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 is 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 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. Participants will be followed for 12 months after administration of the booster dose. Stage 2: The duration of each participant's participation is approximately 6 months. Treatment Duration: 1 IM injection. Participants will be followed for approximately 6 months post vaccination