On Day 1, all eligible participants will be randomized and receive the first subcutaneous (SC) administration of ALN-PCSSC or placebo. After the first study drug administration, the participant will be observed in the clinic for at least 4 hours post injection before being discharged. Participants will return at Day 14 and then at monthly intervals for 6 months. Participants randomized to receive a second dose of study drug will receive the second injection of ALN-PCSSC or placebo at the Day 90 visit.