An unsolicited AE was any AE reported by the participant that was not specified as a solicited adverse reaction (AR) or was specified as a solicited AR but started outside the protocol-defined period for reporting solicited ARs (that is, for the 7 days after each dose of vaccine). A summary of serious adverse events (SAEs) and all nonserious AEs ("Other") reported up to the end of the study (up to Day 419), regardless of causality, is located in the Reported "Adverse Events" section.

An unsolicited AE was any AE reported by the participant that was not specified as a solicited AR or was specified as a solicited AR but started outside the protocol-defined period for reporting solicited ARs (that is, for the 7 days after each dose of vaccine). A summary of SAEs and all nonserious AEs ("Other") reported up to the end of the study (up to Day 419), regardless of causality, is located in the Reported "Adverse Events" section.

An MAAE was an AE that led to an unscheduled visit to an healthcare practitioner (HCP). This included visits to a study site for unscheduled assessments (for example, abnormal laboratory test results follow-up, COVID-19) and visits to HCPs external to the study site (for example, urgent care, primary care physician). MAAEs were also required by protocol for routine surveillance of participants with symptoms for COVID-19 infection (fever, shortness of breath, cough, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, and diarrhea). All confirmed symptomatic COVID-19 cases were recorded as MAAEs. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

An MAAE was an AE that led to an unscheduled visit to an HCP. This included visits to a study site for unscheduled assessments (for example, abnormal laboratory test results follow-up, COVID-19) and visits to HCPs external to the study site (for example, urgent care, primary care physician). MAAEs were also required by protocol for routine surveillance of participants with symptoms for COVID-19 infection (fever, shortness of breath, cough, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, and diarrhea). All confirmed symptomatic COVID-19 cases were recorded as MAAEs. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

An AE (including an AR) was considered an SAE if, in the view of either the investigator or Sponsor, it results in any of the following outcomes: death, was life-threatening, inpatient hospitalization or prolongation of existing hospitalization, persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, congenital anomaly or birth defect, or medically important event. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

An AE (including an AR) was considered an SAE if, in the view of either the investigator or Sponsor, it results in any of the following outcomes: death, was life-threatening, inpatient hospitalization or prolongation of existing hospitalization, persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, congenital anomaly or birth defect, or medically important event. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

An AESI was an AE (serious or non-serious) of scientific and medical concern specific to the Sponsor's product or program. AESIs included thrombocytopenia, new onset of or worsening of the protocol specified neurologic diseases, anaphylaxis, and myocarditis/pericarditis. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

An AESI was an AE (serious or non-serious) of scientific and medical concern specific to the Sponsor's product or program. AESIs included thrombocytopenia, new onset of or worsening of the protocol specified neurologic diseases, anaphylaxis, and myocarditis/pericarditis. 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 it is considered drug-related. 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 it is considered drug-related. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

Solicited ARs, including local and systemic ARs were collected in the eDiary. Local ARs included: pain at injection site, erythema (redness) at injection site, swelling/induration (hardness) at injection site, and localized axillary swelling or tenderness ipsilateral to the injection arm. Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, rash, body temperature (potentially fever), and chills. All solicited ARs (local and systemic) considered causally related to injection. ARs were graded 0-4 as reviewed and confirmed by Investigator; lower score indicates lower severity and a higher score indicates greater severity. Note, not all solicited ARs were considered AEs. The Investigator reviewed whether the solicited AR was also to be recorded as an AE. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

Solicited ARs, including local and systemic ARs were collected in the eDiary. Local ARs included: pain at injection site, erythema (redness) at injection site, swelling/induration (hardness) at injection site, and localized axillary swelling or tenderness ipsilateral to the injection arm. Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, rash, body temperature (potentially fever), and chills. All solicited ARs (local and systemic) considered causally related to injection. ARs were graded 0-4 as reviewed and confirmed by Investigator; lower score indicates lower severity and a higher score indicates greater severity. Note, not all solicited ARs were considered AEs. The Investigator reviewed whether the solicited AR was also to be recorded as an AE. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

