Part A: The purpose is to evaluate the safety, reactogenicity, and immunogenicity of mRNA-1345 in children aged 2 to <5 years (Cohort 1) and in children at high risk of respiratory syncytial virus (RSV) disease 5 to <18 years of age (Cohort 2) to inform the dose level selection for the next phase of development (Phase 3).

Part B: The purpose is to provide surveillance for RSV disease for the next RSV season (6 months after re-enrollment) and safety follow-up for Cohort 1 participants that were enrolled and dosed in Part A.

Key Inclusion Criteria:

Part A Cohort 1:

• 2 to <5 years of age at Day 1.
• Healthy, or with stable chronic conditions increasing the risk of RSV disease, per the clinical judgment of the Investigator.

Cohort 2:

• 5 to <18 years of age at Day 1.
• Participants with stable chronic conditions increasing the risk of RSV disease.
• 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 injection (Day 1); 2) has practiced adequate contraception or has abstained from all activities that could lead to pregnancy for 28 days prior to Day 1; 3) has agreed to continue adequate contraception through 90 days following injection; and 4) is not currently breastfeeding.

Part B: Cohort 1 Re-enrollment

Participants are eligible to be included in the study only if all of the following criteria apply:

1. Enrolled and dosed in Part A of Cohort 1; either reached EoS for Part A or were dosed and subsequently discontinued from study for various reasons. This includes participants who were lost to follow-up, if they can be re-engaged.
2. Participant's parent(s)/LAR(s) has provided written informed consent for participation in this study.

Key Exclusion Criteria (All Cohorts):

• Acutely ill or febrile (temperature ≥38.0°Celsius [100.4°Fahrenheit]) within 72 hours prior to or at the Screening Visit or Day 1.
• History of a diagnosis or condition that, in the judgment of the Investigator, may affect study assessment or compromise participant safety.
• Has received or plans to receive any licensed or authorized vaccine ≤14 days prior to the study vaccine injection (Day 1) or plans to receive a licensed or authorized vaccine within 14 days after the study vaccine injection.
• Receipt of any prior systemic immunosuppressants. Short courses (<7 days) of oral corticosteroids are allowed if completed at least 3 months prior to enrollment.
• Receipt of RSV monoclonal antibodies within 6 months prior to enrollment in the study.
• Participated in an interventional clinical study within 28 days (6 months for a study assessing a product unlicensed/unauthorized in this age group in country of residence at time of enrollment) prior to the day of enrollment or plans to do so while enrolled in this study.

Part B: Cohort 1 Re-enrollment

1. Participant is currently enrolled in another interventional clinical study.

Note: Other protocol-defined inclusion and exclusion criteria may apply.