Proxy Results (Form 8-K)

Item 5.07 Submission of Matters to a Vote of Security Holders.

Upstream Bio, Inc. (the "Company") held its 2026 Annual Meeting of Stockholders (the "Annual Meeting") on June 9, 2026. As of the close of business on April 13, 2026, the record date for the Annual Meeting, there were 54,419,986 shares of the Company's common stock outstanding and entitled to vote at the Annual Meeting, of which 48,100,767 shares were present virtually or represented by proxy at the Annual Meeting, constituting a quorum for the transaction of business at the Annual Meeting. The Company's stockholders voted on the following matters, which are described in detail in the Company's Definitive Proxy Statement filed with the U.S. Securities and Exchange Commission on April 16, 2026: (i) to elect each of H. Edward Fleming, Jr., M.D. and Liam Ratcliffe, M.B.Ch.B., Ph.D., M.B.A. as Class II directors of the Company, each to serve until the Company's 2029 annual meeting of stockholders and until his respective successor has been duly elected and qualified, or until his earlier death, resignation or removal ("Proposal 1"), and (ii) to ratify the appointment of PricewaterhouseCoopers LLP ("PwC") as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2026 ("Proposal 2"). The voting results reported below are final.

Proposal 1 - Election of Class II Director Nominees

The Company's stockholders approved the Class II director nominees recommended for election in Proposal 1 at the Annual Meeting. The results of Proposal 1 were as follows:

Proposal 2 - Ratification of Appointment of the Company's Independent Registered Public Accounting Firm

The Company's stockholders ratified the appointment of PwC as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2026. The results of Proposal 2 were as follows:

No other matters were submitted to or voted on by the Company's stockholders at the Annual Meeting.