Additional Proxy Soliciting Materials (Form DEFA14A)

Your Vote Counts! Smartphone users Point your camera here and vote without entering a control number For complete information and to vote, visit www.ProxyVote.com Control # V95794-P48504 SIONNA THERAPEUTICS, INC. 21 HICKORY DRIVE, SUITE 500 WALTHAM, MA 02451 SIONNA THERAPEUTICS, INC. 2026 Annual Meeting Vote by June 16, 2026 11:59 PM ET You invested in SIONNA 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 June 17, 2026. Vote Virtually at the Meeting* June 17, 2026 12:00 PM ET If you have registered in advance, you may vote virtually during the meeting at: www.virtualshareholdermeeting.com/SION2026 *Please check the meeting materials for any special requirements for meeting attendance. 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 June 3, 2026. 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 [email protected]. 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. Stockholder Meeting Registration In order to attend the meeting, you must register in advance at www.proxyvote.com by 11:59 p.m. Eastern Time on June 16, 2026.

Vote at www.ProxyVote.com Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click "Delivery Settings". Voting Items Board Recommends V95795-P48504 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 on these important matters. 1. To elect four class II directors to serve until our 2029 annual meeting of stockholders and until his or her successor has been duly elected and qualified, or until his or her earlier death, resignation or removal. For Nominees: 01) H. Edward Fleming, Jr., M.D. 02) Marcella Kuhlman Ruddy, M.D. 03) Peter A. Thompson, M.D. 04) Joanne Louise Viney, Ph.D. 2. To ratify the appointment of Deloitte & Touche LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026. For NOTE: In their discretion, the proxies are authorized to vote upon such other business as may properly come before the meeting or any continuation, adjournment or postponement thereof.