The study will be conducted in 2 phases: Phase 2 and Phase 3. In the Phase 2 segment, up to 2,000 participants will be randomly assigned to receive a single injection of either mRNA-1345 vaccine at the selected dose or placebo in a 1:1 randomization ratio.~In the Phase 3 segment, approximately 35,000 participants will be randomly assigned to receive a single injection of either mRNA-1345 vaccine at the selected dose or placebo in a 1:1 randomization ratio.
The main purpose of this study is to evaluate the safety and tolerability of mRNA-1345 vaccine and to demonstrate the efficacy of a single dose of mRNA-1345 vaccine in the prevention of a first episode of RSV-associated lower respiratory tract disease (RSV-LRTD) as compared with placebo from 14 days postinjection through 12 months.