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.

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.

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.

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.

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.

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. LLOQ was 10 IU/mL and ULOQ was 112476 for RSV-B.

Influenza A strains included H1N1 and H3N2 and influenza B strains included Austria 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.

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 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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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

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.

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.

Pregnancy outcomes will include stillbirth, live birth, vaginal delivery, and cesarean section delivery.

Birth outcomes will include gestational age and anthropometric measurements.

VE of mRNA-1345 to prevent reverse transcription polymerase chain reaction (RT-PCR) confirmed protocol-defined RSV-LRTD, defined as 100\*(1-HR ratio), where HR is the hazard ratio (mRNA-1345 versus placebo).

VE of mRNA-1345 to prevent reverse transcription polymerase chain reaction (RT-PCR) confirmed protocol-defined RSV-LRTD, defined as 100\*(1-HR ratio), where HR is the hazard ratio (mRNA-1345 versus placebo).