The primary objectives of this Phase 1 study are to evaluate the tolerability and reactogenicity of a single injection of 1 of 3 dose levels of mRNA-1345 in younger adults; of 3 injections of the middle dose level of mRNA-1345 given 56 days apart in younger adults; of a booster injection of mRNA-1345 given approximately 12 months after the primary injection in older adults; and of 3 injections of 1 of 2 dose levels of mRNA-1345 given 56 days apart in children who are RSV-seropositive.