An SAE was defined as any untoward medical occurrence that: results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, Results in persistent or significant disability/incapacity, leads to a congenital anomaly/birth defect in the offspring of a participant, or is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

An AESI was defined as AEs that will be specifically highlighted to the Investigator. AESIs for the study included the Potential Immune Mediated Medical Conditions (PIMMC) and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Percentage of participants with any AE leading to withdrawal from the trial until Day 29 was reported.

Percentage of participants with SARS-CoV-2 infection until Day 29 of the main part of the trial were reported in this outcome measure.

GMFR was calculated as the ratio of the post-vaccination titer level to the baseline titer level. Where baseline was defined as the last measurement taken before the first dose of trial vaccination.

SCR was defined as percentage of participants with 4-fold or more rises from baseline in titer. Baseline was defined as the last measurement taken before the first dose of trial vaccination.

The neutralization titer was expressed as the reciprocal of the highest dilution at which greater than or equal to (>=) 50 percent (%) of the replicate wells were protected from infection (microneutralization [MN] with an inhibitory concentration of 50% [MN50]). GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ where LLOQ was equal to 20.

The neutralization titer was expressed as the reciprocal of the highest dilution at which >=50% of the replicate wells were protected from infection (MN50). GMFR was calculated as the ratio of the post-vaccination titer level to the baseline titer level. Baseline was defined as the last measurement taken before the first dose of trial vaccination.

The neutralization titer was expressed as the reciprocal of the highest dilution at which greater than or equal to (>=) 50% of the replicate wells were protected from infection (MN50). SCR was defined as percentage of participants with 4-fold or more rises in from baseline. Baseline was defined as the last measurement taken before the first dose of trial vaccination.

An SAE was defined as any untoward medical occurrence that: Results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, Results in persistent or significant disability/incapacity, leads to a congenital anomaly/birth defect in the offspring of a participant, or is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

An AESI was defined as AEs that will be specifically highlighted to the Investigator. AESIs for the study included the PIMMC and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Percentage of participants with any AE leading to participant's withdrawal from the trial from the day of the first single booster vaccination throughout the main part of trial was reported.

Percentage of participants with SARS-CoV-2 infection throughout the main part of trial was reported.

GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ. LLOQ was equal to 200 EU/mL.

The GMFR was calculated as the ratio of the post-second-booster-vaccination titer level to extension part baseline titer level. Where extension part baseline was defined as last measurement taken before the second booster vaccination.

SCR was defined as percentage of participants with 4-fold or more rises in titer from extension part baseline. Where extension part baseline was defined as last measurement taken before the second booster vaccination.

The neutralization titer was expressed as the reciprocal of the highest dilution at >= 50% of the replicate wells were protected from infection (MN50). GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ where LLOQ was equal to 20.

The neutralization titer was expressed as the reciprocal of the highest dilution at >= 50% of the replicate wells were protected from infection (MN50). The GMFR was calculated as the ratio of the post-second-booster-vaccination titer level to extension part baseline titer level. Where extension part baseline was defined as last measurement taken before the second booster vaccination.

The neutralization titer was expressed as the reciprocal of the highest dilution at >= 50% of the replicate wells were protected from infection (MN50). SCR was defined as percentage of participants with 4-fold or more rises in titer from extension part baseline.

AE was defined as any untoward medical occurrence in a clinical investigation participant administered an investigational medicinal product (IMP); it did not necessarily have to have a causal relationship with IMP administration. Reported solicited local AEs were defined as injection site pain, tenderness, erythema/redness, induration, and swelling.

Solicited systemic AEs included were defined as fever, fatigue, malaise, myalgia, arthralgia, nausea/vomiting and headache.

Unsolicited AEs defined as AEs other than solicited local AEs and solicited systemic AEs.

An SAE was defined as any untoward medical occurrence that: results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, leads to a congenital anomaly/birth defect in the offspring of a participant, or is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

An AESI was defined as AEs that will be specifically highlighted to the investigator. AESIs for the study included the PIMMC and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Percentage of participants with any AEs leading to withdrawal from the trial until extension part Day 29 was reported.

Percentage of participants with SARS-CoV-2 infection until extension part Day 29 was reported.

An SAE was defined as any untoward medical occurrence that: Results in death, Is life-threatening, Requires inpatient hospitalization or prolongation of existing hospitalization, Results in persistent or significant disability/incapacity, Leads to a congenital anomaly/birth defect in the offspring of a participant, or Is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

An AESI was defined as AEs that will be specifically highlighted to the Investigator. AESIs for the study included the PIMMC and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Percentage of participants with any AEs leading to withdrawal from the trial from the day of the second single booster vaccination throughout the extension part of the trial was reported.

