New mRNA vaccine aims to shield toddlers from two winter viruses

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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 administration of the first dose of IP. * 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 (Parts A, B, and C) or hMPV (Parts A and B) infection or if the participant's mother received an investigational or approved vaccine for the prevention of RSV (Parts A, B, and C) or hMPV (Parts A and B) infection during pregnancy. * Has received investigational or approved agents for prophylaxis against RSV or hMPV (for example, monoclonal antibodies) or is intending to receive these during the course of the study. For Part C (Cohort 7 only), use of nirsevimab ≥6 months before Day 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 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. Inclusion Criteria: * For Part B Extension: participants enrolled and dosed in Part B (Cohorts 3 to 6); either reached the end of study of the Main Study or were dosed and subsequently discontinued from the Main Study. This includes participants who were lost to follow-up, if they can be re-engaged. * For Part B Extension: participant's parent(s)/LAR(s) has provided written informed consent for participation in Part B of the Main Study and the Part B Extension. Exclusion Criteria: • For Part B Extension, there are no specific exclusion criteria.