A Study of Fitusiran (ALN-AT3SC) in Severe Hemophilia A and B Patients Without Inhibitors

Primary Objective: -To evaluate the efficacy of fitusiran compared to on-demand treatment with factor concentrates, as determined by the frequency of bleeding episodes. Secondary Objectives: * To evaluate the efficacy of fitusiran compared to on-demand treatment with factor concentrates, as determined by: * The frequency of spontaneous bleeding episodes. * The frequency of joint bleeding episodes. * Health-related quality of life (HRQOL) in participants \>=17 years of age. * To determine the frequency of bleeding episodes during the onset period. * To determine the safety and tolerability of fitusiran.