance with such
laws, and (b) the filing of a registration statement and all amendments thereto
with the SEC as contemplated by Section 7.1 of this Agreement.

     3.7  NO MATERIAL ADVERSE CHANGE. Since June 30, 2000, there have not been
any changes in the assets, liabilities, financial condition or operations of the
Company from that reflected in the SEC Documents except changes in the ordinary
course of business or which have not been, either individually or in the
aggregate, materially adverse.

SECTION 4. REPRESENTATIONS, WARRANTIES AND COVENANTS OF PURCHASER

     Purchaser hereby represents and warrants to the Company, effective as of
the Closing Date, as follows:

     4.1  AUTHORIZATION. Purchaser represents and warrants to the Company that:
(i) Purchaser has all requisite legal and corporate or other power and capacity
and has taken all requisite corporate or other action to execute and deliver
this Agreement, to purchase the Shares and to carry out and perform all of its
obligations under this Agreement; and (ii) this Agreement constitutes the legal,
valid and binding obligation of the Purchaser, enforceable in accordance with
its terms, except (a) as limited by applicable bankruptcy, insolvency,
reorganization, or similar laws relating to or affecting the enforcement of
creditors' rights generally and (b) as limited by equitable principles
generally.

     4.2  INVESTMENT EXPERIENCE. Purchaser is an "accredited investor" as
defined in Rule 501(a) under the Securities Act of 1933, as amended (the
"Securities Act"). Purchaser is aware of the Company's business affairs and
financial condition and has had access to and has acquired sufficient
information about the Company to reach an informed and knowledgeable decision to
acquire the Shares. Purchaser has such business and financial experience as is
required to give it the capacity to protect its own interests in connection with
the purchase of the Shares.

     4.3  INVESTMENT INTENT. Purchaser is purchasing the Shares for its own
account as principal, for investment purposes only, and not with a view to, or
for, resale, distribution or

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fractionalization thereof, in whole or in part, within the meaning of the
Securities Act. Purchaser understands that its acquisition of the Shares has not
been registered under the Securities Act or registered or qualified under any
state securities law in reliance on specific exemptions therefrom, which
exemptions may depend upon, among other things, the bona fide nature of
Purchaser's investment intent as expressed herein. Purchaser has completed or
caused to be completed the Purchaser Certificates/Questionnaire attached hereto
as Exhibits A-1 through A-3 for use in connection with the sale of Shares and in
preparation of the Registration Statement (as defined below), will deliver the
such Certificates/Questionnaires to the Company on or prior to the Closing Date,
and the responses provided therein shall be true and correct as of the Closing
Date. Purchaser has, in connection with its decision to purchase the Shares,
relied solely upon the SEC Documents and the representations and warranties of
the Company contained herein.

     4.4  REGISTRATION OR EXEMPTION REQUIREMENTS. Purchaser further acknowledges
and understands that the Shares must be held for investment purposes, and they
may not be resold or otherwise transferred except in a transaction registered
under the Securities Act or an exemption from such registration is available.
Purchaser understands that the certificate(s) evidencing the Shares will be
imprinted with a legend that prohibits the transfer of the Shares unless (i)
they are registered or such registration is not required, and (ii) if the
transfer is pursuant to an exemption from registration other than Rule 144 under
the Securities Act ("Rule 144") and, if the Company shall so request in writing,
an opinion of counsel satisfactory to the Company is obtained to the effect that
the transaction is so exempt and in compliance with applicable state law.

     4.5  RESTRICTION ON SHORT SALES. Purchaser represents and warrants to and
covenants with the Company that Purchaser has not engaged and will not engage in
any short sales of the Company's Common Stock prior to the effectiveness of the
Registration Statement, except to the extent that any such short sale is fully
covered by shares of Common Stock of the Company other than the Shares.

     4.6  NO LEGAL, TAX OR INVESTMENT ADVICE. Purchaser understands that nothing
in this Agreement or any other materials presented to Purchaser in connection
with the purchase and sale of the Shares constitutes legal, tax or investment
advice and that no independent legal counsel has reviewed these documents and
materials on Purchaser's behalf. Purchaser has consulted such legal, tax and
investment advisors as it, in its sole discretion, has deemed necessary or
appropriate in connection with its purchase of the Shares.

SECTION 5. CONDITIONS TO CLOSING OF PURCHASER

     Purchaser's obligation to purchase the Initial Shares at the Closing is, at
the option of Purchaser, subject to the fulfillment or waiver as of the Closing
Date of the following conditions:

     5.1  REPRESENTATIONS AND WARRANTIES. The representations and warranties
made by the Company in Section 3 hereof shall be true and correct in all
material respects when made, and shall be true and correct in all material
respects on the Closing Date with the same force and effect as if they had been
made on and as of said date.

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     5.2  LEGAL OPINION. The Company shall have delivered a legal opinion from
Cooley Godward LLP, counsel to the Company, addressed to Purchaser in the form
attached hereto as Exhibit C with respect to the sale of the Shares by the
Company hereunder.

     5.3  OFFICERS' CERTIFICATE. The Company shall deliver to Purchaser a
certificate, dated as of the Closing Date, signed by the President and Chief
Financial Officer of the Company, stating that the representations and
warranties set forth in Section 3 are true as of and all of the closing
conditions set forth in Section 5 have been satisfied on the Closing Date.

     5.4  COVENANTS. All covenants, agreements and conditions contained in this
Agreement to be performed by the Company on or prior to the Closing Date shall
have been performed or complied with in all material respects.

