UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. )

Filed by the Registrant ☒        Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐	Preliminary Proxy Statement
☐	Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))
☐	Definitive Proxy Statement
☒	Definitive Additional Materials
☐	Soliciting Material Under §240.14a-12

Spero Therapeutics, Inc.

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box all boxes that apply):

☒	No fee required.
☐	Fee paid previously with preliminary materials.
☐	Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-1.

You invested in SPERO THERAPEUTICS, INC. and it’s time to vote!

You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on May 29, 2024.

Get informed before you vote

View the Notice and Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 15, 2024. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy.

*Please check the meeting materials for any special requirements for meeting attendance.

Vote at www.ProxyVote.com

THIS IS NOT A VOTABLE BALLOT

This is an overview of the proposals being presented at the

upcoming stockholder meeting. Please follow the instructions on

the reverse side to vote these important matters.

Voting Items		Board Recommends
1.	To elect three Class I directors to serve until the Company’s 2027 Annual Meeting of Stockholders or until their respective successors are elected and qualified.
	Nominees:
1a.	Scott Jackson		For
1b.	John C. Pottage, Jr., M.D.		For
1c.	Cynthia Smith		For
2.	To ratify the appointment of PricewaterhouseCoopers LLP as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2024.		For
3.	To approve by an advisory vote the compensation of the Company’s named executive officers, as disclosed in the proxy statement.		For
4.	To approve an amendment to the Company’s 2017 Stock Incentive Plan, as amended, to increase the total number of shares of common stock authorized for issuance thereunder by 3,000,000 shares.		For
NOTE: To transact such other business that is properly presented at the annual meeting and any adjournments or postponements thereof.

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