Percentage of participants with SARS-CoV-2 infection throughout the extension part of trial was reported.

AE was defined as any untoward medical occurrence in a clinical investigation participant administered an investigational medicinal product (IMP); it did not necessarily have to have a causal relationship with IMP administration. Reported solicited local AEs were defined as injection site pain, tenderness, erythema/redness, induration, and swelling.

Main Part: The safety analysis set included all participants who received at least 1 dose of the trial vaccination.

Solicited systemic AEs were defined as fever, fatigue, malaise, myalgia, arthralgia, nausea/vomiting, and headache.

Main Part: The safety analysis set included all participants who received at least 1 dose of the trial vaccination.

Unsolicited AEs defines as other AEs than solicited local AEs and solicited systemic AEs.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An SAE was defined as any untoward medical occurrence that: results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, Results in persistent or significant disability/incapacity, leads to a congenital anomaly/birth defect in the offspring of a participant, or is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An AESI was defined as AEs that will be specifically highlighted to the Investigator. AESIs for the study included the Potential Immune Mediated Medical Conditions (PIMMC) and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with any AE leading to withdrawal from the trial until Day 29 was reported.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with SARS-CoV-2 infection until Day 29 of the main part of the trial were reported in this outcome measure.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ. LLOQ was equal to 200 EU/mL.

Per-protocol Set: PPS was defined to include participants in the FAS and who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "number analyzed" signifies participants who were evaluable at specified timepoints.

GMFR was calculated as the ratio of the post-vaccination titer level to the baseline titer level. Where baseline was defined as the last measurement taken before the first dose of trial vaccination.

Per-protocol Set: PPS was defined to include participants in the FAS and who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, number analyzed signifies participants who were evaluable at specified timepoints.

SCR was defined as percentage of participants with 4-fold or more rises from baseline in titer. Baseline was defined as the last measurement taken before the first dose of trial vaccination.

Per-protocol Set: PPS was defined to include participants in the FAS and who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, number analyzed signifies participants who were evaluable at specified timepoints.

The neutralization titer was expressed as the reciprocal of the highest dilution at which greater than or equal to (>=) 50 percent (%) of the replicate wells were protected from infection (microneutralization [MN] with an inhibitory concentration of 50% [MN50]). GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ where LLOQ was equal to 20.

Per-protocol Set: PPS was defined to include participants in the FAS and who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, number analyzed signifies participants who were evaluable at specified timepoints.

The neutralization titer was expressed as the reciprocal of the highest dilution at which >=50% of the replicate wells were protected from infection (MN50). GMFR was calculated as the ratio of the post-vaccination titer level to the baseline titer level. Baseline was defined as the last measurement taken before the first dose of trial vaccination.

Per-protocol Set: PPS was defined to include participants in the FAS and who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, number analyzed signifies participants who were evaluable at specified timepoints.

The neutralization titer was expressed as the reciprocal of the highest dilution at which greater than or equal to (>=) 50% of the replicate wells were protected from infection (MN50). SCR was defined as percentage of participants with 4-fold or more rises in from baseline. Baseline was defined as the last measurement taken before the first dose of trial vaccination.

Per-protocol Set: PPS was defined to include participants in the FAS and who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, number analyzed signifies participants who were evaluable at specified timepoints.

An SAE was defined as any untoward medical occurrence that: Results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, Results in persistent or significant disability/incapacity, leads to a congenital anomaly/birth defect in the offspring of a participant, or is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An AESI was defined as AEs that will be specifically highlighted to the Investigator. AESIs for the study included the PIMMC and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Main Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with any AE leading to participant's withdrawal from the trial from the day of the first single booster vaccination throughout the main part of trial was reported.

Main Part: The safety analysis set included all participants who received at least 1 dose of the trial vaccination.

Percentage of participants with SARS-CoV-2 infection throughout the main part of trial was reported.

Main Part: The safety analysis set included all participants who received at least 1 dose of the trial vaccination.

GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ. LLOQ was equal to 200 EU/mL.

Extension Part PPS was defined to include participants in the FAS in the Extension Part who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "overall number of participants analyzed" signified participants who were evaluable for this outcome measure and "number analyzed" signifies participants who were evaluable at specified timepoints.

The GMFR was calculated as the ratio of the post-second-booster-vaccination titer level to extension part baseline titer level. Where extension part baseline was defined as last measurement taken before the second booster vaccination.

