This clinical study will assess the safety and immunogenicity of 2 dose levels of mRNA-1653, a combined human metapneumovirus and human parainfluenza virus type 3 vaccine, in healthy adults (18 to 49 years of age) and 2 dose levels in children (12 to 59 months of age) with serologic evidence of prior exposure. The safety profile of the adult cohort will permit enrollment of the pediatric cohort.

Key Inclusion Criteria:

• Adults 18 to 49 years of age and children 12 to 59 months of age at the time of consent who, in the opinion of the Investigator, are in good health based on review of medical history and screening physical examination
• Adult participant or parent(s)/legal guardian(s) has provided written informed consent for participation in this trial, including all evaluations and procedures as specified by this protocol
• Screening laboratory values Grade ≤1

Specific inclusion criteria for adults 18 to 49 years of age:

• Body mass index (BMI) from ≥18 kg/m^2 and ≤35 kg/m^2
• Female participants must be either of non-childbearing potential or if of childbearing potential may be enrolled if the participant: 1) has a negative pregnancy test at Screening and on the day of vaccination, and 2) has practiced adequate contraception or has abstained from all activities that could lead to pregnancy for 28 days prior to vaccination, and 3) has agreed to continue adequate contraception through 3 months following the last vaccination, and 4) is not currently breastfeeding

Specific inclusion criteria for children 12 to 59 months of age:

• Seropositive for both hMPV and PIV3 neutralizing antibody at Screening
• Has received routine immunizations appropriate for age per the Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP)
• Current height and weight above the third percentile for age

Key Exclusion Criteria:

Adult and pediatric participants eligible for this study must not meet any of the following criteria:

• Acutely ill or febrile (temperature ≥38.0℃/100.4°F, regardless of route) on the day of the first vaccination
• Any ongoing, symptomatic acute or chronic illness requiring medical or surgical care
• Receipt of, or plans for receipt of an inactivated vaccine(s) 14 days prior to though 14 days following each study injection or a live virus vaccine(s) within 28 days prior to, or plans to through 28 days following each study injection. The exception is any COVID-19 vaccine (regardless of type of vaccine) that becomes available to the participant during the study; efforts should be made to space study vaccinations and COVID-19 vaccinations by at least 7 and preferably 14 days, but COVID-19 vaccinations should not be delayed.
• Any chronic administration of an immunosuppressant or other immune modifying drug
• Prior administration of investigational agent using lipid nanoparticle formulations
• Donation of blood or blood products ≥450 mL within 28 days of the Screening visit (for the Adult Cohort)
• Intravenous blood products (red cells, platelets, immunoglobulins) within 3 months prior to enrollment
• Participated in an interventional clinical trial within 28 days prior to the day of enrollment, or plans to do so while enrolled in this trial
• Has a family member or household contact who is an employee of the research center or otherwise involved with the conduct of the study

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