The purpose of this study is to evaluate the efficacy and safety of vutrisiran (ALN-TTRSC02) in participants with hereditary transthyretin amyloidosis (hATTR amyloidosis). Participants will receive vutrisiran subcutaneous (SC) injection once every 3 months (q3M) or the reference comparator patisiran intravenous (IV) injection once every 3 weeks (q3w) during the 18 month Treatment Period. This study will use the placebo arm of the APOLLO study (NCT01960348) as an external comparator for the primary and most other efficacy endpoints during the 18 Month Treatment Period. Following the 18 Month Treatment Period, all participants will be randomized to receive vutrisiran 50 mg SC injection once every 6 months (q6M) or vutrisiran 25 mg q3M in the Randomized Treatment Extension (RTE) Period. Upon implementation of Amendment 6, participants receiving vutrisiran SC 50 mg q6M will transition to vutrisiran SC 25 mg q3M at their next scheduled dosing.