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-neutralizing Abs at Screening. * Has received routine immunizations appropriate for age per local guidance. * Current height and weight above the third percentile for age. Specific inclusion criteria for Japanese older adults: * Japanese participants are defined as individuals born in Japan, with both parents and 4 grandparents who were born in Japan. Key Exclusion Criteria: * Has Screening laboratory values Grade ≥1 (younger adult, women of child-bearing potential, and pediatric participants) or \>Grade 1 (older adult participants, including Japanese older adult participants). * Is acutely ill or febrile on the day of the first injection. * Has a significant medical history, including but not limited to: * Congenital or acquired immunodeficiency, including human immunodeficiency virus (HIV) infection. * Chronic hepatitis or suspected active hepatitis. * Bleeding disorder that is considered a contraindication to intramuscular (IM) injection or phlebotomy. * Dermatologic conditions that could affect local solicited AR assessments. * Any history of allergic or anaphylactic reactions following a vaccination that required medical intervention. * Autoimmune disease except for Hashimoto's disease. * Systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to the day of enrollment. Topical tacrolimus is allowed if not used within 14 days prior to the day of enrollment. Inhaled, nasal, and topical steroids are allowed. * Intravenous blood products (red cells, platelets, and immunoglobulins \[Ig\]) within 3 months prior to enrollment. * Has received or plans to receive any licensed or authorized vaccine, to include COVID-19 vaccines, ≤ 28 days prior to the first injection (Day 1) or plans to receive a licensed vaccine within 28 days before or after any study vaccine injection, with the exception of licensed influenza vaccines, which may be received more than 14 days before or after any study vaccine injection. Nonstudy vaccinations should not be delayed. * Has a history of myocarditis, pericarditis, or myopericarditis. Specific exclusion criteria for older adults (Cohorts 7-11 and 15): * Known history of poorly controlled hypertension (per determination of the Investigator) or systolic blood pressure \>160 millimeters of mercury (mmHg) at the Screening visit. * Known history of hypotension or systolic blood pressure \<85 mmHg at the Screening visit. * Poorly controlled diabetes mellitus (per determination of the Investigator). * Diagnosis of significant chronic pulmonary disease (per determination of the Investigator) (such as, chronic obstructive pulmonary disease, asthma). * Significant chronic cardiovascular disease (per determination of the Investigator). * Resides in a nursing home. * Anticipates the need for immunosuppressive treatment at any time during participation in the study. * Diagnosis of malignancy within previous 10 years (excluding non-melanoma skin cancer and cervical carcinoma in-situ). Specific exclusion criteria for children 12 to 59 months of age (Cohorts 5 and 6): * Has received any monoclonal antibody at any time prior to Screening. * Prior hospitalization for RSV disease in the last 2 years. * Receipt of any prior systemic immunosuppressants or immune-modifying drugs. * Any history of febrile seizures (inclusive of single simple febrile seizure). * History of epilepsy. * History of meningitis.