Sec Form 4 Filing - RA CAPITAL MANAGEMENT, L.P. @ Icosavax, Inc. - 2022-06-10

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person RA CAPITAL MANAGEMENT, L.P. 2. Issuer Name and Ticker or Trading Symbol Icosavax, Inc. [ ICVX] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 200 BERKELEY STREET, 18TH FLOOR, 3. Date of Earliest Transaction (MM/DD/YY) 06/10/2022 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 06/10/2022 A 5,543( 1 ) A $ 0 5,543 I See footnote( 1 )( 2 )( 3 ) Common Stock 5,324,897 I See footnote( 2 )( 4 ) Common Stock 638,668 I See footnote( 2 )( 5 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 6.25 06/10/2022 A 22,278 ( 6 ) 06/09/2032 Common Stock 22,278 $ 0 22,278 I See footnote( 2 )( 3 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other RA CAPITAL MANAGEMENT, L.P. 200 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X RA Capital Healthcare Fund LP C/O RA CAPITAL MANAGEMENT, L.P. 200 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X RA Capital Nexus Fund II, L.P. C/O RA CAPITAL MANAGEMENT, L.P. 200 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X X Kolchinsky Peter C/O RA CAPITAL MANAGEMENT, L.P. 200 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X X Shah Rajeev M. C/O RA CAPITAL MANAGEMENT, L.P. 200 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X X Signatures /s/ Peter Kolchinsky, Manager of RA Capital Management, L.P. 06/14/2022 Signature of Reporting Person Date /s/ Peter Kolchinsky, Manager of RA Capital Healthcare Fund GP, LLC the General Partner of RA Capital Healthcare Fund, L.P. 06/14/2022 Signature of Reporting Person Date /s/ Peter Kolchinsky, Manager of RA Capital Nexus Fund II GP, LLC the General Partner of RA Capital Nexus Fund II, L.P. 06/14/2022 Signature of Reporting Person Date /s/ Peter Kolchinsky, individually 06/14/2022 Signature of Reporting Person Date /s/ Rajeev Shah, individually 06/14/2022 Signature of Reporting Person Date

Explanation of Responses:

( 1 )These shares represent restricted stock units and will be settled into common stock upon vesting. Each restricted stock unit would be converted into one share of ICVX common stock. These restricted stock units shall vest at the earlier of the first anniversary of the grant date or the date of the next annual meeting of the Company's stock holders, subject to the continued service of Dr. Peter Kolchinsky's service on the Issuer's board of directors through the vesting date. ( 2 )RA Capital Management, L.P. (the "Adviser") is the investment manager for RA Capital Healthcare Fund, L.P. (the "Fund") and RA Capital Nexus Fund II, L.P. (the "Nexus Fund II"). The general partner of the Adviser is RA Capital Management GP, LLC (the "Adviser GP"), of which Dr. Peter Kolchinsky and Mr. Rajeev Shah are the managing members. The Adviser, the Adviser GP, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of any of the reported securities, except to the extent of their pecuniary interest therein. ( 3 )Under Dr. Kolchinsky's arrangement with the Adviser, Dr. Kolchinsky holds the option and restricted stock units for the benefit of the Fund and the Nexus Fund II. Dr. Kolchinsky is obligated to turn over to the Adviser any net cash or stock received upon sale of the common stock underlying the option and restricted stock units, which will offset advisory fees owed by the Fund and the Nexus Fund II to the Adviser. The Reporting Persons therefore disclaim beneficial ownership of the option and restricted stock units and respective underlying common stock except to the extent of their pecuniary interest. ( 4 )Held by the Fund. ( 5 )Held by the Nexus Fund II. ( 6 )The option shall vest in substantially equal monthly installments over the twelve (12) months following the date of grant (or, in the event the next annual meeting of the Company's stockholders occurs prior to the first anniversary of the date of grant, any remaining unvested portion of the option will vest on the date of such annual meeting of the Company's stockholders), subject to Dr. Kolchinsky's continued service through the applicable vesting date.

Remarks: Dr. Kolchinsky, a Managing Partner of the Adviser, serves on the Issuer's board of directors

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

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