A Study of Fitusiran in Severe Hemophilia A and B Patients Previously Receiving Factor or Bypassing Agent Prophylaxis

Primary Objective: To characterize the frequency of bleeding episodes (BE) while receiving fitusiran treatment, relative to the frequency of bleeding episodes while receiving factor concentrate or bypassing agent (BPA) prophylaxis. Secondary Objectives: * To characterize the following while receiving fitusiran treatment, relative to receiving factor or BPA prophylaxis: * the frequency of spontaneous bleeding episodes * the frequency of joint bleeding episodes * health related quality of life (HRQOL) in participants greater than or equal to (\>=) 17 years of age * To characterize the frequency of bleeding episodes during the onset and treatment periods in participants receiving fitusiran. * To characterize the safety and tolerability of fitusiran. * To characterize the annualized weight-adjusted consumption of factor/BPA while receiving fitusiran treatment, relative to receiving factor or BPA prophylaxis.