X0508 4 2026-02-02 0001701108 Spero Therapeutics, Inc. SPRO 0001903716 Rajavelu Esther C/O SPERO THERAPEUTICS, INC. 675 MASSACHUSETTS AVENUE, 14TH FLOOR CAMBRIDGE MA 02139 false true false false See Remarks false Common Stock 2026-02-02 4 A false 249000 0 A 1118450 D Common Stock 2026-02-02 4 S false 18442 2.20 D 1100008 D Stock Option (Right to Buy) 2.23 2026-02-02 4 A false 498000 0 A 2036-02-02 Common Stock 498000 498000 D Consists of restricted stock units ("RSUs") granted to Reporting Person under Issuer's 2017 Stock Incentive Plan. Each RSU represents the right to receive one share of common stock upon vesting. The RSUs vest in four equal annual installments beginning on February 2, 2027, subject to the Reporting Person's continued service through the applicable vesting date. Represents shares required to be sold by the Reporting Person to cover tax withholding obligations in connection with the vesting of RSUs. The sale occurred automatically to satisfy the tax withholding obligations to be funded by a "sell to cover" transaction and does not represent a discretionary trade by the Reporting Person. The shares underlying this option vest as to 25% on February 2, 2027, with the remainder vesting in 36 equal monthly installments thereafter, subject to the Reporting Person's continued service through the applicable vesting date. Chief Executive Officer, Chief Financial Officer and Chief Business Officer /s/ Maegan Deare, Attorney-in-Fact for Esther Rajavelu 2026-02-04