Molecularly confirmed moderate to severe/critical COVID-19 was defined as a severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) positive reverse transcription/polymerase chain reaction (RT-PCR) or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample. Moderate included one sign or symptom such as respiratory rate >= 20 breaths per minute and symptoms such as shortness of breath or two signs or symptoms such as heart rate >= 90 beats per minute (beats/minute) and symptoms such as cough from a list of signs and symptoms and severe/critical included one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute, oxygen saturation (SpO2) <= 93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance.

Per-protocol Efficacy (PP) set: participants of the FAS (all randomized participants with double-blind study vaccine administration, regardless of protocol deviations and serostatus at enrollment) who received double-blind study vaccine and who were seronegative at the time of vaccination and who had no other major protocol deviations to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded.

Molecularly confirmed moderate to severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample. Moderate included one sign or symptom such as respiratory rate >=20 breaths per minute and symptoms such as shortness of breath or two signs or symptoms such as heart rate >= 90 beats/minute and symptoms such as cough from a list of signs and symptoms and severe/critical included one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute, SpO2 less than or equal to (<=) 93 percent (%) on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the Intensive Care Unit (ICU), death defined as per Food and Drug Administration (FDA) guidance.

PP set included participants of the Full Analysis Set (FAS) who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 28 in PP set were excluded.

Participants who received the booster dose were asked to note in the e-Diary occurrences of injection site pain/tenderness, erythema, and swelling at the study vaccine injection site daily for 7 days post-booster vaccination (day of vaccination and the subsequent 7 days).

Full analysis booster set included all randomized participants with a documented study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment, who received the Ad26.COV2.S booster vaccination at the booster visit.

Participants recorded the temperature in the e-Diary in the evening of the day of vaccination, and then daily for the next 7 days approximately at the same time each day. If more than 1 measurement was made on any given day, the highest temperature of that day was recorded in the e-Diary. Fever was defined as endogenous elevation of body temperature >= 38.0 degree Celsius or >=100.4-degree Fahrenheit, as recorded in at least 1 measurement. Participants also noted the signs and symptoms in the e-Diary on a daily basis for 7 days post-booster vaccination (day of vaccination and the subsequent 7 days), if feasible, for the following events: fatigue, headache, nausea, myalgia.

Full analysis booster set included all randomized participants with a documented study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment, who received the Ad26.COV2.S booster vaccination at the booster visit.

Unsolicited AEs were all AEs for which the participant is not specifically questioned in the participant diary.

Full analysis booster set included all randomized participants with a documented study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment, who received the Ad26.COV2.S booster vaccination at the booster visit.

Molecularly confirmed severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample and one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute,SpO2 <=93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance. Seronegative is defined as N-serology seronegative at the time of boosting or at the Year 1 visit if not boosted.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded.

Severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample and one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute,SpO2 <=93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 28 in PP set were excluded. Here, N (Overall number of participants analyzed) signifies participants evaluable for this outcome measure.

Molecularly confirmed moderate to severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample. Moderate included one sign or symptom such as respiratory rate >= 20 breaths per minute and symptoms such as shortness of breath or two signs or symptoms such as heart rate >= 90 beats per minute (beats/minute) and symptoms such as cough from a list of signs and symptoms and severe/critical included one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute,SpO2 <=93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance.

FAS included all randomized participants with a documented double-blind study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment.

Molecularly confirmed moderate to severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample. Moderate included one sign or symptom such as respiratory rate >= 20 breaths per minute and symptoms such as shortness of breath or two signs or symptoms such as heart rate >= 90 beats per minute (beats/minute) and symptoms such as cough from a list of signs and symptoms and severe/critical included one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute,SpO2 <=93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance.

PP set included participants of the FAS who received double-blind study vaccine, regardless of their serostatus at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. This excludes participants who had a COVID-19 case with an onset before Day 15.

Molecularly confirmed moderate to severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample. Moderate included one sign or symptom such as respiratory rate >= 20 breaths per minute and symptoms such as shortness of breath or two signs or symptoms such as heart rate >= 90 beats per minute (beats/minute) and symptoms such as cough from a list of signs and symptoms and severe/critical included one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute,SpO2 <=93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance.

