A Safety, Tolerability, and Immunogenicity Study of mRNA-1345 and mRNA-1365 in Participants Aged 5 Months to <24 Months

Inclusion Criteria: * The participant is 8 months to \<24 months (Part A), 5 months to \<8 months (Part B), or 8 months to \<12 months (Part C) of age at the time of randomization (Day 1/Baseline visit), who is in good general health, in the opinion of the Investigator, based on review of medical history and screening physical examination. * In the Investigator's opinion, the parent(s)/ legally authorized representative (LAR)(s) understand and are willing and physically able to comply with protocol-mandated follow up, including all procedures, and provide written informed consent. * The participant is growing normally for age in the opinion of the site clinician in the months prior to enrollment. * The participant was born at full-term (≥37 weeks gestation) with a minimum birth weight of 2.5 kilograms (kg). * For Part C Cohort 7: participant must have received nirsevimab ≥6 months prior to Day 1 Visit. * For Part C Cohort 8: participant was eligible at any time since birth, according to national guidelines, to receive nirsevimab prior to Day 1 Visit but did not do so. Exclusion Criteria: * Has a known history of symptomatic RSV (Part A: within 3 months; Part B and Part C: since birth) or hMPV infection (Part A: within 3 months; Part B: since birth) prior to administration of the first dose of investigational product (IP) or has a known close contact with anyone with laboratory-confirmed RSV (Parts A, B, and C) or hMPV infection (Parts A or B) within 14 days prior to admini…