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

The purpose of this study is to assess the safety and immunogenicity of mRNA-1365, an mRNA vaccine targeting respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) and mRNA-1345, an mRNA vaccine targeting RSV, in participants aged 5 months to <24 months.

Inclusion Criteria:The participant is 8 months to <24 months (Part A) or 5 months to <8 months (Part B) 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 kg.Exclusion Criteria:Has a known history of symptomatic RSV 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 or hMPV infection within 14 days prior to administration of the first dose of IP (Part A and Part B).Is acutely ill or febrile 24 hours prior to or at the screening visit. Fever is defined as a body temperature ≥38.0°Celsius/≥100.4°Fahrenheit. Participants who meet this criterion may have visits rescheduled within the relevant study visit windows.Has previously been administered an investigational or approved vaccine for prevention of RSV or hMPV infection or if the participant's mother received an investigational or approved vaccine for the prevention of RSV or hMPV infection during pregnancy.Has undergone treatment with investigational or approved agents for prophylaxis against RSV or hMPV (for example, receipt of monoclonal antibodies).Has a known hypersensitivity to a component of the vaccine or its excipients. Hypersensitivity includes, but is not limited to, anaphylaxis or immediate allergic reaction of any severity to a previous dose of an mRNA vaccine or any of its components (including polyethylene glycol or immediate allergic reaction of any severity to polysorbate).Has a medical condition that, according to the Investigator's judgment, may pose additional risk as a result of participation, interfere with safety assessments, or interfere with interpretation of results.Note: Other protocol-defined inclusion/exclusion criteria apply.