A Study of mRNA-1345, an mRNA Vaccine Targeting Respiratory Syncytial Virus, in Children 2 to <18 Years of Age at High Risk of Respiratory Syncytial Virus

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.