A Phase 2, Randomized, Observer-blind Study to Evaluate the Safety, Reactogenicity, and Immunogenicity 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 Disease

Part A: Number of Participants with Solicited Local and Systemic Adverse Reactions (ARs)

时间窗: Up to 7 days postinjection

Part A: Number of Participants with Unsolicited Adverse Events (AEs)

时间窗: Up to 28 days postinjection

Part A: Number of Participants With Medically Attended AEs (MAAEs)

时间窗: Day 1 through end of study (EOS; Month 6)

Part A: Number of Participants With Adverse Events of Special Interest (AESIs)

时间窗: Day 1 through EOS (Month 6)

Part A: Number of Participants With Serious Adverse Events (SAEs)

时间窗: Day 1 through EOS (Month 6)

Part A: Number of Participants With AEs Leading to Discontinuation

时间窗: Day 1 through EOS (Month 6)

Part B: Number of participants With Respiratory syncytial virus-Respiratory tract disease (RSV-RTD), Respiratory syncytial virus- Lower Respiratory tract disease (RSV-LRTD), Severe RSV-LRTD, Very Severe RSV-LRTD and RSV Hospitalization

时间窗: Day 1 through EOS (Month 6)

Part A (Cohorts 1 and 2): Number of Participants With Solicited Local and Systemic Adverse Reactions (ARs)

时间窗: Up to 7 days post-injection

Solicited ARs were collected in an electronic diary (eDiary). Local ARs: injection site pain, erythema (redness), swelling/induration (hardness); and axillary (underarm) swelling or tenderness ipsilateral to the side of injection. Systemic ARs: fever, headache, fatigue, myalgia, arthralgia, nausea/vomiting, and chills. Note, not all solicited ARs were considered adverse events (AEs). Investigator reviewed whether the solicited AR was also to be recorded as an AE. A summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part A (Cohorts 1 and 2): Number of Participants With Unsolicited Adverse Events (AEs)

时间窗: Up to 28 days post-injection

An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. Any abnormal laboratory test result (hematology, clinical chemistry, or prothrombin time \[PT\]/partial thromboplastin time \[PTT\]) or other safety assessment (for example, electrocardiogram, radiological scan, vital sign measurement), including one that worsened from baseline and was considered clinically significant in the medical and scientific judgment of the Investigator was recorded as an AE. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part A (Cohorts 1 and 2): Number of Participants With Medically Attended AEs (MAAEs), Adverse Events of Special Interest (AESIs), Serious Adverse Events (SAEs), and AEs Leading to Study Discontinuation

时间窗: Day 1 through end of Part A (Month 6)

A MAAE was an AE that led to an unscheduled visit to a healthcare practitioner. An AESI was an AE (serious or nonserious) of scientific and medical concern specific to the Sponsor's product or program, for which ongoing monitoring and immediate notification by the Investigator to the Sponsor are required. An SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability, was a congenital anomaly/birth defect, or was an important medical event. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part B (Cohort 1): Number of Participants With RSV-RTD, Respiratory Syncytial Virus- Lower Respiratory Tract Disease (RSV-LRTD), Severe RSV-LRTD, Very Severe RSV-LRTD and RSV Hospitalization Classified by Clinical Assessment Team (CAT)

时间窗: Day 1 through end of Part B (Month 6)

RSV-RTD: Runny nose or blocked nose or cough and confirmed RSV infection. RSV-LRTD: Cough or difficulty breathing (Based on Investigator's observation; difficulty breathing included signs of wheezing, stridor, tachypnoea, chest in-drawing or subcostal or intercostal retractions) and peripheral oxygen saturation (SpO2) \<95%, or respiratory rate (RR) increased and confirmed RSV infection. RSV Severe-LRTD: Meeting the definition of RSV-LRTD and SpO2 \<93%, or lower chest wall in-drawing. RSV Very Severe LRTD: Meeting the definition of RSV-LRTD and SpO2 \<90%, or failure to respond/unconscious. RSV Hospitalization: Confirmed RSV and hospitalized for acute medical condition.