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 theInvestigator, 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 withprotocol-mandated follow up, including all procedures, and provide written informedconsent.

• The participant is growing normally for age in the opinion of the site clinician inthe months prior to enrollment.

• The participant was born at full-term (≥37 weeks gestation) with a minimum birthweight of 2.5 kilograms (kg).

• For Part C Cohort 7: participant must have received nirsevimab ≥6 months prior toDay 1 Visit.

• For Part C Cohort 8: participant was eligible at any time since birth, according tonational 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) priorto administration of the first dose of investigational product (IP) or has a knownclose contact with anyone with laboratory-confirmed RSV (Parts A, B, and C) or hMPVinfection (Parts A or B) within 14 days prior to administration of the first dose ofIP.

• Is acutely ill or febrile 24 hours prior to or at the screening visit. Fever isdefined as a body temperature ≥38.0°Celsius/≥100.4°Fahrenheit. Participants who meetthis criterion may have visits rescheduled within the relevant study visit windows.

• Has previously been administered an investigational or approved vaccine forprevention of RSV (Parts A, B, and C) or hMPV (Parts A and B) infection or if theparticipant's mother received an investigational or approved vaccine for theprevention of RSV (Parts A, B, and C) or hMPV (Parts A and B) infection duringpregnancy.

• Has received investigational or approved agents for prophylaxis against RSV or hMPV (for example, monoclonal antibodies) or is intending to receive these during thecourse of the study. For Part C (Cohort 7 only), use of nirsevimab ≥6 months beforeDay 1 Visit is allowed.

• Has a known hypersensitivity to a component of the vaccine or its excipients.Hypersensitivity includes, but is not limited to, anaphylaxis or immediate allergicreaction of any severity to a previous dose of an mRNA vaccine or any of itscomponents (including polyethylene glycol or immediate allergic reaction of anyseverity to polysorbate).

• Has a medical condition that, according to the Investigator's judgment, may poseadditional risk as a result of participation, interfere with safety assessments, orinterfere with interpretation of results.

Note: Other protocol-defined inclusion/exclusion criteria apply.