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 Safety Set: all participants in Safety Set (Safety Set of Part 1 and Part 3 included all dosed participants and of Part 2 included all randomized participants who received any study injection) who contributed any solicited AR data, that is, had at least 1 post-baseline solicited safety assessment. Per prespecified analysis, data for this endpoint was not collected for the mRNA-1273.214 treatment groups.

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. COVID-19/SARS-CoV-2 infections were considered clinical events for efficacy and not AEs. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section and presented by each dose group separately.

Safety Set of Part 1 and Part 3 included all dosed participants and of Part 2 included all randomized participants who received any study injection. Per prespecified analysis, data for this endpoint was not collected for the mRNA-1273.214 and placebo only treatment groups.

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. AESIs for mRNA-1273 were identified based upon medical concepts that may be related to COVID-19 or were of interest in COVID-19 vaccine safety surveillance. An MAAE is an AE that led to an unscheduled visit to a healthcare practitioner. This included visits to a study site for unscheduled assessments and visits to healthcare practitioners external to the study site. COVID-19/SARS-CoV-2 infections were considered clinical events for efficacy and not AEs. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section and presented by each dose group separately.

Safety Set of Part 1 and Part 3 included all dosed participants and of Part 2 included all randomized participants who received any study injection. Per prespecified analysis, the data for this outcome measure was not collected for participants who received only placebo.

Antibody values reported as below lower limit of quantification (LLOQ) were replaced by 0.5*LLOQ and values greater than upper limit of quantification (ULOQ) were replaced by ULOQ if actual values were not available. LLOQ was 18.5 arbitrary units (AU)/milliliter (mL) and ULOQ was 45118 AU/mL for ID50 titer. Per-Protocol (PP) Immunogenicity Subset: all enrolled participants who received planned doses of the study vaccine per schedule, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment for analysis endpoint, complied with immunogenicity window based on 2nd injection timing; had negative reverse transcriptase polymerase chain reaction (RT-PCR) test for SARS-CoV-2 and negative serology test based on binding antibody (bAb) specific to SARS-CoV-2 nucleocapsid protein at baseline, not receiving highly active antiretroviral therapy (HAART) for participants with HIV; and had no major protocol deviations that impacted key or critical data.

PP immunogenicity subset. Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = participants evaluable for this endpoint. As planned, Part 1 immunogenicity assessment did not serve as formal noninferiority hypothesis testing. It was intended to guide the dose selection only.

Antibody values reported as below LLOQ were replaced by 0.5*LLOQ and values >ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset: all enrolled participants who received planned doses of the study vaccine per schedule, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment, complied with immunogenicity window based on 2nd injection timing; had negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline in Part 2, not receiving HAART in participants with HIV; and had no major protocol deviations that impacted key/critical data. Since the number of participants enrolled in Part 3 was substantially smaller than the planned sample size required for immunogenicity hypothesis testing after Dose 2 of mRNA-1273 25 μg primary series and after a 3rd dose of mRNA-1273 25 μg, the hypothesis testing was not performed.

PP immunogenicity subset. Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = participants evaluable for this endpoint.

Percentage of participants with seroresponse for pseudovirus neutralizing antibody ID50 are reported. Seroresponse: change from below LLOQ to equal above 4*LLOQ, or at least a 4-fold rise if baseline is ≥LLOQ. LLOQ=18.5 AU/mL and ULOQ=45118 AU/mL for ID50 titer. PP Immunogenicity Subset: all enrolled participants who received planned doses of study vaccine, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment, complied with immunogenicity window based on 2nd injection timing; had negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 at baseline, not receiving HAART; and had no major protocol deviations that impacted key/critical data. Since the number of participants enrolled in Part 3 was substantially smaller than the planned sample size required for immunogenicity hypothesis testing after Dose 2 of mRNA-1273 25 μg primary series and after a 3rd dose of mRNA-1273 25 μg, the hypothesis testing was not performed.

PP immunogenicity subset. Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = participants evaluable for this endpoint. As planned, Part 1 immunogenicity assessment did not serve as formal noninferiority hypothesis testing. It was intended to guide the dose selection only.

Percentage of participants with seroresponse for Pseudovirus Neutralizing Antibody VAC62 are reported. Seroresponse was defined as a change from below the LLOQ to equal above 4 * LLOQ, or at least a 4-fold rise if baseline is equal to or above the LLOQ. LLOQ was 10 and ULOQ AU/mL was 111433 AU/mL. PP Immunogenicity Subset: all enrolled participants who received planned doses of the study vaccine per schedule, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment for analysis endpoint, complied with immunogenicity window based on 2nd injection timing; had negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline in Part 2, not receiving HAART in participants with HIV; and had no major protocol deviations that impacted key or critical data.

PP immunogenicity subset. Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = participants evaluable for this endpoint.

