A Dose Escalation Study to Evaluate Safety, Reactogenicity, and Immunogenicity of mRNA-1345 in Healthy Adults and in Children Who Are Respiratory Syncytial Virus (RSV)-Seropositive

Key Inclusion Criteria: * Healthy young adults ≥18 to ≤49 years of age, women of child-bearing potential ≥18 to ≤40 years of age, healthy older adults ≥65 to \<80 years of age, Japanese older adults ≥ 60 years of age, and children ≥12 to \< 60 months of age. * Willing and physically able to comply with protocol-mandated follow-up, including all procedures. * Adult participant or parent(s)/legal guardian(s) of pediatric participants has provided written informed consent for participation in this study, including all evaluations and procedures as specified by the protocol. Specific inclusion criteria for adults (younger adults, women of child-bearing potential, and older adults \[including Japanese older adults\]): * Has a body mass index (BMI) from ≥18 kilogram (kg)/meter (m)\^2 to ≤35 kg/m\^2. * Female participants of non-child-bearing potential. This criterion does not apply for women of child-bearing potential Cohorts 12, 13, and 14. * Female participants of child-bearing potential may be enrolled in the study, if the participant: 1) has a negative urine pregnancy test at Screening and on the day of vaccination; 2) has practiced adequate contraception or has abstained from all activities that could lead to pregnancy for 28 days prior to vaccination; 3) has agreed to continue adequate contraception through 3 months following the last injection; and 4) is not currently breastfeeding. Specific inclusion criteria for children 12 to 59 months of age: * Seropositive for RSV-…