<PAGE>   1
                                                                    EXHIBIT 4.18


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

                                     AVIRON

               WARRANT FOR THE PURCHASE OF SHARES OF COMMON STOCK

No. CSW- 4                                                          9,398 Shares


         FOR VALUE RECEIVED, AVIRON, a Delaware corporation (the "Company"),
with its principal office at 297 North Bernardo Avenue, Mountain View, CA 94043,
hereby certifies that The Procter & Gamble Company, or its assigns (the
"Holder"), is entitled, subject to the provisions of this Warrant, to purchase
from the Company, at any time after the Eligibility Date (as defined below) and
prior to the Expiration Date (as defined below), nine thousand three hundred
ninety-eight (9,398) fully paid and nonassessable shares of Common Stock of the
Company, at an exercise price per share equal to fifty-three dollars ($53.00)
(the "Exercise Price"). The term "Common Stock" shall mean the aforementioned
Common Stock of the Company, together with any other equity securities that may
be issued by the Company in addition thereto or in substitution therefor as
provided herein.

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

SECTION 1. EXERCISE OF WARRANT. Subject to Section 11 below, this Warrant shall
be immediately exercisable in whole, but not in part, upon the earlier of: (i)
the date of receipt by the Company of written acceptance by the United States
Food and Drug Administration of the Company's filing of its Biologics License
Application for its product, FluMist(TM), or (ii) ten (10) days prior to the
Expiration Date (the "Eligibility Date"). This Warrant may be exercised in
whole, but not in part, by presentation and surrender to the Company at its
principal office at the address set forth in the initial paragraph hereof (or at
such other address as the Company may hereafter notify the Holder in writing)
with the Purchase Form annexed hereto duly executed and accompanied by proper
payment of the Exercise Price in lawful money of the United States of America in
the form of a check, subject to collection, for the number of Warrant Shares
specified


<PAGE>   2

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

         SECTION 2. NET ISSUE EXERCISE. Notwithstanding any provisions herein to
the contrary, in lieu of exercising this Warrant for cash, the Holder may elect
to receive shares equal to the value (as determined below) of this Warrant (or
the portion thereof being canceled) by surrender of this Warrant at the
principal office of the Company together with notice of such election in which
event the Company shall issue to the Holder a number of shares of Common Stock
computed using the following formula:

                                  X = Y (A - B)
                                      ---------
                                          A

                Where X = the number of shares of Common Stock to be issued to
                the Holder

                Y = the number of shares of Common Stock purchasable under the
                Warrant (at the date of such calculation)

                A = the fair market value as determined by the closing sales
                price on the date of exercise of one share of the Company's
                Common Stock as quoted on NASDAQ

                B = Exercise Price (as adjusted to the date of such calculation)

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

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

         SECTION 5. ASSIGNMENT OR LOSS OF WARRANT.

                (a) Except as provided in Section 9, the Holder of this Warrant
shall be entitled, without obtaining the consent of the Company, to assign its
interest in this Warrant in whole to any person or persons. Subject to the
provisions of Section 9, upon surrender of this Warrant to the Company or at the
office of its stock transfer agent or warrant agent, with the Assignment Form
annexed hereto duly executed and funds sufficient to pay any transfer tax, the


                                       2.
<PAGE>   3

Company shall, without charge, execute and deliver a new Warrant or Warrants in
the name of the assignee or assignees named in such instrument of assignment,
and this Warrant shall promptly be canceled.

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

         SECTION 6. RIGHTS OF THE HOLDER. The Holder shall not, by virtue
hereof, be entitled to any rights of a shareholder in the Company, either at law
or equity, and the rights of the Holder are limited to those expressed in this
Warrant. Nothing contained in this Warrant shall be construed as conferring upon
the Holder hereof the right to vote or to consent or to receive notice as a
shareholder of the Company on any matters or with respect to any rights
whatsoever as a shareholder of the Company. No dividends or interest shall be
payable or accrued in respect of this Warrant or the interest represented hereby
or the Warrant Shares purchasable hereunder until, and only to the extent that,
this Warrant shall have been exercised in accordance with its terms.

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

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

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

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


                                       3.
<PAGE>   4

in effect immediately prior to such date of determination by a fraction (a) the
numerator of which shall be the total number of shares of Common Stock
outstanding immediately prior to such dividend or distribution, and (b) the
denominator of which shall be the total number of shares of Common Stock
outstanding immediately after such dividend or distribution and the number of
Warrant Shares subject to this Warrant shall be proportionately adjusted.

