Inclusion Criteria:

• - Healthy young adults ≥18 to ≤49 years of age, healthy older adults ≥65 to <80 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 younger and older adults:

• - Has a body mass index (BMI) from ≥18 kilogram (kg)/meter (m)^2 and ≤35 kg/m^2.

• - Female participants of nonchildbearing potential.

• - Female participants of childbearing 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.

Exclusion Criteria:

• - Has Screening laboratory values Grade ≥1.

• - 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.

• - Receipt of: - Inactivated vaccine(s) within 14 days prior to first injection; or, plans to receive inactivated vaccine(s) within 14 days prior to and through 14 days following each study injection, with the exception of 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.

• - Live virus vaccine(s) within 28 days prior to first injection; or, plans to receive live virus vaccine(s) within 28 days prior to and through 28 days following each study injection.

• - 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 a drug product containing lipid nanoparticles (LNPs) within 14 days before enrollment, with the exception of 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.

Specific exclusion criteria for older adults 65 to 79 years of age:

• - 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:

• - Has received any monoclonal antibody at any time prior to Screening.

• - Prior hospitalization for RSV disease (confirmed by polymerase chain reaction).

• - 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.

Arms

Experimental: Cohort 1: Dose A in Younger Adults

Single injection of Dose A of mRNA-1345 or matching-placebo on Day 1.

Experimental: Cohort 2: Dose B in Younger Adults

Single injection of Dose B of mRNA-1345 or matching-placebo on Day 1.

Experimental: Cohort 3: Dose B in Younger Adults

Three total injections, 1 injection of either Dose B of mRNA-1345 or matching-placebo per day on Day 1, Day 57, and Day 113.

Experimental: Cohort 4: Dose C in Younger Adults

Single injection of Dose C of mRNA-1345 or matching-placebo on Day 1.

Experimental: Cohort 5: Dose D in Children

Three total injections, 1 injection of either Dose D of mRNA-1345 or matching-placebo per day on Day 1, Day 57, and Day 113.

Experimental: Cohort 6: Dose B in Children

Three total injections, 1 injection of either Dose B of mRNA-1345 or matching-placebo per day on Day 1, Day 57, and Day 113.

Experimental: Cohort 7: Dose A in Older Adults

Two total injections, 1 injection of either Dose A of mRNA-1345 or matching-placebo per day on Day 1 and approximately 12 months later

Experimental: Cohort 8: Dose B in Older Adults

Two total injections, 1 injection of either Dose B of mRNA-1345 or matching-placebo per day on Day 1 and approximately 12 months later

Experimental: Cohort 9: Dose C in Older Adults

Two total injections, 1 injection of either Dose C of mRNA-1345 or matching-placebo per day on Day 1 and approximately 12 months later

Interventions

Biological: - mRNA-1345

Formulation for injection

Drug: - Placebo

0.9% sodium chloride (normal saline) injection