A Phase 3 Randomized, Observer-Blind, Study to Evaluate Safety, Tolerability, and Immunogenicity of mRNA-1345, an mRNA Vaccine Targeting Respiratory Syncytial Virus (RSV), When Given Alone or Coadministered With a Seasonal Influenza Vaccine or SARS-CoV-2 Vaccine and When Given as an Open-label Boost at 1 Year Following a Primary Dose in Adults ≥ 50 Years of Age

Part A: Number of Participants With Solicited Local and Systemic Adverse Reactions (ARs) Within 7 Days After Day 1 Injection

Time Frame: Within 7 days after Day 1 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: Number of Participants With Unsolicited Adverse Events (AEs) After Day 1 Injection

Time Frame: Day 1 through Day 28 (28 days after Day 1 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: Number of Participants With Medically Attended AEs (MAAEs), SAEs, Adverse Events of Special Interest (AESIs), and AEs Leading to Withdrawal

Time Frame: Day 1 through Day 181 (end of Study Part A)

A MAAE is an AE that leads to an unscheduled visit to a healthcare practitioner. An AESI is 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 A: Geometric Mean Titer (GMT) of Serum Respiratory Syncytial Virus Subtype A (RSV-A) Neutralizing Antibodies (NAbs) at Day 29

Time Frame: Day 29

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) and ULOQ was 259061 IU/mL for RSV-A. As prespecified, only arms where participants received mRNA-1345 are presented for this outcome measure.

Part A: Percentage of Participants With Seroresponse in RSV-A NAbs at Day 29

Time Frame: Day 29

Seroresponse was defined as ≥4 × lower limit of quantification (LLOQ) if baseline was \<LLOQ or 4-fold or greater increase from baseline if baseline was ≥LLOQ. As prespecified, only arms where participants received mRNA-1345 are presented for this outcome measure.

Part A: GMT of Serum Anti-hemagglutination (HA) Ab Level, as Measured by Hemagglutination Inhibition (HAI) Assay for Influenza at Day 29

Time Frame: Day 29

Influenza A strains included H1N1 and H3N2 and influenza B strains included Washington and Phuket strains. Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ. LLOQ was 10 and ULOQ was 2560 for Influenza A. LLOQ was 10 and ULOQ was 640 for Influenza B. As prespecified, only arms where participants received Afluria® Quadrivalent are presented for this outcome measure.

Part B: Number of Participants With Solicited Local and Systemic ARs Within 7 Days After Day 1 Injection

Time Frame: Within 7 days after Day 1 injection

Solicited ARs were collected in an 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 AEs. Investigator reviewed whether the solicited AR was also to be recorded as an AE. A Summary of SAEs and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part B: Number of Participants With Solicited Local and Systemic ARs Within 7 Days After Day 29 Injection

Time Frame: Within 7 days after Day 29 injection

Solicited ARs were collected in an 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 AEs. Investigator reviewed whether the solicited AR was also to be recorded as an AE. A Summary of SAEs and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part B: Number of Participants With Unsolicited AEs After Day 1 Injection

Time Frame: Day 1 through Day 28 (28 days after Day 1 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 PT/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 B: Number of Participants With Unsolicited AEs After Day 29 Injection

Time Frame: Day 29 through Day 57 (28 days after Day 29 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 B: Number of Participants With MAAEs, SAEs, AESIs, and AEs Leading to Withdrawal

Time Frame: Day 1 through Day 211

A MAAE is an AE that leads to an unscheduled visit to a healthcare practitioner. An AESI is 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: GMT of Serum RSV-A at Day 29

Time Frame: Day 29

Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the upper limit of quantification (ULOQ) were replaced by the ULOQ. LLOQ was 13 IU/mL and ULOQ was 259061 IU/mL for RSV-A. As prespecified, only arms where participants received mRNA-1345 are presented for this outcome measure.

Part B: Percentage of Participants With Seroresponse for RSV-A Neutralizing Abs From Baseline to Day 29

Time Frame: Baseline to Day 29

Seroresponse was defined as ≥4 × LLOQ if baseline was \<LLOQ or 4-fold or greater increase from baseline if baseline was ≥LLOQ. As prespecified, only arms where participants received mRNA-1345 are presented for this outcome measure.

Part B: Geometric Mean Concentration (GMC) of Serum Ab Level, as Measured by Neutralization Assay for SARS-Cov-2 at Day 29

Time Frame: Day 29

The model-based GM titer was estimated on ANCOVA model. SARS-CoV-2 variants included Wuhan-Hu-1 and B.1.1.529. Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ. LLOQ was 10 arbitrary units (AU)/milliliters (mL) and ULOQ was 111433 AU/mL for Wuhan-Hu-1. LLOQ was 8 and ULOQ was B.1.1.529 for B1.1.529. As prespecified, only arms where participants received mRNA-1273.214 are presented for this outcome measure.

Part B: Percentage of Participants With Seroresponse for SARS-Cov-2 NAbs From Baseline to Day 29

Time Frame: Baseline to Day 29

Seroresponse was defined as ≥4 × LLOQ if baseline was \<LLOQ or 4-fold or greater increase from baseline if baseline was ≥LLOQ. mRNA-As prespecified, only arms where participants received mRNA-1273.214 are presented for this outcome measure.

Part C: Number of Participants With Solicited Local and Systemic Within 7 Days After Revaccination Day 1

Time Frame: Within 7 days after Day 1 revaccination

Solicited ARs were collected in an electronic 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 AEs. Investigator reviewed whether the solicited AR was also to be recorded as an AE. A Summary of SAEs and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part C: Number of Participants With Unsolicited AEs Within 28 Days After Revaccination Day 1

Time Frame: Revaccination Day 1 through Day 28 (28 days after revaccination Day 1)

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 PT/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 C: Number of Participants With MAAEs

Time Frame: Revaccination Day 1 through Day 181

A MAAE is an AE that leads to an unscheduled visit to a healthcare practitioner.

Part C: Number of Participants With SAEs, AESIs, and AEs Leading to Withdrawal

Time Frame: Revaccination Day 1 through Day 361

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. An AESI is 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. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Part C: GMT of Serum RSV-A and RSV-B NAbs mRNA-1345 Revaccination Day 29 Compared to Primary Vaccination Day 29

Time Frame: Primary Vaccination Day 29 to Revaccination Day 29

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 and ULOQ was 259061 IU/mL for RSV-A. LLOQ was 10 IU/mL and ULOQ was 112476 IU/mL for RSV-B. mRNA-1345 Revaccination Day 29 and mRNA-1345 primary vaccination Day 29 (received in Part B) are presented.