                (d) NOTICE OF RECORD DATE. In the event of any taking by the
Company of a record of its shareholders for the purpose of determining
shareholders who are entitled to receive payment of any dividend (other than a
cash dividend) or other distribution, any right to subscribe for, purchase or
otherwise acquire any share of any class or any other securities or property, or
to receive any other right, or for the purpose of determining shareholders who
are entitled to vote in connection with any proposed merger or consolidation of
the Company with or into any other corporation, or any proposed sale, lease or
conveyance of all or substantially all of the assets of the Company, or any
proposed liquidation, dissolution or winding up of the Company, the Company
shall mail to the Holder of this Warrant, at least ten days prior to the date
specified therein, a notice specifying the date on which any such record is to
be taken for the purpose of such dividend, distribution or right, and the amount
and character of such dividend, distribution or right.

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

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

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

         THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
         SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), OR UNDER THE SECURITIES
         LAWS OF ANY STATE. THESE SECURITIES ARE SUBJECT TO RESTRICTIONS ON
         TRANSFERABILITY AND RESALE AND MAY NOT BE TRANSFERRED OR RESOLD EXCEPT
         AS PERMITTED UNDER THE ACT AND THE


                                       4.
<PAGE>   5

         APPLICABLE STATE SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION
         THEREFROM. THE ISSUER OF THESE SECURITIES MAY REQUIRE AN OPINION OF
         COUNSEL IN FORM AND SUBSTANCE SATISFACTORY TO THE ISSUER TO THE EFFECT
         THAT ANY PROPOSED TRANSFER OR RESALE IS IN COMPLIANCE WITH THE ACT AND
         ANY APPLICABLE STATE SECURITIES LAWS.

Any certificate for any Warrant Shares issued at any time in exchange or
substitution for any certificate for any Warrant Shares bearing such legend
(except a new certificate for any Warrant Shares issued after the acquisition of
such Warrant Shares pursuant to a registration statement that has been declared
effective under the Act) shall also bear such legend unless, in the opinion of
counsel for the Company, the Warrant Shares represented thereby need no longer
be subject to the restriction contained herein. The provisions of this Section 9
shall be binding upon all subsequent Holders of certificates for Warrant Shares
bearing the above legend and all subsequent Holders of this Warrant, if any. In
addition in connection with the issuance of this Warrant, the Holder
specifically represents to the Company by acceptance of this Warrant as follows:

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

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

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

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

                (e) The Holder further understands that in the event all of the
requirements of Rule 144 are not satisfied, registration under the Act,
compliance with Regulation A, or some other registration exemption will be
required; and that, notwithstanding the fact that Rule 144 is not exclusive, the
staff of the Securities and Exchange Commission (the "SEC") has expressed


                                       5.
<PAGE>   6

its opinion that persons proposing to sell private placement securities other
than in a registered offering and otherwise than pursuant to Rule 144 will have
a substantial burden of proof in establishing that an exemption from
registration is available for such offers or sales, and that such persons and
their respective brokers who participate in such transactions do so at their own
risk.

         SECTION 10. REGISTRATION RIGHTS. If, after this Warrant has been
exercised in full, the Holder is unable to sell or otherwise dispose of all the
Warrant Shares delivered upon such exercise without restriction pursuant to Rule
144, then with respect to such Warrant Shares, the Holder shall be entitled to
registration rights as provided in this Section 10.

                (a) REGISTRATION REQUIREMENTS.

                    (i) Except as provided in paragraph (iv) below, if the
Company shall receive a written request from the Holder that the Company file a
registration statement under the Act covering the registration of those Warrant
Shares which Holder cannot sell or otherwise dispose pursuant to Rule 144, then
the Company shall use its best efforts to prepare and file a registration
statement on Form S-3 with the SEC under the Act within 30 days of the receipt
of such written request, to register such Warrant Shares by the Holder (the
"Registration Statement") 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 (or other
available form for similar type securities registration) for purposes of
registering for resale the Warrant Shares, the Company shall cause a
registration statement on Form S-1 (or other available form for similar type
securities registration) to be filed as soon as practicable after its receipt of
the request specified in this Section 10(a)(i). The Holder agrees to furnish
promptly to the Company in writing all information reasonably required by the
Company to file such Registration Statement. The Company shall only be obligated
to effect one (1) registration pursuant to this Section 10.

