Study of a Respiratory Syncytial Virus Candidate Encapsulated in a Lipid Nanoparticle Based Formulation in Adults Aged 18 to 50 Years and 60 Years and Older

Stage 1: The duration of each participant's participation was 12 months for the Sentinel and Main Cohorts, 24 months overall for the subset of participants enrolled in the Booster Cohort. Treatment Duration: * Sentinel Cohort: 1 intra-muscular (IM) injection. Participants were followed for 12 months post-vaccination. * Main Cohort: 1 IM injection. Participants were followed for 12 months post-vaccination. * Booster Cohort: 1 IM injection 12 months after the primary vaccination. Participants were followed for 12 months after administration of the booster dose. Stage 2: The duration of each participant's participation was approximately 6 months. Treatment Duration: 1 IM injection. Participants were followed for approximately 6 months post vaccination