Safety and Immunogenicity of mRNA-1653, a Combined Human Metapneumovirus (hMPV) and Parainfluenza Virus Type 3 (PIV3) Vaccine, in Healthy Adults, and Children 12 to 59 Months of Age With Serologic Evidence of Prior Exposure

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 p…