SECTION 6. CONDITIONS TO CLOSING OF COMPANY

     The Company's obligation to sell and issue the Shares is, at the option of
the Company, subject to the fulfillment or waiver of the following conditions:

     6.1  REPRESENTATIONS AND WARRANTIES. The representations made by Purchaser
in Section 4 hereof shall be true and correct in all material respects when
made, and shall be true and correct in all material respects on the Closing Date
with the same force and effect as if they had been made on and as of such date.

     6.2  COVENANTS. All covenants, agreements and conditions contained in this
Agreement to be performed by Purchaser on or prior to the Closing Date shall
have been performed or complied with in all material respects.

SECTION 7. REGISTRATION RIGHTS

     7.1  REGISTRATION REQUIREMENTS

          (a)  Except as provided in paragraph (d) below, the Company shall use
its best efforts to prepare and file a registration statement on Form S-3 with
the SEC under the Securities Act to register the resale of the Shares by
Purchaser (the "Registration Statement") on the twentieth (20th) business day
after the fifth month anniversary of the date hereof or as soon thereafter as is
practicable and to use its best efforts to cause the Registration Statement to
be declared effective as soon as practicable. In the event that at any time the
filing of such Registration Statement is undertaken or is required to be
undertaken the Company fails to qualify for use of Form S-3 for purposes of
registering for resale the Shares, the Company shall cause a registration
statement on Form S-1 to be filed as soon as practicable thereunder. The
Purchaser agrees to furnish promptly to the Company in writing all information
reasonably required by the Company to file such Registration Statement.

          (b)  The Company shall pay all Registration Expenses (as defined
below) in connection with any registration, qualification or compliance
hereunder, and Purchaser shall pay all Selling Expenses (as defined below) and
other expenses that are not Registration Expenses relating to the Shares resold
by Purchaser. "Registration Expenses" shall mean all expenses,

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except for Selling Expenses, incurred by the Company in complying with the
registration provisions herein described, including, without limitation, all
registration, qualification and filing fees, printing expenses, escrow fees,
fees and disbursements of counsel and independent public accountants for the
Company, blue sky fees, transfer agent fees and expenses and the expense of any
special audits incident to or required by any such registration. "Selling
Expenses" shall mean selling commissions, underwriting fees and stock transfer
taxes applicable to the Shares and, subject to Section 9.10, all fees and
disbursements of counsel for Purchaser.

          (c)  In the case of the registration effected by the Company pursuant
to these registration provisions, the Company will use its best efforts to: (i)
keep such registration effective until the earlier of (A) the second anniversary
of the Closing Date, (B) such date as all of the Shares have been resold or (C)
such time as all of the Shares held by Purchaser can be sold within a given
three-month period without compliance with the registration requirements of the
Securities Act pursuant to Rule 144; (ii) prepare and file with the SEC such
amendments and supplements to the Registration Statement and the prospectus used
in connection with the Registration Statement as may be necessary to comply with
the provisions of the Securities Act with respect to the disposition of all
securities covered by the Registration Statement; (iii) furnish such number of
prospectuses and other documents incident thereto, including any amendment of or
supplement to the prospectus, as Purchaser from time to time may reasonably
request in order to facilitate the public sale or other disposition of all or
any of the Shares held by Purchaser; (iv) cause all Shares registered as
described herein to be listed on each securities exchange and quoted on each
quotation service on which similar securities issued by the Company are then
listed or quoted; (v) provide a transfer agent and registrar for all Shares
registered pursuant to the Registration Statement and a CUSIP number for all
such Shares; (vi) otherwise use its best efforts promptly to comply with all
applicable rules and regulations of the SEC; and (vii) file the documents
required of the Company and otherwise use its best efforts promptly to obtain,
if applicable, and maintain requisite blue sky clearance in (A) all
jurisdictions in which any of the Shares are originally sold and (B) all other
states specified in writing by Purchaser, provided as to clause (B), however,
that the Company shall not be required to qualify to do business or consent to
service of process in any state in which it is not now so qualified or has not
so consented. The Company shall use its best efforts to qualify for use of Form
S-3 under the Securities Act to register the resale of the Shares and to
maintain such qualification during the periods described in paragraph (i).

          (d)  The Company may delay the filing of the Registration Statement
for up to six months by giving written notice to Purchaser if the Company shall
have determined that the Company may be required to disclose any material
corporate development which disclosure may have a material effect on the
Company.

          (e)  Following the effectiveness of the Registration Statement, the
Company may, at any time, but not more than once in any six-month period,
suspend the effectiveness of such registration statement for up to 45 days, as
appropriate (a "Suspension Period"), by giving notice to Purchaser, if the
Company shall have determined that the Company may be required to disclose any
material corporate development which disclosure may have a material effect on
the Company. The Company agrees to use commercially reasonable efforts to
minimize the length of any suspension. The duration of any Suspension Period
shall be added to the period of time

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that the Company agrees to keep the Registration Statement effective. Purchaser
agrees that, upon receipt of any notice from the Company of a Suspension Period,
Purchaser shall forthwith discontinue disposition of shares covered by such
Registration Statement or prospectus until Purchaser (i) is advised in writing
by the Company that the use of the applicable prospectus may be resumed, (ii)
has received copies of a supplemental or amended prospectus, if applicable, and
(iii) has received copies of any additional or supplemental filings which are
incorporated or deemed to be incorporated by reference in such prospectus.

          (f)  The Company will, as expeditiously as possible, notify Purchaser
(i) of the effective date of the Registration Statement and the date when any
post-effective amendment the Registration Statement becomes effective; (ii) of
any stop order or notification from Securities and Exchange Commission or any
other jurisdiction as to the suspension of the effectiveness of the Registration
Statement; and (iii) of the end of any suspension hereunder.