Antibody values reported as below the LLOQ were replaced by 0.5*LLOQ and values greater than the ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset (Booster Dose Analysis): all enrolled participants who received 2 doses of planned doses of mRNA-1273 vaccination in Part 1 open-label phase or Part 2 blinded phase per schedule, received booster dose in Booster Dose Analysis, not receiving HAART in participants with HIV, had a negative SARS-CoV-2 status at baseline (pre-dose 1 of mRNA-1273), had BD-Day 29 Ab assessment for the analysis endpoint, no major protocol deviations that impacted key or critical data, and had not received off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Booster Dose Analysis). Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = Pre-booster SARS-CoV-2 negative participants evaluable for this endpoint. 'Part 1 6-23 months group' and 'Part 1 2-5 years group' combined for noninferiority hypothesis testing. 'Part 1 and Part 2 groups of 6-11 years' combined for noninferiority hypothesis testing.

Antibody values reported as below the LLOQ were replaced by 0.5*LLOQ and values greater than the ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset (Third Dose Analysis): all enrolled participants who received first 2 doses of planned doses of mRNA-1273 vaccination in Part 3 open-label phase per schedule, received third dose in Third Dose Analysis, not receiving HAART in participants with HIV, had BD-Day 29 antibody assessment for the analysis endpoint, had no major protocol deviations that impacted key or critical data, and had not received off-study COVID-19 vaccination prior to BD-Day 29 visit. Since the number of participants enrolled in Part 3 was substantially smaller than the planned sample size required for immunogenicity hypothesis testing after Dose 2 of mRNA-1273 25 μg primary series and after a 3rd dose of mRNA-1273 25 μg, the hypothesis testing was not performed.

PP immunogenicity subset (Third Dose Analysis). Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = participants evaluable for this endpoint.

Percentage of participants with seroresponse for Pseudovirus Neutralizing Antibody VAC62 are reported. Seroresponse was defined as a change from below the LLOQ to equal above 4 * LLOQ, or at least a 4-fold rise if baseline is equal to or above the LLOQ. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset (Booster Dose Analysis): all enrolled participants who received 2 doses of planned doses of mRNA-1273 vaccination in Part 1 open-label phase or Part 2 blinded phase per schedule, received booster dose in Booster Dose Analysis, not receiving HAART in participants with HIV, had a negative SARS-CoV-2 status at baseline (pre-dose 1 of mRNA-1273), had BD-Day 29 Ab assessment for the analysis endpoint, no major protocol deviations that impacted key or critical data, and had not received off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Booster Dose Analysis). Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = Pre-booster SARS-CoV-2 negative participants evaluable for this endpoint.

Percentage of participants with seroresponse for Pseudovirus Neutralizing Antibody VAC62 are reported. Seroresponse was defined as a change from below the LLOQ to equal above 4 * LLOQ, or at least a 4-fold rise if baseline is equal to or above the LLOQ. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset (Third Dose Analysis): all enrolled participants who received first 2 doses of planned doses of mRNA-1273 vaccination in Part 3 open-label phase per schedule, received third dose in Third Dose Analysis, not receiving HAART in participants with HIV were not receiving HAART, had BD-Day 29 antibody assessment for the analysis endpoint, had no major protocol deviations that impacted key or critical data, and had not received off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Third Dose Analysis). Study P301 mRNA-1273 100 μg: PPIS of randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria. 'Overall number of participants analyzed' = participants evaluable for this endpoint.

GM level of SARSCOV2S2P immunoglobulin G (IgG) antibody VAC123/VAC72, as measured by ECL multiplex assay specific to SARS-CoV-2 spike protein is reported. Antibody values reported as <LLOQ were replaced by 0.5*LLOQ and values >ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 23 AU/mL and ULOQ was 14000000 AU/mL for VAC72. LLOQ was 69 AU/mL and ULOQ was 14400000 AU/mL for VAC123. PP Immunogenicity Subset: all enrolled participants who received planned doses of study vaccine per schedule, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment, complied with immunogenicity window based on 2nd injection timing; had negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 protein at baseline, not receiving HAART in participants with HIV; and had no major protocol deviations.

PP immunogenicity subset. Overall number of participants analyzed= participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

GM level of SARSCOV2S2P IgG antibody VAC123/VAC72 is reported. Antibody values reported as <LLOQ were replaced by 0.5*LLOQ and values >ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 23 and ULOQ was 14000000 AU/mL for VAC72. LLOQ was 69 and ULOQ was 14400000 AU/mL for VAC123. PP Immunogenicity Subset (Booster Dose Analysis): all enrolled participants who received 2 doses of planned doses of mRNA-1273 in Part 1 open-label phase or Part 2 blinded phase per schedule, received booster dose in Booster Dose Analysis, , not receiving HAART in participants with HIV, had a negative SARS-CoV-2 status at baseline, had BD-Day 29 antibody assessment for the analysis endpoint, no major protocol deviations that impacted key or critical data, and had not receive off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Booster Dose Analysis). Overall number of participants analyzed = participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

GM level of SARSCOV2S2P IgG antibody against B.1.1.529 strain is reported. Antibody values reported as below the LLOQ were replaced by 0.5*LLOQ and values greater than the ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 102 and ULOQ was 1180000 AU/mL for VAC123. PP Immunogenicity Subset (Third Dose Analysis): all enrolled participants who received first 2 doses of planned doses of mRNA-1273 vaccination in Part 3 open-label phase per schedule, received third dose in Third Dose Analysis, participants with HIV were not receiving HAART, had BD-Day 29 antibody assessment for the analysis endpoint, had no major protocol deviations that impact key or critical data, and had not receive off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Third Dose Analysis). 'Overall number of participants analyzed = participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

Antibody values reported as below LLOQ were replaced by 0.5*LLOQ and values greater than ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 18.5 AU/mL and ULOQ was 45118 AU/mL for ID50 titer. PP Immunogenicity Subset: all enrolled participants who received planned doses of the study vaccine per schedule, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment for analysis endpoint, complied with immunogenicity window based on 2nd injection timing; had negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline, not receiving HAART in participants with HIV; and had no major protocol deviations that impacted key or critical data.

