Pfizer informed investigators and regulators that a participant in a Hympavzi hemophilia trial died after a cerebellar infarction followed by a cerebral hemorrhage. The company disclosed the fatality in a letter to hemophilia stakeholders and is cooperating with safety reviews and regulatory inquiries. Pfizer emphasized ongoing monitoring and investigational‑drug safety procedures while continuing trial activities under oversight. The disclosure will prompt careful evaluation of causality, trial protocol safeguards, and potential impacts on ongoing enrollment and development timelines for Hympavzi. Sponsors and investigators will track regulatory feedback and any recommended amendments to monitoring, inclusion criteria, or enrollment to mitigate risk and preserve participant safety across hemophilia gene‑therapy and bleeding‑disorder programs.