A Clinical Trial of Study Medicine (Marstacimab) in Pediatric Patients With Hemophilia A or Hemophilia B

The purpose of this clinical trial is to learn about the safety and effects of the study medicine (called marstacimab) for the potential treatment of hemophilia in pediatric patients. This study will enroll pediatric participants from ages 1 to 17 years in a sequential manner. The study opened the enrollment to adolescent participants aged 12 to 17 years first. Then children aged 6 to 11 years erere permitted to enroll. Lastly, children aged 1 to 5 years are permitted to enroll. This study will enroll male participants who: * have severe Hemophilia A or moderately severe to severe Hemophilia B (with or without inhibitors) * have accurate historical records documenting all factor VIII, factor IX, or bypass agent infusions and hemophilia bleed events for at least 1 year prior to entering the study for participants from 6 to 17 years of age, and at least 3 months prior to entering the study for participants from 1 to 5 years of age * if a non-inhibitor patient, must be on a stable routine prophylaxis regimen with factor VIII or factor IX replacement products for at least 12 months prior to study entry for participants from 6 to 17 years of age, and least 3 months for participants aged from 1 to 5 years of age * if an inhibitor patient, must be on an on-demand bypass treatment regimen during the 12 months prior to study entry for participants from 6 to 17 years of age, and during the 3 months prior to study entry for participants aged from 1 to 5 years of age All participants in this study will receive marstacimab to use prophylactically. Marstacimab will be given once a week as a subcutaneous (under the skin) shot. The first dose of marstacimab will be given at the study site by the study site staff. During the 12-month treatment period, weekly doses of marstacimab can be given at home, or if preferred, the doses may be given by the study site staff. To help us determine if the study medicine is safe and effective, we will compare participant experiences when they are taking the study medicine to a historical period when they were not. Researchers want to see if the study medicine works to prevent the bleeding episodes commonly experienced by patients with Hemophilia. Participants will be in this study for about 14 months (approximately 1 month in a Screening period, 12 months receiving treatment, and 1 month in a follow-up period) during which they will visit the study site at least 10 times. If preferred, and if local regulations allow it, 2 of the study visits can be completed at the participant's home instead of at the study site. There will also be 6 scheduled telephone calls approximately every 2 months.