          (g)  With a view to making available to Purchaser the benefits of Rule
144 and any other rule or regulation of the SEC that may at any time permit
Purchaser to sell Shares to the public without registration or pursuant to
registration, the Company covenants and agrees to: (i) make and keep public
information available, as those terms are understood and defined in Rule 144,
until the earlier of (A) the second anniversary of the Closing Date or (B) such
date as all of the Shares shall have been resold; (ii) file with the SEC in a
timely manner all reports and other documents required of the Company under the
Securities Exchange Act of 1934, as amended (the "Exchange Act") and maintain
registration of its Common Stock under Section 12 of the Exchange Act; and (iii)
furnish to Purchaser upon request, as long as Purchaser owns any Shares, (A) a
written statement by the Company that it has complied with the reporting
requirements of the Exchange Act, (B) a copy of the most recent annual or
quarterly report of the Company, and (C) such other information as may be
reasonably requested in order to avail Purchaser of any rule or regulation of
the SEC that permits the selling of any such Shares without registration.

     7.2  INDEMNIFICATION AND CONTRIBUTION.

          (a)  The Company agrees to indemnify Purchaser and hold Purchaser
harmless from and against any losses, claims, damages or liabilities (or actions
or proceedings in respect thereof) to which Purchaser may become subject (under
the Securities Act Exchange Act, state securities laws or otherwise) insofar as
such losses, claims, damages or liabilities (or actions proceedings or
settlements in respect thereof) arise out of, or are based upon, (i) any untrue
statement (or alleged untrue statement) of a material fact contained in the
Registration Statement, on the effective date thereof or any preliminary
prospectus or final prospectus contained therein, or any amendment or supplement
thereto, (ii) the omission or the alleged omission of a material fact required
to be stated therein or necessary to make the statements therein not misleading
or (iii) any failure by the Company (or its agents) to fulfill any undertaking
included in the Registration Statement, and the Company will, as incurred,
reimburse Purchaser for any legal or other expenses reasonably incurred in
investigating, defending or preparing to defend any such action, loss, damage,
proceeding or claim; provided, however, that the Company shall not be liable in
any such case to the extent that such loss, claim, damage or liability arises
out of, or is based upon (i) an untrue statement (or omission) made in such
Registration Statement in reliance upon and in conformity with written
information furnished to the Company by or on behalf of

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Purchaser specifically for use in preparation of the Registration Statement,
(ii) the failure of Purchaser to comply with the covenants and agreements
contained in Section 7.1 or 8.3 hereof, or (iii) any untrue statement (or
omission) in any Prospectus that is corrected in any subsequent Prospectus that
was delivered to Purchaser by the Company prior to the pertinent sale or sales
by Purchaser. The Company will reimburse Purchaser for any legal or other
expenses reasonably incurred in investigating, defending or preparing to defend
any such action, proceeding or claim notwithstanding the absence of a judicial
determination as to the propriety and enforceability of the obligations under
this section and the possibility that such payments might later be held to be
improper, provided, that (i) to the extent any such payment is ultimately held
to be improper, the persons receiving such payments shall promptly refund them
and (ii) such persons shall provide to the Company, upon request, reasonable
assurances of their ability to effect any refund, when and if due.

          (b)  Purchaser agrees to indemnify and hold harmless the Company from
and against any losses, claims, damages or liabilities (or actions or
proceedings in respect thereof) to which the Company may become subject (under
the Securities Act or otherwise) insofar as such losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) arise out of, or are
based upon (i) an untrue statement made in such Registration Statement in
reliance upon and in conformity with written information furnished to the
Company by or on behalf of Purchaser specifically for use in preparation of the
Registration Statement, provided, however, that Purchaser shall not be liable in
any such case for any untrue statement included in any Prospectus which
statement has been corrected, in writing, by Purchaser and delivered to the
Company before the sale from which such loss occurred, (ii) the failure of
Purchaser to comply with the covenants and agreements contained in Section 7.1
or 8.3 hereof, or (iii) any untrue statement in any Prospectus that is corrected
in any subsequent Prospectus that was delivered to the Purchaser prior to the
pertinent sale or sales by Purchaser, provided, further, however, that the
liability of Purchaser hereunder shall be limited to the proceeds received by
Purchaser from the sale of the Shares covered by such Registration Statement;
and provided, further, however, that the obligations of Purchaser hereunder
shall not apply to amounts paid in settlement of any such loss, claim, damage,
liability, or action settlement is effected without the consent of Purchaser.
Purchaser will reimburse the Company for any legal or other expenses reasonably
incurred in investigating, defending or preparing to defend any such action,
proceeding or claim notwithstanding the absence of a judicial determination as
to the propriety and enforceability of the obligations under this section and
the possibility that such payments might later be held to be improper, provided,
that (i) to the extent any such payment is ultimately held to be improper, the
persons receiving such payments shall promptly refund them and (ii) such persons
shall provide to Purchaser, upon request, reasonable assurances of their ability
to effect any refund, when and if due.

          (c)  Promptly after receipt by any indemnified person of a notice of a
claim or the commencement of any action in respect of which indemnity is to be
sought against an indemnifying person pursuant to this Section 7.2, such
indemnified person shall notify the indemnifying person in writing of such claim
or of the commencement of such action, and, subject to the provisions
hereinafter stated, in case any such action shall be brought against an
indemnified person and the indemnifying person shall have been notified thereof,
the indemnifying person shall be entitled to participate therein, and, to the
extent that it shall wish, to

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assume and undertake the defense thereof, with counsel reasonably satisfactory
to the indemnified person. After notice from the indemnifying person to such
indemnified person of the indemnifying person's election to assume and undertake
the defense thereof, the indemnifying person shall not be liable to such
indemnified person for any legal expenses subsequently incurred by such
indemnified person in connection with the defense thereof; provided, however,
that if there exists or shall exist a conflict of interest that would make it
inappropriate in the reasonable judgment of the indemnified person for the same
counsel to represent both the indemnified person and such indemnifying person or
any affiliate or associate thereof, the indemnified person shall be entitled to
retain its own counsel at the expense of such indemnifying person.

