Pfizer disclosed the death of a participant in a long‑term extension trial of Hympavzi (marstacimab), reporting a cerebellar infarction followed by a cerebral hemorrhage. The company communicated the event in a letter to the hemophilia community and said it is reviewing the case with investigators. The report prompted scrutiny from clinicians and regulators and may affect safety monitoring plans, ongoing enrollment and labeling discussions. Pfizer said it will cooperate with investigators and regulators as they assess causality and any implications for trial conduct and patient risk mitigation.