Trial Outcomes & Findings for 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 (NCT NCT06097299)

This study consisted of 2 parts (Parts A and B). Part A consisted of 2 cohorts. For Part B, all Cohort 1 participants who were enrolled and dosed in Part A, were offered to re-enroll into the safety follow-up study. Participants who were in Part B were not administered any study drug.

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

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.

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.

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.

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.

Antibody values reported as below lower limit of quantification (LLOQ) were replaced by 0.5\*LLOQ. Values greater than the upper limit of quantification (ULOQ) were replaced by the ULOQ. LLOQ was 13 international units (IU)/milliliter (mL) for RSV-A and 10 IU/mL for RSV-B. ULOQ was 259061 IU/mL for RSV-A and 112476 IU/mL for RSV-B. 95% confidence interval (CI) for geometric mean (GM) value was calculated based on the t-distribution of the log-transformed values, then back transformed to the original scale for presentation.

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than ULOQ were replaced by the ULOQ. LLOQ was 35 arbitrary units (AU)/mL and ULOQ was 580553 AU/mL for RSV Pre-F immunoglobulin G (IgG) antibody. 95% CI for GM value was calculated based on the t-distribution of the log-transformed values, then back transformed to the original scale for presentation.

Antibody values reported as below lower LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ. LLOQ was 13 IU/mL for RSV-A and 10 IU/mL for RSV-B. ULOQ was 259061 IU/mL for RSV-A and 112476 IU/mL for RSV-B. 95% CI for GMFR (post-injection/baseline titers) was calculated based on the t-distribution of the differences in the log-transformed values between analysis timepoint and baseline, then back transformed to the original scale for presentation.

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than ULOQ were replaced by the ULOQ. LLOQ was 35 AU/mL and ULOQ was 580553 AU/mL for RSV Pre-F IgG antibody. 95% CI for GMFR (post-injection/baseline titers) was calculated based on the t-distribution of the differences in the log-transformed values between analysis timepoint and baseline, then back transformed to the original scale for presentation.

Seroresponse was defined as a change from below the LLOQ to equal or above 4 \* LLOQ, or at least a 4-fold increase if baseline was equal to or above the LLOQ.

Seroresponse was defined as a change from below the LLOQ to equal or above 4 \* LLOQ, or at least a 4-fold increase if baseline was equal to or above the LLOQ.

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ. LLOQ was 13 IU/mL for RSV-A and 15 IU/mL for RSV-B. ULOQ was 259061 IU/mL for RSV-A and 162163 IU/mL for RSV-B. 95% CI for GM value was calculated based on the t-distribution of the log-transformed values, then back transformed to the original scale for presentation.

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than ULOQ were replaced by the ULOQ. LLOQ was 35 AU/mL and ULOQ was 580553 AU/mL for RSV Pre-F IgG antibody. 95% CI for GM value was calculated based on the t-distribution of the log-transformed values, then back transformed to the original scale for presentation.

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ. LLOQ was 13 IU/mL for RSV-A and 15 IU/mL for RSV-B. ULOQ was 259061 IU/mL for RSV-A and 162163 IU/mL for RSV-B. 95% CI for GMFR (post-injection/baseline titers) was calculated based on the t-distribution of the differences in the log-transformed values between analysis timepoint and baseline, then back transformed to the original scale for presentation.

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than ULOQ were replaced by the ULOQ. LLOQ was 35 AU/mL and ULOQ was 580553 AU/mL for RSV Pre-F IgG antibody. 95% CI for GMFR (post-injection/baseline titers) was calculated based on the t-distribution of the differences in the log-transformed values between analysis timepoint and baseline, then back transformed to the original scale for presentation.

Seroresponse was defined as a change from below the LLOQ to equal or above 4 \* LLOQ, or at least a 4-fold increase if baseline was equal to or above the LLOQ.

Seroresponse was defined as a change from below the LLOQ to equal or above 4 \* LLOQ, or at least a 4-fold increase if baseline was equal to or above the LLOQ.

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.