          (d)  If the indemnification provided for in this Section 7.2 is
unavailable to or insufficient to hold harmless an indemnified party under
subsection (a) or (b) above in respect of any losses, claims, damages or
liabilities (or actions proceedings or settlements in respect thereof) referred
to therein, then the indemnifying party shall contribute to the amount paid or
payable by such indemnified party as result of such losses, claims, damages or
liabilities (or actions in respect thereof) in such proportion as is appropriate
to reflect the relative fault of the Company on the one hand and the Purchaser
on the other in connection with the statements or omissions which resulted in
such losses, claims, damages or liabilities (or actions in respect thereof), as
well as any other relevant equitable considerations. The relative fault shall be
determined by reference to, among other things, whether the untrue or alleged
untrue statement of a material fact or the omission or alleged omission to state
a material fact relates to information supplied by the Company on the one hand
or Purchaser on the other and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such statement or omission.
The Company and Purchaser agree that it would not be just and equitable if
contribution pursuant to this subsection (d) were determined by pro rata
allocation or by any other method of allocation which does not take account of
the equitable considerations referred to above in this subsection (d). The
amount paid or payable by an indemnified party as a result of the losses,
claims, damages, or liabilities (or actions in respect thereof) referred to
above in this subsection (d) shall be deemed to include any legal or other
expenses reasonably incurred by such indemnified party in connection with
investigating or defending any such action or claim. Notwithstanding the
provisions of this subsection (d), Purchaser shall not be required to contribute
any amount in excess of the amount by which the amount received by Purchaser
(net of Selling Expenses) from the sale of the Shares to which such loss relates
exceeds the amount of any damages which Purchaser has otherwise been required to
pay by reason of such untrue or alleged untrue statement or omission or alleged
omission. No person guilty of fraudulent misrepresentation (within the meaning
of Section 11(f) of the Securities Act) shall be entitled to contribution from
any person who was not guilty of such fraudulent misrepresentation.

          (e)  The obligations of the Company and Purchaser under this Section
7.2 shall be in addition to any liability which the Company and Purchaser may
otherwise have and shall extend, upon the same terms and conditions, to each
person, if any, who controls the Company or Purchaser within the meaning of the
Securities Act.

                                       9

<PAGE>   10

SECTION 8. RESTRICTIONS ON TRANSFERABILITY OF SHARES: COMPLIANCE WITH SECURITIES
           ACT

          8.1  RESTRICTIONS ON TRANSFERABILITY. The Shares shall not be
transferable in the absence of a registration under the Securities Act or an
exemption therefrom or in the absence of compliance with any term of this
Agreement. The Company shall be entitled to give stop transfer instructions to
its transfer agent with respect to the Shares in order to enforce the foregoing
restrictions.

          8.2  RESTRICTIVE LEGEND. Each certificate representing Shares shall
bear substantially the following legends (in addition to any legends required
under applicable securities laws):

          THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
          INVESTMENT PURPOSES ONLY AND HAVE NOT BEEN REGISTERED UNDER THE
          SECURITIES ACT OF 1933 OR ANY STATE SECURITIES LAWS. THE SHARES MAY
          NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN
          EXEMPTION THEREFROM.

          ADDITIONALLY, THE TRANSFER OF THE SHARES REPRESENTED BY THIS
          CERTIFICATE IS SUBJECT TO CERTAIN RESTRICTIONS SPECIFIED IN THE COMMON
          STOCK PURCHASE AGREEMENT DATED OCTOBER 10, 2000 BETWEEN THE COMPANY
          AND THE ORIGINAL PURCHASER, AND NO TRANSFER OF SHARES SHALL BE VALID
          OR EFFECTIVE ABSENT COMPLIANCE WITH SUCH RESTRICTIONS. ALL SUBSEQUENT
          HOLDERS OF THIS CERTIFICATE WILL HAVE AGREED TO BE BOUND BY CERTAIN OF
          THE TERMS OF THE AGREEMENT, INCLUDING SECTIONS 7.1 AND 8.3 OF THE
          AGREEMENT. COPIES OF THE AGREEMENT MAY BE OBTAINED AT NO COST BY
          WRITTEN REQUEST MADE BY THE REGISTERED HOLDER OF THIS CERTIFICATE TO
          THE SECRETARY OF THE COMPANY.

     Upon the request of Purchaser, the Company shall remove the foregoing
legend from the certificates evidencing the Shares and issue to Purchaser new
certificates free of any transfer legend if with such request, and at the
request of the Company, the Company shall have received an opinion of counsel
satisfactory to the Company, to the effect that any transfers by Purchaser of
such Shares may be made to the public without compliance with either Section 5
of the Securities Act or Rule 144 thereunder and applicable state securities
laws.

          8.3  TRANSFER OF SHARES AFTER REGISTRATION. Purchaser hereby covenants
with the Company not to make any sale of the Shares except either (i) in
accordance with the Registration Statement, in which case Purchaser covenants to
comply with the requirement of delivering a current prospectus, or (ii) in
accordance with Rule 144, in which case Purchaser covenants to comply with Rule
144. Purchaser further acknowledges and agrees that such Shares are not
transferable on the books of the Company unless the certificate submitted to the
Company's transfer agent evidencing such Shares is accompanied by a separate
certificate executed by an officer of, or other person duly authorized by, the
Purchaser in the form attached hereto as Exhibit B.