Extension part PPS was defined to include participants in the FAS in the Extension Part who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "overall number of participants analyzed" signified participants who were evaluable for this outcome measure and "number analyzed" signifies participants who were evaluable at specified timepoints.

SCR was defined as percentage of participants with 4-fold or more rises in titer from extension part baseline. Where extension part baseline was defined as last measurement taken before the second booster vaccination.

Extension part PPS was defined to include participants in the FAS in the Extension Part who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "overall number of participants analyzed" signified participants who were evaluable for this outcome measure and "number analyzed" signifies participants who were evaluable at specified timepoints.

The neutralization titer was expressed as the reciprocal of the highest dilution at >= 50% of the replicate wells were protected from infection (MN50). GMT was the immunogenicity outcome expressed as reciprocal antibody titer with average. Titer values was measured as below LLOQ were imputed to a value that was half of the LLOQ where LLOQ was equal to 20.

Extension part PPS was defined to include participants in the FAS in the Extension Part who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "overall number of participants analyzed" signified participants who were evaluable for this outcome measure and "number analyzed" signifies participants who were evaluable at specified timepoints.

The neutralization titer was expressed as the reciprocal of the highest dilution at >= 50% of the replicate wells were protected from infection (MN50). The GMFR was calculated as the ratio of the post-second-booster-vaccination titer level to extension part baseline titer level. Where extension part baseline was defined as last measurement taken before the second booster vaccination.

Extension part PPS was defined to include participants in the FAS in the Extension Part who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "overall number of participants analyzed" signified participants who were evaluable for this outcome measure and "number analyzed" signifies participants who were evaluable at specified timepoints.

The neutralization titer was expressed as the reciprocal of the highest dilution at >= 50% of the replicate wells were protected from infection (MN50). SCR was defined as percentage of participants with 4-fold or more rises in titer from extension part baseline.

Extension part PPS was defined to include participants in the FAS in the Extension Part who had evaluable immunogenicity data and did not have significant protocol deviations which influenced the immunogenicity assessment. Here, "overall number of participants analyzed" signified participants who were evaluable for this outcome measure and "number analyzed" signifies participants who were evaluable at specified timepoints.

AE was defined as any untoward medical occurrence in a clinical investigation participant administered an investigational medicinal product (IMP); it did not necessarily have to have a causal relationship with IMP administration. Reported solicited local AEs were defined as injection site pain, tenderness, erythema/redness, induration, and swelling.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Solicited systemic AEs included were defined as fever, fatigue, malaise, myalgia, arthralgia, nausea/vomiting and headache.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Unsolicited AEs defined as AEs other than solicited local AEs and solicited systemic AEs.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An SAE was defined as any untoward medical occurrence that: results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, leads to a congenital anomaly/birth defect in the offspring of a participant, or is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An AESI was defined as AEs that will be specifically highlighted to the investigator. AESIs for the study included the PIMMC and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with any AEs leading to withdrawal from the trial until extension part Day 29 was reported.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with SARS-CoV-2 infection until extension part Day 29 was reported.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An SAE was defined as any untoward medical occurrence that: Results in death, Is life-threatening, Requires inpatient hospitalization or prolongation of existing hospitalization, Results in persistent or significant disability/incapacity, Leads to a congenital anomaly/birth defect in the offspring of a participant, or Is an important medical event. Solicited SAEs and unsolicited SAEs were reported.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

An AESI was defined as AEs that will be specifically highlighted to the Investigator. AESIs for the study included the PIMMC and AEs specific to COVID-19. PIMMC is categorized as following; neuroinflammatory disorders, musculoskeletal and connective tissue disorders, vasculitides, gastrointestinal disorders, hepatic disorders, renal disorders, cardiac disorders, skin disorder, hematologic disorders, metabolic disorders, and other disorders.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

MAAEs were defined as AEs leading to an unscheduled visit to or by a healthcare professional including visits to an emergency department, but not fulfilling seriousness criteria.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with any AEs leading to withdrawal from the trial from the day of the second single booster vaccination throughout the extension part of the trial was reported.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Percentage of participants with SARS-CoV-2 infection throughout the extension part of trial was reported.

Extension Part: The safety analysis set included all participants who received at least 1 dose of trial vaccination.

Participants received a second booster vaccination of TAK-019 0.5 mL, intramuscular injection in the mid deltoid, preferable in the non-dominant upper arm at Day 1 of Extension Part.