Study of a Respiratory Syncytial Virus/Human Metapneumovirus Vaccine Candidate

Study duration per participant: 12 months for the Sentinel and Main Cohorts and 24 months overall for the subset of participants enrolled in the Booster Cohort. Treatment Duration: Sentinel Cohort: 1 IM injection. Participants will be followed for 12 months post-vaccination. Main Cohort: 1 IM injection. Participants will be followed for 12 months post-vaccination. Booster Cohort: 1 IM injection 12 months after the primary vaccination (Main Cohort). Participants will be followed for 12 months after administration of the booster dose.