                                       10

<PAGE>   11

          8.4  PURCHASER INFORMATION. Purchaser covenants that it will promptly
notify the Company in writing of any changes in the information set forth in the
Registration Statement regarding Purchaser.

SECTION 9. MISCELLANEOUS

          9.1  WAIVERS AND AMENDMENTS. The terms of this Agreement may be waived
or amended with the written consent of the Company and Purchaser.

          9.2  GOVERNING LAW. This Agreement shall be governed in all respects
by and construed in accordance with the laws of the State of California without
any regard to conflicts of laws principles.

          9.3  SURVIVAL. The representations, warranties, covenants and
agreements made in this Agreement shall survive any investigation made by the
Company or Purchaser and the Closing. With respect to any registration made
pursuant to this Agreement, the covenants and agreements set forth in section
7.1 shall continue in effect until all obligations hereunder with respect
thereto are fulfilled, and provided that the indemnification and contribution
obligations as set forth in Section 7.2 shall survive for the period of the
statute of limitations with respect thereto.

          9.4  SUCCESSORS AND ASSIGNS. The provisions hereof shall inure to the
benefit of, and be binding upon, the successors, assigns, heirs, executors and
administrators of the parties to this Agreement. Notwithstanding the foregoing,
Purchaser shall not assign this Agreement without the prior written consent of
the Company.

          9.5  ENTIRE AGREEMENT. This Agreement constitutes the full and entire
understanding and agreement between the parties with regard to the subjects
thereof.

          9.6  NOTICES, ETC. All notices and other communications required or
permitted under this Agreement shall be effective upon receipt and shall be in
writing and may be delivered in person, by telecopy, overnight delivery service
or registered or certified United States mail, addressed to the Company or
Purchaser, as the case may be, at their respective addresses set forth below:

                                       11

<PAGE>   12

If to the Company:

          Aviron
          297 North Bernardo Avenue
          Mountain View, CA  94043
          Attn:                     C. Boyd Clarke
                                    President and Chief Executive Officer
          Telephone:                (650) 919-6500
          Facsimile:                (650) 919-6612

With a copy to:

          Cooley Godward LLP
          Five Palo Alto Square
          3000 El Camino Real
          Palo Alto, CA  94306-2155
          Attn:                     Robert J. Brigham, Esq.
          Telephone:                (650) 843-5000
          Facsimile:                (650) 849-7400

If to Purchaser:

          Biotech Invest, S.A.
          Swiss Bank Tower
          Panama 1
          Republic of Panama

With copies to:
          Bellevue Asset Management AG
          Grafenauweg 4
          CH-6301 Zug
          SWITZERLAND
          Attn:                     Dr. Nora Frey
          Telephone:                011-41-724-5920
          Facsimile:                011-41-724-5958

                                       12

<PAGE>   13

and:

          Bellevue Research, Inc.
          1 Cambridge Center,
          Cambridge, MA  02142
          Attn:                     Dr. Anders Hove
          Telephone:                (617) 679-9888
          Facsimile:                (617) 679-9887
and:

          Baker & McKenzie
          815 Connecticut Avenue, N.W.
          Washington, D.C.  20006

          Attn:                     Daniel L. Goelzer, Esq.
          Telephone:                (202) 452-7000
          Facsimile:                (202) 452-7072

     All notices and other communications shall be effective upon the earlier of
actual receipt thereof by the person to whom notice is directed or (i) in the
case of notices and communications sent by personal delivery or telecopy, one
business day after such notice or communication arrives at the applicable
address or was successfully sent to the applicable telecopy number, (ii) in the
case of notices and communications sent by overnight delivery service, at noon
(local time) on the second business day following the day such notice or
communication was sent, and (iii) in the case of notices and communications sent
by United States mail, seven days after such notice or communication shall have
been deposited in the United States mail. Any notice delivered to a party
hereunder shall be sent simultaneously, by the same means, to such party's
counsel as set forth above.

          9.7  SEVERABILITY OF THIS AGREEMENT. If any provision of this
Agreement shall be judicially determined to be invalid, illegal or
unenforceable, the validity, legality and enforceability of the remaining
provisions shall not in any way be affected or impaired thereby.

          9.8  COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

          9.9  FURTHER ASSURANCES. Each party to this Agreement shall do and
perform or cause to be done and performed all such further acts and things and
shall execute and deliver all such other agreements, certificates, instruments
and documents as the other party hereto may reasonably request in order to carry
out the intent and accomplish the purposes of this Agreement and the
consummation of the transactions contemplated hereby.

          9.10 EXPENSES. The Company agrees to bear the cost of reasonable fees
and expenses of one counsel for the Purchaser, in an amount not to exceed
$15,000, and reasonable fees and

                                       13

<PAGE>   14

expenses of one counsel for the Purchaser in connection with its review of the
Registration Statement.

                                       14

<PAGE>   15

     The foregoing agreement is hereby executed as of the date first above
written.

AVIRON                                  BIOTECH INVEST, S.A.

By: /s/ FRED KURLAND                    By:
   -----------------------------------     -------------------------------------
   Fred Kurland                            Dr. Anders Hove
   Senior Vice President and Chief         Authorized Signatory
   Financial officer

                                        By:
                                           -------------------------------------
                                           Dr. Nora Frey
                                           Authorized Signatory

                                       15

<PAGE>   16

                                    EXHIBIT A

                         INSTRUCTION SHEET FOR PURCHASER

                   (to be read in conjunction with the entire
                        Common Stock Purchase Agreement)

A.   Complete the following items in the Common Stock Purchase Agreement:

     1.   Provide the information regarding the Purchaser requested on the
          signature page. The Agreement must be executed by an individual
          authorized to bind the Purchaser.

