X0306 4 2021-08-17 0 0001701108 Spero Therapeutics, Inc. SPRO 0001030575 FORMELA JEAN FRANCOIS C/O SPERO THERAPEUTICS, INC. 675 MASSACHUSETTS AVENUE, 14TH FLOOR CAMBRIDGE MA 02139 1 0 0 0 Stock Option (right to buy) 13.69 2021-08-17 4 A 0 7500 0.00 A 2031-08-17 Common Stock 7500 7500 D The shares underlying this option will vest and become exercisable in full on August 17, 2022 subject to the Reporting Person's continued service as a director through the vesting date. The reporting person is a member of Atlas Venture Associates IX, LLC and is obligated to transfer the economic benefit, if any, received upon the sale of the shares issuable upon exercise of the equity grants to Atlas Venture Advisors, L.P. As such, the Reporting Person disclaims beneficial ownership to the securities reported herein for purposes of Section 16 of the Securities Exchange Act of 1934, as amended, except to the extent of his pecuniary interest therein, if any. /s/ Matthew J. Gardella, as Attorney-in-Fact for Jean Francois Formela 2021-08-18