PP immunogenicity subset. 'Overall number of participants analyzed' = participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

Antibody values reported as below the LLOQ were replaced by 0.5*LLOQ and values greater than the ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. Data are reported per Baseline SARS-CoV-2 status: Negative and Positive. PP Immunogenicity Subset: all enrolled participants who received planned doses of the study vaccine per schedule, had baseline SARS-CoV-2 status, had baseline and Day 57 antibody assessment for analysis endpoint, complied with immunogenicity window based on 2nd injection timing; had negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline in Part 2, not receiving HAART in participants with HIV; and had no major protocol deviations that impacted key or critical data.

PP immunogenicity subset. 'Overall number of participants analyzed' = participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

Antibody values reported as below the LLOQ were replaced by 0.5*LLOQ and values greater than the ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset (Booster Dose Analysis): all enrolled participants who received 2 doses of planned doses of mRNA-1273 vaccination in Part 1 open-label phase or Part 2 blinded phase per schedule, received booster dose in Booster Dose Analysis, not receiving HAART in participants with HIV, had a negative SARS-CoV-2 status at baseline (pre-dose 1 of mRNA-1273), had BD-Day 29 Ab assessment for the analysis endpoint, no major protocol deviations that impacted key or critical data, and had not received off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Booster Dose Analysis). 'Overall number of participants analyzed' = participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

Antibody values reported as below the LLOQ were replaced by 0.5*LLOQ and values greater than the ULOQ were replaced by ULOQ if actual values were not available. LLOQ was 10 AU/mL and ULOQ was 111433 AU/mL. PP Immunogenicity Subset (Third Dose Analysis): all enrolled participants who received first 2 doses of planned doses of mRNA-1273 vaccination in Part 3 open-label phase per schedule, received third dose in Third Dose Analysis, not receiving HAART in participants with HIV, had BD-Day 29 antibody assessment for the analysis endpoint, had no major protocol deviations that impacted key or critical data, and had not received off-study COVID-19 vaccination prior to BD-Day 29 visit.

PP immunogenicity subset (Third Dose Analysis). 'Overall number of participants analyzed' = participants evaluable for this endpoint. Number analyzed = participants evaluable at specified timepoint.

SARS-CoV-2 infection was defined in participants with negative SARS-CoV-2 at baseline: bAb level against SARS-CoV-2 nucleocapsid protein negative at Day 1, that became positive (as measured by Roche Elecsys) postbaseline; OR positive RT-PCR postbaseline. PP Set for Efficacy included all enrolled participants who received planned doses of study drug per schedule, complied with the 2nd injection timing, had no major protocol deviations that impacted key or critical efficacy data, and had a negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline.

PP set for efficacy. 'Overall number of participants analyzed' = participants evaluable for this endpoint.

Asymptomatic SARS-CoV-2 infection was identified by absence of symptoms and infections as detected by RT-PCR or serology tests: Absence of COVID-19 symptoms AND at least 1 from following: bAb level against SARS-CoV-2 nucleocapsid protein negative at Day 1 that became positive post-baseline, OR positive RT-PCR test post-baseline. PP Set for Efficacy included all enrolled participants who received planned doses of study drug per schedule, complied with the 2nd injection timing, had no major protocol deviations that impacted key or critical efficacy data, and had a negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline.

PP set for efficacy. 'Overall number of participants analyzed' = participants evaluable for this endpoint.

A COVID-19 case was identified as a positive post-baseline RT-PCR test result together with at least 1 of following systemic symptoms: fever (≥ 38 degrees Celsius [°C]/≥ 100.4 degree Fahrenheit [°F]) or chills, fatigue, headache, myalgia, nasal congestion or rhinorrhea, new loss of taste or smell, sore throat, abdominal pain, diarrhoea, nausea/vomiting, poor appetite/poor feeding; or at least 1 of following respiratory signs/symptoms: cough, shortness of breath or difficulty breathing. PP Set for Efficacy included all enrolled participants who received planned doses of study drug per schedule, complied with the 2nd injection timing, had no major protocol deviations that impacted key or critical efficacy data, and had a negative RT-PCR test for SARS-CoV-2 and negative serology test based on bAb specific to SARS-CoV-2 nucleocapsid protein at baseline.

PP set for efficacy.

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.

Safety Set of Part 1 and Part 3 included all dosed participants and of Part 2 included all randomized participants who received any study injection. Data are reported by study part and by age categories of "6-11 years" and "6 months-5 years".