     2.   Exhibit A-1 - Stock Certificate Questionnaire: Provide the information
          requested by the Stock Certificate Questionnaire;

     3.   Exhibit A-2 - Registration Statement Questionnaire: Provide the
          information requested by the Registration Statement Questionnaire.

     4.   Exhibit A-3 - Purchaser Certificate: Provide the information requested
          by the Certificate.

     5.   Return the signed Agreement including the properly completed Exhibit A
          to:

               Cooley Godward LLP
               Five Palo Alto Square
               3000 El Camino Real
               Palo Alto, CA  94306
               Attn: Annie O. Loo, Esq.
               Fax: (650) 849-7400

B.   Instructions regarding the transfer of funds for the purchase of Shares
     will be telecopied to Purchaser by the Company at a later date.

C.   Upon the resale of the Shares by Purchaser after the Registration Statement
     covering the Shares is effective, as described in the Purchase Agreement,
     Purchaser:

          (i)  must deliver a current prospectus, and annual and quarterly
               reports of the Company to the buyer (prospectuses, and annual and
               quarterly reports may be obtained from the Company at the
               Purchaser's request); and

          (ii) must send a letter in the form of Exhibit B to the Company so
               that the Shares may be properly transferred.

                                       16

<PAGE>   17

                                   EXHIBIT A-1

                                     AVIRON

                         STOCK CERTIFICATE QUESTIONNAIRE

     Pursuant to Section 4.3 of the Agreement, please provide us with the
following information:

1.   The exact name that the Shares are
     to be registered in (this is the
     name that will appear on the stock
     certificate(s)). You may use a
     nominee name if appropriate:
                                          --------------------------------------

2.   The relationship between the
     Purchaser of the Shares and the
     Registered Holder listed in
     response to item 1 above:
                                          --------------------------------------

3.   The mailing address of the
     Registered Holder listed in
     response to item 1 above:
                                          --------------------------------------

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

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

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

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

4.   The Tax Identification Number of
     the Registered Holder listed in
     response to item 1 above:
                                          --------------------------------------

<PAGE>   18

                                   EXHIBIT A-2

                                     AVIRON

                      REGISTRATION STATEMENT QUESTIONNAIRE

     In connection with the preparation of the Registration Statement, please
provide us with the following information regarding the Purchaser.

     1.   Please state your organization's name exactly as it should appear in
the Registration Statement:

     2.   Have you or your organization had any position, office or other
material relationship within the past three years with the Company?

                         Yes                           No
               ---------                     ---------

     If yes, please indicate the nature of any such relationships below:

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

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

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

<PAGE>   19

                                   EXHIBIT A-3

                                     AVIRON

                     CERTIFICATE FOR CORPORATE, PARTNERSHIP,
                     TRUST, FOUNDATION AND JOINT PURCHASERS

     If the investor is a corporation, partnership, trust, pension plan,
foundation, joint purchaser (other than a married couple) or other entity, an
authorized officer, partner, or trustee must complete, date and sign this
Certificate.

                                   CERTIFICATE

     The undersigned certifies that the representations and responses below are
true and accurate:

     (a)  The investor has been duly formed and is validly existing and has full
power and authority to invest in the Company. The person signing on behalf of
the undersigned has the authority to execute and deliver the Common Stock
Purchase Agreement on behalf of the Purchaser and to take other actions with
respect thereto.

     (b)  Indicate the form of entity of the undersigned:

                    Limited Partnership
          --------

                    General Partnership
          --------

                    Corporation
          --------

                    Revocable Trust (identify each grantor and indicate under
          --------  what circumstances the trust is revocable by the grantor):

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

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

                       (Continue on a separate piece of paper, if necessary.)

                    Other Type of Trust (indicate type of trust and, for
          --------  trusts other than pension trusts, name the grantors and
                    beneficiaries):

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

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

                       (Continue on a separate piece of paper, if necessary.)

                    Other form of organization (indicate form of organization):
          --------

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

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

     (c)  Indicate the date the undersigned entity was formed:
                                                              ------------------

<PAGE>   20

     (d)  In order for the Company to offer and sell the Shares in conformance
with state and federal securities laws, the following information must be
obtained regarding your investor status. Please initial each category applicable
to you as an investor in the Company.

          ____ 1.   A bank as defined in Section 3(a)(2) of the Securities Act,
          or any savings and loan association or other institution as defined in
          Section 3(a)(5)(A) of the Securities Act whether acting in its
          individual or fiduciary capacity;

          ____ 2.   A broker or dealer registered pursuant to Section 15 of the
          Securities Exchange Act of 1934;

          ____ 3.   An insurance company as defined in Section 2(13) of the
          securities Act;

          ____ 4.   An investment company registered under the Investment
          Company Act of 1940 or a business development company as defined in
          Section 2(a)(48) of that Act;

          ____ 5.   A Small Business Investment Company licensed by the U.S.
          Small Business Administration under Section 301(c) or (d) of the Small
          Business Investment Act of 1958;

          ____ 6.   A plan established and maintained by a state, its political
          subdivisions, or any agency or instrumentality of a state or its
          political subdivisions, for the benefit of its employees, if such plan
          has total assets in excess of $5,000,000;

          ____ 7.   An employee benefit plan within the meaning of the Employee
          Retirement Income Security Act of 1974, if the investment decision is
          made by a plan fiduciary, as defined in Section 3(21) of such act,
          which is either a bank, savings and loan association, insurance
          company, or registered investment adviser, or if the employee benefit
          plan has total assets in excess of $5,000,000 or, if a self-directed
          plan, with investment decisions made solely by persons that are
          accredited investors;

          ____ 8.   A private business development company as defined in Section
          202(a)(22) of the Investment Advisers Act of 1940;

