Long-term Safety and Efficacy Study of Fitusiran in Patients With Hemophilia A or B, With or Without Inhibitory Antibodies to Factor VIII or IX

Primary Objective: To characterize the long-term safety and tolerability of fitusiran Secondary Objectives: * To characterize the efficacy and long-term efficacy of fitusiran as assessed by the frequency of: * Bleeding episodes * Spontaneous bleeding episodes * Joint bleeding episodes * To characterize the effects of fitusiran on health-related quality of life (HRQOL) measures in participants ≥17 years of age

The estimated total time on the study for a participant is up to 90 months (including screening, treatment period and safety follow up period).

Inclusion criteria : * Participant must be at least 12 years of age inclusive, at the time of signing the informed consent * Participants with severe hemophilia A or B who have completed a Phase 3 fitusiran clinical trial * Male * Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in the protocol. In countries where legal age of majority is above 18 years, a specific ICF must also be signed by the participant's legally authorized representative Exclusion criteria: * Completion of a surgical procedure within 14 days prior to screening, or currently receiving additional factor concentrate or BPA infusion for postoperative hemostasis * Current participation in immune tolerance induction treatment (ITI) * Current use of factor concentrates or bypassing agents (BPAs) as regularly administered prophylaxis designed to prevent spontaneous bleeding episodes except for participants requiring factor concentrates or BPAs prophylaxis during the study dosing pause period * Use of compounds other than factor concentrates or BPAs for hemophilia treatment * Current or prior participation in a gene therapy trial * Alanine aminotransferase (ALT) and/or Aspartate aminotransferase (AST) \>1.5 × upper limit of normal reference range (ULN) for patients who are naïve to fitusiran at study start; ALT and/or AST \> 5 x ULN for patients who were in the fitusiran arm in the parent study * Additional exclusions for participants not currently participating in a fitusiran trial at the time of enrollment in the lower dose cohort: * Clinically significant liver disease * History of arterial or venous thromboembolism The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.