A Study to Test a Medicine (Fitusiran) for Preventing Bleeds in People With Severe Hemophilia Who Previously Received Preventive Treatment With Emicizumab

Inclusion Criteria: * Male participants must be ≥12 years of age inclusive, at the time of signing the informed consent * Diagnosis of severe congenital hemophilia A (FVIII \< 1%) as evidenced by a central laboratory measurement at screening or documented medical record evidence. * Inhibitor titer of ≥0.6 BU/mL at Screening, or * Inhibitor titer of \<0.6 BU/mL at Screening with medical record evidence of 2 consecutive titers ≥0.6 BU/mL, or * Inhibitor titer of \<0.6 BU/mL at Screening with medical record evidence of anamnestic response. * Participants who are currently on the full labeled dose of emicizumab prophylaxis, irrespective of inhibitor/non-inhibitor status. * Signed informed consent/assent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: * Known coexisting bleeding disorders * History of antiphospholipid antibody syndrome. * History of arterial or venous thromboembolism, atrial fibrillation, significant valvular disease, myocardial infarction, angina, transient ischemic attack, or stroke. Participants who have experienced thrombosis associated with indwelling venous access may be enrolled. * Presence of clinically significant liver disease * Current or prior participation in a fitusiran trial * Current or prior participation in a gene therapy trial * AT activity \<60% at Screen…