The GMC of VAC62 antibodies, as measured by pseudovirus neutralization assay (PsVNA) specific to the SARS-CoV-2 spike protein is reported. Antibody values reported as below the lower limit of quantification (LLOQ) were replaced by 0.5\*LLOQ. Values that were greater than the upper limit of quantification (ULOQ) were converted to the ULOQ if actual values were not available. LLOQ was 10 and ULOQ was 111433 arbitrary units (AU)/milliliter (mL). The 95% confidence intervals (CIs) were calculated based on the t-distribution of the log-transformed values or the difference in the log-transformed values for GMC value, then back transformed to the original scale for presentation. Results in healthy participants were included as reference not for any comparisons.

The GMC of VAC62 antibodies, as measured by PsVNA specific to the SARS-CoV-2 spike protein is reported. Antibody values reported as below the LLOQ were replaced by 0.5\*LLOQ. Values that were greater than the ULOQ were converted to the ULOQ if actual values were not available. LLOQ was 10 and ULOQ was 111433 AU/mL. The 95% CIs were calculated based on the t-distribution of the log-transformed values or the difference in the log-transformed values for GMC value, then back transformed to the original scale for presentation.

The GMC of VAC62 antibodies, as measured by PsVNA specific to the SARS-CoV-2 spike protein is reported. Antibody values reported as below the LLOQ were replaced by 0.5\*LLOQ. Values that were greater than the ULOQ were converted to the ULOQ if actual values were not available. LLOQ was 10 and ULOQ was 111433 AU/mL. The 95% CIs were calculated based on the t-distribution of the log-transformed values or the difference in the log-transformed values for GMC value, then back transformed to the original scale for presentation.

COVID-19 cases were defined as participants meeting clinical criteria based both on symptoms for COVID-19 and on RT-PCR detection of SARS-CoV-2 from samples collected within 72 hours of the study participant reporting symptoms that met the definition of COVID-19. An adjudication committee was assembled for the purpose of reviewing potential cases to determine if the criteria for COVID-19 were met.

Solicited ARs (local and systemic) were collected in electronic diary (eDiary) within 7 days of dosing. Local ARs included: pain at injection site, erythema (redness) at injection site, swelling/induration (hardness) at injection site, and localized axillary swelling or tenderness ipsilateral to the injection arm. Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, rash, body temperature (potentially fever), and chills. Severity grading for solicited ARs is based on modified Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventative Vaccine Clinical Trials. Severity was graded 0-4; a lower score indicated lower severity and a higher score indicated greater severity. The Investigator determined if solicited AR was also to be recorded as an AE. Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is in Reported "Adverse Events" section.

Solicited ARs (local and systemic) were collected in eDiary within 7 days of dosing. Local ARs included: pain at injection site, erythema (redness) at injection site, swelling/induration (hardness) at injection site, and localized axillary swelling or tenderness ipsilateral to the injection arm. Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, rash, body temperature (potentially fever), and chills. Severity grading for solicited ARs is based on modified Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventative Vaccine Clinical Trials. Severity was graded 0-4; lower score indicated lower severity and a higher score indicated greater severity. The Investigator determined if solicited AR was also to be recorded as an AE. Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is in Reported "Adverse Events" section

An MAAE is an AE that leads to an unscheduled visit to a healthcare practitioner (HCP). A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

An SAE was defined as any AE that resulted in death, is life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability/permanent damage, 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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Solicited ARs (local and systemic) were collected in an electronic diary (eDiary). Local ARs included injection site pain, injection site erythema (redness), injection site swelling/induration (hardness), and axillary (underarm) swelling or tenderness ipsilateral to the side of injection. Systemic ARs included fever, headache, fatigue, myalgia, arthralgia, nausea/vomiting, and chills. Solicited ARs considered causally related to injection were graded 1-4 (per Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials); lower score indicated lower severity, and higher score indicated greater severity. Investigator reviewed if the solicited AR was recorded as an adverse event (AE) as detailed in the AE section. A Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is located in the AE section.