          ____ 9.   An organization described in Section 501(c)(3) of the
          Internal Revenue Code, a corporation, Massachusetts or similar
          business trust, or partnership, not formed for the specific purpose of
          acquiring the Shares, with total assets in excess of $5,000,000;

          ____ 10.  A trust, with total assets in excess of $5,000,000, not
          formed for the specific purpose of acquiring the Shares, whose
          purchase is directed by a sophisticated person who has such knowledge
          and experience in financial and

                                       20

<PAGE>   21

          business matters that such person is capable of evaluating the merits
          and risks of investing in the Company;

          ____ 11.  An entity in which all of the equity owners qualify under
          any of the above subparagraphs. If the undersigned belongs to this
          investor category only, list the equity owners of the undersigned, and
          the investor category which each such equity owner satisfies:

          ----------------------------------------------------------------------
                  (Continue on a separate piece of paper, if necessary.)

     (e)  The state of incorporation or formation of the investor is ___________
and the investor's principal office is located in the state of ________________.

Dated: _____________, 20__

----------------------------------------
Name of investor

----------------------------------------
Signature and title of authorized
officer, partner or trustee

                                       21

<PAGE>   22

                                    EXHIBIT B

                   PURCHASER'S CERTIFICATE OF SUBSEQUENT SALE

To: Aviron
    297 N. Bernardo Avenue
    Mountain View, CA  94043

     The undersigned, the Purchaser or an officer of, or other person duly
authorized by the Purchaser, hereby certifies that BIOTECH INVEST, S.A. was the
Purchaser of the shares evidenced by the attached certificate, and as such,
proposes to transfer such shares on or about ______ either (check the applicable
box): (i) in accordance with the registration statement, file number ___________
in which case the Purchaser certifies that the requirement of delivering a
current prospectus has been complied with or will be complied with in connection
with such sale, or: (ii) in accordance with Rule 144 under the Securities Act of
1933 ("Rule 144"), in which case the Purchaser certifies that it has complied
with or will comply with the requirements of Rule 144.

Print or type:

Name of Purchaser: Biotech Invest, S.A.

Name of Individual
representing Purchaser:
                       ---------------------------------------------------------

Title of Individual
representing Purchaser:
                       ---------------------------------------------------------

Signature by:
Individual representing
Purchaser:
          ----------------------------------------------------------------------

<PAGE>   23
                                    EXHIBIT C
                                 FORM OF OPINION

October __, 2000

Biotech Invest, S.A.
Swiss Bank Tower
Panama 1
Republic of Panama

RE: SALE AND PURCHASE OF AVIRON COMMON STOCK

Gentlemen:

We have acted as counsel for Aviron, a Delaware corporation (the "Company"), in
connection with the issuance and sale of 450,000 shares of the Company's Common
Stock to Biotech Invest, S.A., a corporation organized under the laws of Panama
("Purchaser"), pursuant to the terms of that certain Common Stock Purchase
Agreement, dated October 10, 2000, by and between the Company and Purchaser (the
"Agreement"). The shares of the Company's Common Stock issued to Purchaser at
the closing (the "Closing") are referred to herein as the "Shares". We are
rendering this opinion pursuant to Section 5.2 of the Agreement. Except as
otherwise defined herein, capitalized terms used but not defined herein have the
respective meanings given to them in the Agreement.

In connection with this opinion, we have examined and relied upon the
representations and warranties as to factual matters contained in and made
pursuant to the Agreement by the parties thereto and originals or copies
certified to our satisfaction, of such records, documents, certificates,
opinions, memoranda and other instruments as in our judgment are necessary or
appropriate to enable us to render the opinion expressed below. Where we render
an opinion "to the best of our knowledge" or concerning an item "known to us" or
our opinion otherwise refers to our knowledge, it is based solely upon (i) an
inquiry of attorneys within this firm who perform legal services for the
Company, (ii) receipt of a certificate executed by an officer of the Company
covering such matters, and (iii) such other investigation, if any, that we
specifically set forth herein.

In rendering this opinion, we have assumed: the genuineness and authenticity of
all signatures on original documents; the authenticity of all documents
submitted to us as originals; the conformity to originals of all documents
submitted to us as copies; the accuracy, completeness and authenticity of
certificates of public officials; and the due authorization, execution and
delivery of all documents where authorization, execution and delivery are
prerequisites to the effectiveness of such documents (except the due
authorization, execution and delivery of the Agreement by the Company). We have
also assumed: that all individuals executing and delivering documents had the
legal capacity to so execute and deliver; that you have received all documents
you were to receive under the Agreement; that the Agreement is an obligation

<PAGE>   24

binding upon you; if you are a corporation or other entity, that you have filed
any required California franchise or income tax returns and have paid any
required California franchise or income taxes; and that there are no extrinsic
agreements or understandings among the parties to the Agreement that would
modify or interpret the terms of the Agreement or the respective rights or
obligations of the parties thereunder.

Our opinion is expressed only with respect to the federal laws of the United
States of America and the laws of the State of California and the General
Corporation Law of the State of Delaware. We express no opinion as to whether
the laws of any particular jurisdiction apply, and no opinion to the extent that
the laws of any jurisdiction other than those identified above are applicable to
the subject matter hereof. We are not rendering any opinion as to compliance
with any antifraud law, rule or regulation relating to securities, or to the
sale or issuance thereof.

With regard to our opinion in paragraph 3 below with respect to the
authorization of the Agreement, we have examined and relied solely upon a
certificate executed by an officer of the Company, to the effect that the
Agreement was duly and validly authorized in accordance with the provisions of
the applicable Board of Directors resolutions, and we have undertaken no
independent verification with respect thereto. With regard to our opinion in
paragraph 4 below, we have examined and relied upon a certificate executed by an
officer of the Company, to the effect that the consideration for all outstanding
shares of capital stock of the Company was received by the Company in accordance
with the provisions of the applicable Board of Directors resolutions and any
plan or agreement relating to the issuance of such shares, and we have
undertaken no independent verification with respect thereto.