                    (ii) The Company shall pay all Registration Expenses (as
defined below) in connection with any registration, qualification or compliance
hereunder, and Holder shall pay all Selling Expenses (as defined below).
"Registration Expenses" shall mean all expenses, 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
Warrant Shares.

                    (iii) In the case of the registration effected by the
Company pursuant to these registration provisions, the Company will use its best
efforts to: (1) keep such registration effective until the earliest of (A) the
first anniversary of the effective date of the Registration Statement, (B) such
date as all of the Warrant Shares have been resold or (C) such time as all of
the Warrant Shares held by Holder can be sold without restriction pursuant to
Rule 144 under the Act; (2) prepare and file with the SEC such amendments and
supplements to the Registration


                                       6.
<PAGE>   7

Statement and the prospectus used in connection with the Registration Statement
as may be necessary to comply with the provisions of the Act with respect to the
disposition of all securities covered by the Registration Statement; (3) furnish
such number of prospectuses and other documents incident thereto, including any
amendment of or supplement to the prospectus, as Holder from time to time may
reasonably request in order to facilitate the public sale or other disposition
of all or any of the Warrant Shares held by Holder; (4) cause all Warrant 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; (5) provide a transfer agent and registrar
for all Warrant Shares registered pursuant to the Registration Statement and a
CUSIP number for all such Warrant Shares; (6) otherwise use its best efforts
promptly to comply with all applicable rules and regulations of the SEC; and (7)
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 Warrant Shares are originally sold and
(B) all other states specified in writing by Holder, 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 or other similar form then available under the Act to register
the resale of the Warrant Shares and to maintain such qualification during the
periods described in paragraph (i).

                    (iv) The Company may delay the filing of the Registration
Statement for up to forty-five (45) days by giving written notice to Holder if
the Board of Directors of the Company shall have determined in good faith that
the Company may be required to disclose any material corporate development which
disclosure may have a material effect on the Company.

                    (v) 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 30
days, as appropriate (a "Suspension Period"), by giving notice to Holder, 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 such suspension. The duration of any Suspension
Period shall be added to the period of time that the Company agrees to keep the
Registration Statement effective. Holder agrees that, upon receipt of any notice
from the Company of a Suspension Period, Holder shall forthwith discontinue
disposition of Warrant Shares covered by such Registration Statement or
prospectus until Holder (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.

                    (vi) The Company will, as expeditiously as possible, notify
Holder (1) of the effective date of the Registration Statement and the date when
any post-effective amendment to the Registration Statement becomes effective;
(2) of any stop order or notification from the SEC or any other jurisdiction as
to the suspension of the effectiveness of the Registration Statement; and (3) of
the end of any suspension hereunder.


                                       7.
<PAGE>   8

                    (vii) With a view to making available to Holder the benefits
of Rule 144 and any other rule or regulation of the SEC that may at any time
permit Holder to sell the Warrant Shares to the public without registration or
pursuant to registration, the Company covenants and agrees to: (1) 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 date hereof or (B)
such date as all of the Warrant Shares shall have been resold; (2) file with the
SEC in a timely manner all reports and other documents required of the Company
under the Exchange Act and maintain registration of its Common Stock under
Section 12 of the Exchange Act until the earlier of (A) the second anniversary
of the date hereof or (B) such date as all of the Warrant Shares shall have been
resold; and (3) furnish to Holder upon request, as long as Holder owns any
Warrant 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 Holder of any rule or regulation of
the SEC that permits the selling of any such Warrant Shares without
registration.

                (b) INDEMNIFICATION AND CONTRIBUTION.

                    (i) The Company agrees to indemnify Holder and hold Holder
harmless from and against any losses, claims, damages or liabilities (or actions
or proceedings in respect thereof) to which Holder may become subject (under the
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, (1) 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, (2) 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 (3) any failure by
the Company (or its agents) to fulfill any undertaking included in the
Registration Statement, and the Company will, as incurred, reimburse Holder 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 (1) 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 Holder specifically for use in preparation of the
Registration Statement, or (2) any untrue statement (or omission) in any
Prospectus that is corrected in any subsequent Prospectus that was delivered to
Holder by the Company prior to the pertinent sale or sales by Holder. The
Company will reimburse Holder 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 (1) to the extent any such payment is ultimately held to be improper, the
persons receiving such payments shall promptly refund them and (2) such persons
shall provide to the Company, upon request, reasonable assurances of their
ability to effect any refund, when and if due.

