A Study of mRNA-1283 Injection Compared With mRNA-1273 Injection in Participants ≥12 Years of Age to Prevent COVID-19

Part 1: Geometric Mean (GM) of Omicron BA.4/5 at Day 29

Timeframe: Day 29

Antibody values reported as below the 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 if actual values were not available. LLOQ was 103 arbitrary units (AU)/milliliter (mL) and ULOQ was 28571 AU/mL.

Part 1: Seroresponse Rate (SRR) Omicron BA.4/5 at Day 29

Timeframe: Day 29

Seroresponse was defined as an antibody value change from baseline below the LLOQ to ≥4\*LLOQ, or at least a 4-fold rise if baseline was ≥LLOQ and \<4\*LLOQ, or at least a 2-fold rise if baseline was ≥4\*LLOQ, where baseline referred to pre-booster. LLOQ was 103 AU/mL and ULOQ was 28571 AU/mL.

Part 1: GM of the Ancestral Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) D614G at Day 29

Timeframe: Day 29

Antibody values reported as below the LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL.

Part 1: SRR of Ancestral SARS-CoV-2 D641G at Day 29

Timeframe: Day 29

Seroresponse was defined as an antibody value change from baseline below the LLOQ to ≥4\*LLOQ, or at least a 4-fold rise if baseline was ≥LLOQ and \<4\*LLOQ, or at least a 2-fold rise if baseline was ≥4\*LLOQ, where baseline referred to pre-booster. LLOQ was 10 AU/mL and ULOQ was 111433AU/mL.

Part 1: Number of Participants With First Event of Centers for Disease Control and Prevention (CDC)-Defined COVID-19

Timeframe: From 14 days after injection up to Day 365

CDC COVID-19 definition: the presence of at least 1 CDC listed symptom and positive reverse transcriptase polymerase chain reaction (RT-PCR) test on a respiratory sample.

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

Timeframe: Up to Day 7 (7-day follow-up after vaccination)

Solicited ARs were recorded daily using electronic diaries (eDiaries). 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 3: Number of Participants With Solicited Local and Systemic ARs

Timeframe: Up to Day 7 (7-day follow-up after vaccination)

Solicited ARs were recorded daily using eDiaries. 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 1: Number of Participants With Unsolicited Adverse Events (AEs)

Timeframe: Up to Day 28 (28-day follow-up after vaccination)

An unsolicited AE was defined as any AE reported by the participant that was not specified as a solicited AR in the protocol. An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Part 3: Number of Participants With Unsolicited AEs

Timeframe: Up to Day 28 (28-day follow-up after vaccination)

An unsolicited AE was defined as any AE reported by the participant that was not specified as a solicited AR in the protocol. An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Part 1: Number of Participants With Any Serious AEs (SAEs), Medically Attended AEs (MAAEs), AEs Leading to Withdrawal From Study, and AEs of Special Interest (AESIs)

Timeframe: Day 1 up to Day 365

SAEs were AEs that resulted in death, were life threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, a congenital anomaly or birth defect, or was a medically important event. MAAEs were AEs that lead to an unscheduled visit to a healthcare provider. AESIs were AEs (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 was required. AEs leading to the withdrawal from study reported for this endpoint also include the AEs that led to death. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Part 3: Number of Participants With Any SAEs, MAAEs, AEs Leading to Withdrawal From Study, and AESIs

Timeframe: Day 1 up to Day 181

SAEs were AEs that resulted in death, were life threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, a congenital anomaly or birth defect, or was a medically important event. MAAEs were AEs that lead to an unscheduled visit to a healthcare provider. AESIs were AEs (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 was required. AEs leading to the withdrawal from study reported for this endpoint also include the AEs that led to death. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Part 1: GMs of Omicron BA.4/5 and Ancestral SARS-CoV-2 D614G

Timeframe: Days 91, 181, and 365

Antibody values reported as below the LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ if actual values were not available. LLOQ was 103 AU/mL and ULOQ was 28571 AU/mL for Omicron BA.4/5. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL for ancestral SARS-CoV-2 D614G.

Part 1: SRR Against Omicron BA.4/5 and Ancestral SARS-CoV-2 D614G at Days 91, 181, and 365

Timeframe: Days 91, 181, and 365

Seroresponse was defined as an antibody value change from baseline below the LLOQ to ≥4\*LLOQ, or at least a 4-fold rise if baseline was ≥LLOQ and \<4\*LLOQ, or at least a 2-fold rise if baseline was ≥4\*LLOQ, where baseline referred to pre-booster. LLOQ was 103 AU/mL and ULOQ was 28571 AU/mL for Omicron BA.4/5. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL for ancestral SARS-CoV-2 D614G.

Part 1: Number of Participants With a SARS-CoV-2 Infection (Symptomatic or Asymptomatic)

Timeframe: From 14 days after injection up to Day 365

SARS-CoV-2 infection (symptomatic or asymptomatic) was defined as (1) negative binding antibody (bAb) level against SARS-CoV-2 nucleocapsid protein and negative RT-PCR at baseline that became positive bAb level against SARS-CoV-2 nucleocapsid protein post-baseline, or (2) positive RT-PCR post-baseline. Asymptomatic SARS-CoV-2 infection was characterized by the absence of COVID-19 symptoms.

Part 3: GM of Omicron XBB.1.5 in Unvaccinated Participants

Timeframe: Day 29

Antibody values reported as below the LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ if actual values were not available. LLOQ was 38 AU/mL and ULOQ was 6960 AU/mL.

Part 3: GM of Omicron XBB.1.5 in All Study Participants

Timeframe: Day 29

Antibody values reported as below the LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were replaced by the ULOQ if actual values were not available. LLOQ was 38 AU/mL and ULOQ was 6960 AU/mL.

Part 3: SRR Against Omicron XBB.1.5 in Unvaccinated Participants

Timeframe: Day 29

Seroresponse was defined as an antibody value change from baseline below the LLOQ to ≥4\*LLOQ, or at least a 4-fold rise if baseline was ≥LLOQ and \<4\*LLOQ, or at least a 2-fold rise if baseline was ≥4\*LLOQ, where baseline referred to last non-missing measurement on or before study vaccination day. LLOQ was 38 AU/mL and ULOQ was 6960 AU/mL.

Part 3: SRR Against Omicron XBB.1.5 in All Study Participants

Timeframe: Day 29

Seroresponse was defined as an antibody value change from baseline below the LLOQ to ≥4\*LLOQ, or at least a 4-fold rise if baseline was ≥LLOQ and \<4\*LLOQ, or at least a 2-fold rise if baseline was ≥4\*LLOQ, where baseline referred to last non-missing measurement on or before study vaccination day. LLOQ was 38 AU/mL and ULOQ was 6960 AU/mL.