On the basis of the foregoing, in reliance thereon and with the foregoing
qualifications, we are of the opinion that:

     1.   The Company has been duly incorporated and is a validly existing
corporation in good standing under the laws of the State of Delaware.

     2.   The Company has the requisite corporate power to own or lease its
property and assets and to conduct its business as it is currently being
conducted and, to the best of our knowledge, is qualified as a foreign
corporation to do business in each jurisdiction in the United States in which
the ownership of its property or the conduct of its business requires such
qualification and where any statutory fines or penalties or any corporate
disability imposed for the failure to qualify would materially or adversely
affect the Company, its assets, financial condition or operations.

     3.   The Agreement has been duly and validly authorized, executed and
delivered by the Company and constitutes a valid, legal and binding agreement of
the Company enforceable against the Company in accordance with its terms, except
as rights to indemnity under Section 7.2 of the Agreement may be limited by
applicable laws and except as enforcement may be limited by applicable
bankruptcy, insolvency, reorganization, arrangement, moratorium or other similar
laws affecting creditors' rights, and subject to general equity principles and
to limitations on availability of equitable relief, including specific
performance.

<PAGE>   25

     4.   The Shares have been duly authorized and, upon issuance and delivery
in accordance with the terms of the Agreement, will be validly issued, fully
paid and nonassessable.

     5.   The issuance and sale of the Shares as contemplated by the Agreement
does not violate any provision of the Company's Certificate of Incorporation or
Bylaws and does not violate or contravene (a) any governmental statute, rule or
regulation applicable to the Company or (b) any order, writ, judgment,
injunction, decree, determination or award which has been entered against the
Company and which is known to us, the violation or contravention of which would
materially and adversely affect the Company, its assets, financial condition or
operations.

     6.   All consents, approvals, authorizations, or orders of, and filings,
registrations, and qualifications with any regulatory authority or governmental
body in the United States required for the issuance and sale of the Shares as
contemplated by the Agreement, have been made or obtained.

     7.   The issuance and sale of the Shares as contemplated by the Agreement
is exempt from the registration requirements of the Securities Act of 1933, as
amended.

This opinion is intended solely for your benefit and is not to be made available
to or be relied upon by any other person, firm, or entity without our prior
written consent.

Very truly yours,

COOLEY GODWARD LLP

By
  ------------------------------------------
  Robert J. Brigham

<PAGE>   26

                         COMMON STOCK PURCHASE AGREEMENT

                          DATED AS OF OCTOBER 10, 20000

                                 BY AND BETWEEN

                                     AVIRON

                                       AND

                              BIOTECH TARGET, S.A.

<PAGE>   27

                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                             PAGE
<S>    <C>                                                                    <C>
Section 1.  Purchase and Sale of Common Stock...............................   1

Section 2.  Closing Date; Delivery..........................................   1

2.1    Closing Date.........................................................   1

2.2    Delivery.............................................................   1

Section 3.  Representations and Warranties of the Company...................   1

3.1    Organization and Standing............................................   1

3.2    Corporate Power; Authorization.......................................   1

3.3    Issuance and Delivery of the Shares..................................   2

3.4    Full Disclosure......................................................   2

3.5    Litigation...........................................................   3

3.6    Governmental Consents................................................   3

3.7    No Material Adverse Change...........................................   3

3.8    Removal of Legends...................................................   3

Section 4.  Representations, Warranties and Covenants of Purchaser..........   3

4.1    Authorization........................................................   3

4.2    Investment Experience................................................   3

4.3    Investment Intent....................................................   3

4.4    Registration or Exemption Requirements...............................   4

4.5    Restriction on Short Sales...........................................   4

4.6    No Legal, Tax or Investment Advice...................................   4

Section 5.  Conditions to Closing of Purchaser..............................   4

5.1    Representations and Warranties.......................................   4

5.2    Legal Opinion........................................................   5

5.3    Officers' Certificate................................................   5

5.4    Covenants............................................................   5

Section 6.  Conditions to Closing of Company................................   5

6.1    Representations and Warranties.......................................   5

6.2    Covenants............................................................   5

Section 7.  Registration Rights.............................................   5

7.1    Registration Requirements............................................   5

7.2    Indemnification and Contribution.....................................   7
</TABLE>

<PAGE>   28

<TABLE>
<S>    <C>                                                                    <C>
Section 8.  Restrictions on Transferability of Shares: Compliance with
            Securities Act..................................................  10

8.1    Restrictions on Transferability......................................  10

8.2    Restrictive Legend...................................................  10

8.3    Transfer of Shares after Registration................................  10

8.4    Purchaser Information................................................  11

Section 9.  Miscellaneous...................................................  11

9.1    Waivers and Amendments...............................................  11

9.2    Governing Law........................................................  11

9.3    Survival.............................................................  11

9.4    Successors and Assigns...............................................  11

9.5    Entire Agreement.....................................................  11

9.6    Notices, etc. .......................................................  11

9.7    Severability of this Agreement.......................................  13

9.8    Counterparts.........................................................  13

9.9    Further Assurances...................................................  13

9.10   Expenses.............................................................  13
</TABLE>

                                       2

<PAGE>   29

<TABLE>
<S>         <C>
Exhibit A   Instruction Sheet for Purchaser

Exhibit B   Purchaser's Certificate of Subsequent Sale

Exhibit C   Form of Legal Opinion
</TABLE>

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