                    (ii) Holder agrees to indemnify and hold harmless the
Company from and against any losses, claims, damages or liabilities (or actions
or proceedings in respect


                                       8.
<PAGE>   9

thereof) to which the Company may become subject (under the 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 (1) 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 Holder
specifically for use in preparation of the Registration Statement, provided,
however, that Holder shall not be liable in any such case for any untrue
statement included in any Prospectus which statement has been corrected, in
writing, by Holder and delivered to the Company before the sale from which such
loss occurred and the Company has had the opportunity to amend or supplement the
prospectus prior to such sale, or (2) any untrue statement in any Prospectus
that is corrected in any subsequent Prospectus that was delivered to the Holder
prior to the pertinent sale or sales by Holder, provided, further, however, that
the liability of Holder hereunder shall be limited to the proceeds received by
Holder from the sale of the Warrant Shares covered by such Registration
Statement; and provided, further, however, that the obligations of Holder
hereunder shall not apply to amounts paid in settlement of any such loss, claim,
damage, liability, or action if such settlement is effected without the consent
of Holder. Holder 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 up to the limits set forth herein 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 Holder, upon
request, reasonable assurances of their ability to effect any refund, when and
if due.

                    (iii) 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 10(b), 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 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.

                    (iv) If the indemnification provided for in this Section
10(b) is unavailable to or insufficient to hold harmless an indemnified party
under paragraph (i) or (ii) 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


                                       9.
<PAGE>   10

relative fault of the Company on the one hand and the Holder 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
Holder 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 Holder agree that it would not be just and equitable if
contribution pursuant to this paragraph (iv) 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 paragraph (iv). 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 paragraph (iv) 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 paragraph (d), Holder shall not be required to contribute any
amount in excess of the amount by which the amount received by Holder (net of
Selling Expenses) from the sale of the Warrant Shares to which such loss relates
exceeds the amount of any damages which Holder 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 Act) shall be entitled to contribution from any person
who was not guilty of such fraudulent misrepresentation.

                    (v) The obligations of the Company and Holder under this
Section 10(b) shall be in addition to any liability which the Company and Holder
may otherwise have and shall extend, upon the same terms and conditions, to each
person, if any, who controls the Company or Holder within the meaning of the
Act.

         SECTION 11. EXPIRATION DATE. This Warrant shall expire and shall be
wholly void and have no effect after 5:00 p.m. (San Francisco time) on the date
(the "Expiration Date") which is the earlier of (a) the fifth anniversary of the
date hereof, or (b) the closing date of a merger or consolidation of the Company
with or into any other entity, including a reverse triangular merger involving
the Company (other than a merger or consolidation in which the Holders of the
voting power of the Company immediately prior to such consolidation or merger
hold a majority of the surviving or resulting entity immediately following such
consolidation or merger), provided that, if the last day on which this Warrant
may be exercised, or on which it may be exercised at a particular Exercise
Price, is a Sunday or a legal holiday or a day on which banking institutions
doing business in the city of San Francisco are authorized by law to close, this
Warrant may be exercised prior to 5:00 p.m. (San Francisco time) on the next
succeeding full business day with the same force and effect and at the same
Exercise Price as if exercised on such last day specified herein.

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


                                      10.
<PAGE>   11

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

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

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

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

         IN WITNESS WHEREOF, the Company has duly caused this Warrant to be
signed by its duly authorized officer and to be dated as of October 13, 2000.

                                       AVIRON

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






                                      11.
<PAGE>   12

                                SUBSCRIPTION FORM

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

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


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




Dated:_______________________________________________

Signature:____________________________________________________

By:___________________________________________________________





<PAGE>   13

                                 ASSIGNMENT FORM

                                                           Dated _________, 20__


         FOR VALUE RECEIVED, _____________________________________ hereby sells,
assigns and transfers unto ________________________________________________ (the
"Assignee"),____________________________________________________________________
                        (please type or print in block letters)

________________________________________________________________________________
                                (insert address)

its right to purchase up to ________ shares of Common Stock of AVIRON
represented by this Warrant and does hereby irrevocably constitute and appoint
____________________________ Attorney, to transfer the same on the books of the
Company, with full power of substitution in the premises.


         Signature__________________________________