An unsolicited AE was 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 (onset after Day 7 of dosing). 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 or other safety assessment, including one that worsened from baseline and was considered clinically significant by the Investigator was recorded as an AE. Non-serious SARs persisting beyond 7 days, leading to discontinuation, or medically attended were classified as AEs in Part 1/2 but not in Part 3. COVID-19/SARS-CoV-2 infections were AEs in Part1/2 but were considered clinical events for efficacy in Part 3 and not AEs. A Summary of SAEs and nonserious AEs ("Other"), regardless of causality, is located in the AE section.

Pseudovirus nAb ID50 titers were measured using pseudovirus neutralization assay (PsVNA) assay. 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: 18.5 arbitrary units (AU)/milliliter (mL), ULOQ: 45118 AU/mL). Antibody levels were analyzed using an analysis of covariance (ANCOVA) model with the group variable (adolescents in P203 and young adults in P301) as fixed effect. PPIS P301: randomly selected participants from Study P301 aged 18-25 meeting pre-specified criteria were used for comparison assessments of immune response.

Percentage of participants with seroresponse for pseudovirus nAb ID50 measured using PsVNA assay 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: 18.5 AU/mL), ULOQ: 45118 AU/mL). PPIS P301: randomly selected participants from Study P301 aged 18-25 meeting pre-specified criteria were used for comparison assessments of immune response.

Pseudovirus nAb were measured using PsVNA assay. 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: 10, ULOQ: 281600). PPIS P301: randomly selected participants from study P301 aged 18-25 meeting pre-specified criteria and SARS-CoV-2 negative were used for comparison assessments of immune response.

Percentage of participants with seroresponse for pseudovirus nAb measured using PsVNA assay are reported. Seroresponse relative to pre-Dose 1 (baseline) at a participant level was defined as a change from below the LLOQ to equal or above 4 \* LLOQ, or at least a 4-fold-rise if baseline was equal to or above LLOQ (LLOQ: 10 AU/mL, ULOQ: 281600 AU/mL). PPIS P301: randomly selected participants from Study P301 aged 18-25 meeting pre-specified criteria and SARS-CoV-2 negative were used for comparison assessments of immune response.

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: 103 AU/mL, ULOQ: 28571 AU/mL). ANCOVA model with the group variable (adolescents in P203 and young adults in P301) as fixed effect. PPIS P301: randomly selected participants from Study P301 aged 18-25 meeting pre-specified criteria and SARS-CoV-2 negative were used for comparison of immune response.

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: 10 AU/mL, ULOQ: 111433 AU/mL).

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: 10 AU/mL, ULOQ: 111433 AU/mL).

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: 10 AU/mL, ULOQ: 111433 AU/mL). ANCOVA model with the group variable (adolescents in P203 and young adults in P301) as fixed effect. PPIS P301: randomly selected participants from Study P301 aged 18-25 meeting pre-specified criteria and SARS-CoV-2 negative were used for comparison of immune response.

Percentage of participants with seroresponse for pseudovirus nAb measured using PsVNA assay are reported. Seroresponse relative to pre-Dose 1 (baseline) at a participant level was defined as a change from below the LLOQ to equal or above 4 \* LLOQ, or at least a 4-fold-rise if baseline was equal to or above the LLOQ (LLOQ: 10 AU/mL, ULOQ: 111433 AU/mL).

SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability/permanent damage, was a congenital anomaly/birth defect, or was an important medical event. AESIs were identified based upon medical concepts that may be related to COVID-19 or were of interest in COVID-19 vaccine safety surveillance. MAAE was an AE that led to an unscheduled visit to a healthcare practitioner, included visits to a study site for unscheduled assessments (for example, abnormal laboratory follow-up, and visits to healthcare practitioners external to the study site \[for example, urgent care, primary care physician\]). Non-serious SARs persisting beyond 7 days, leading to discontinuation, or medically attended were defined as AEs in Part 1/2 but not in Part 3. COVID-19/SARS-CoV-2 infections were AEs in Part 1/2 but were considered clinical events for efficacy in Part 3 and not AEs.