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.

Measured in the Solicited Safety Set, which consisted of all participants in the Safety Set who contributed any solicited AR data.

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.

Measured in the Safety Set, which consisted of all randomized/enrolled (Part A: randomized, Part B: enrolled) participants who received one dose of IP. Data in the Safety Set is presented per actual dose received. There was 1 participant in the mRNA-1283.211 low dose group who had a dosing error and received mRNA-1283 at medium dose.

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.

Measured in the Safety Set, which consisted of all randomized/enrolled (Part A: randomized, Part B: enrolled) participants who received one dose of IP. Data in the Safety Set is presented per actual dose received. There was 1 participant in the mRNA-1283.211 low dose group who had a dosing error and received mRNA-1283 at medium dose.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the full analysis set (FAS) who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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.

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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.

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

The GMT (ID50) of nAb against ancestral SARS-CoV-2 and against SARS-CoV-2 Variant B.1.351 are reported.

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

The GMT of bAb against ancestral SARS-CoV-2 and against SARS-CoV-2 Variant B.1.351 are reported.

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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.

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

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.

Measured in the Per-Protocol Set for Immunogenicity - Pre-booster SARS-CoV-2 Negative, which consisted of all participants in the FAS who received the planned dose of IP and who had no major protocol deviations that impact key or critical data who were pre-booster SARS-CoV-2 negative and had evaluable data for the endpoint. Here, number analyzed signifies those participants who were evaluable for specified categories only.

A death that occurred during the study or that came to the attention of the investigator during the study was reported to the Sponsor, whether or not it was considered related to study drug. The investigator assessed causality (that is, whether there is a reasonable possibility that the study drug caused the death). The relationship was characterized using the following classifications: Not related: There was not a reasonable possibility of a relationship to the study drug. The temporal sequence of the death relative to administration of the study drug was not reasonable AND/OR the death was more likely explained by a cause other than the study drug. Related: There was a reasonable possibility of a relationship to the study drug. There was evidence of exposure to the study drug. The temporal sequence of the death relative to the administration of the study drug was reasonable. The death was more likely explained by the study drug than by another cause.

Measured in the Safety Set, which consisted of all randomized/enrolled (Part A: randomized, Part B: enrolled) participants who received one dose of IP. Data in the Safety Set is presented per actual dose received. There was 1 participant in the mRNA-1283.211 low dose group who had a dosing error and received mRNA-1283 at medium dose.