mRNA-1283

An AR is any adverse event (AE) related to the IP injection. Solicited local ARs included pain at injection site, erythema (redness) at injection site, swelling (hardness) at injection site, localized axillary swelling or tenderness ipsilateral to the injection arm, and groin or underarm swelling or tenderness ipsilateral to the side of injection. Solicited systemic ARs included headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, fever, chills, irritability/crying, sleepiness, and loss of appetite. Note, not all solicited ARs were considered AEs. The Investigator determined if solicited AR was also to be recorded as an AE. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

An unsolicited AE was defined as any AE reported by the participant that was not specified as a solicited AR in the protocol or was specified as a solicited AR but started outside the protocol-defined period for reporting solicited ARs. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

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. A summary of all Serious Adverse Events and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

The GMT (50% inhibitory dose \[ID50\]) of nAb against ancestral SARS-CoV-2 and against SARS-CoV-2 variant B.1.351 are reported.

The GMFR measures the changes in immunogenicity titers or levels within participants.

Seroresponse was defined as an increase of SARS-CoV-2 specific binding antibody (bAb) level or nAb titer to at least 4x lower limit of quantification (LLOQ) if the baseline is below the LLOQ, or a 4-fold or greater rise if pre-booster ≥ LLOQ.

The GMT (ID50) of nAb against SARS-CoV-2 omicron variant (B.1.1.529) are reported.

The GMFR measures the changes in immunogenicity titers or levels within participants.

Seroresponse was defined as an increase of SARS-CoV-2 specific bAb level or nAb titer to at least 4x LLOQ if the baseline is below the LLOQ, or a 4-fold or greater rise if pre-booster ≥ LLOQ.