Pfizer disclosed the death of a participant in a long‑term extension trial of Hympavzi (marstacimab), reporting a cerebellar infarction followed by cerebral hemorrhage. The company communicated the event in a letter to the hemophilia community and regulators; the incident is under investigation and has raised concerns in the bleeding disorders field about safety signals associated with the tissue factor pathway inhibitor antagonist class. Clinical trial monitors and sponsors typically review such events for causality and trial conduct implications; regulators may request additional safety data or restrict enrollment depending on findings. Pfizer’s public disclosure aims to inform clinicians and patients while the company completes its internal review. Stakeholders will watch for formal safety updates, any protocol amendments, and potential impacts on regulatory filings and ongoing studies.