PP set included participants of the FAS who received double-blind study vaccine, regardless of their serostatus at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. This excludes participants who had a COVID-19 case with an onset before Day 29.

Number of participants with first occurrence of COVID-19 requiring medical intervention (such as a composite endpoint of hospitalization, ICU admission, mechanical ventilation, and extracorporeal membrane oxygenation [ECMO], linked to objective measures such as decreased oxygenation, X-ray or computed tomography [CT] findings) or linked to any molecularly confirmed, COVID-19 at least 14 days post vaccination were reported.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded.

Number of participants with first occurrence of COVID-19 requiring medical intervention (such as a composite endpoint of hospitalization, ICU admission, mechanical ventilation, and ECMO, linked to objective measures such as decreased oxygenation, X-ray or CT findings) or linked to any molecularly confirmed, COVID-19 at least 28 days post vaccination were reported.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 28 in PP set were excluded.

AUC of SARS-CoV-2 Viral Load was assessed in confirmed COVID-19 cases using RT-PCR. Nasal swabs were used to detect and/or quantify SARS-CoV-2.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded. Here N (Overall number of participants analyzed) signifies participants evaluable for this OM.

Molecularly confirmed mild Covid-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample and one of the following signs and symptoms: fever (>=38°C or >=100.4°F), sore throat, malaise (loss of appetite, generally unwell, fatigue, physical weakness), headache, muscle pain (myalgia), gastrointestinal symptoms, cough, chest congestion, runny nose, wheezing, skin rash, eye irritation or discharge, chills, new or changing olfactory or taste disorders, red or bruised looking feet or toes, or shaking chills or rigors.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded.

Molecularly confirmed mild Covid-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample and one of the following signs and symptoms: fever (>=38°C or >=100.4°F), sore throat, malaise (loss of appetite, generally unwell, fatigue, physical weakness), headache, muscle pain (myalgia), gastrointestinal symptoms, cough, chest congestion, runny nose, wheezing, skin rash, eye irritation or discharge, chills, new or changing olfactory or taste disorders, red or bruised looking feet or toes, or shaking chills or rigors.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 28 in PP set were excluded.

Molecularly confirmed COVID-19 was defined as a positive SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample; and COVID-19 symptoms consistent with those defined by the US FDA harmonized case definition at the time of finalization of the study protocol: fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, diarrhea.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded.

Molecularly confirmed COVID-19 was defined as a positive SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample; and COVID-19 symptoms consistent with those defined by the US FDA harmonized case definition at the time of finalization of the study protocol: fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, diarrhea.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 28 in PP set were excluded.

BOD is a weighted version of the mild, moderate, and severe/critical vaccine efficacies and was evaluated based on the first occurrence of molecularly confirmed COVID-19, including mild, moderate or severe/critical COVID-19 case.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 14 in PP set were excluded.

BOD is a weighted version of the mild, moderate, and severe/critical vaccine efficacies and was evaluated based on the first occurrence of molecularly confirmed COVID-19, including mild, moderate or severe/critical COVID-19 case.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a positive PCR test between Day 1 and Day 28 in PP set were excluded.

Number of participants with SARS-CoV-2 seroconversion based on antibodies to nucleocapsid (N) protein using enzyme-linked immunosorbent assay (ELISA) and/or SARS-CoV- 2 immunoglobulin assay that is dependent on the SARS-CoV-2 N protein was reported.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Participants who had a COVID-19 case with an onset or discontinued before day 29 in PP set were excluded.

Number of participants with asymptomatic infection detected by RT-PCR at the time of the Month 6/unblinding visit were reported.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine.

Number of participants with first occurrence of SARS-CoV-2 infection (serologically and/or molecularly confirmed) were reported.

PP set included participants of the FAS who received double-blind study vaccine and who were seronegative at the time of double-blind vaccination and who had no other major protocol deviations that were judged to possibly impact the efficacy of the vaccine. Here "N" signifies the excluded participants who had a COVID-19 case with an onset or discontinued before Day 29.

Molecularly confirmed moderate to severe/critical COVID-19 was defined as a SARS-CoV-2 positive RT-PCR or molecular test result from any available respiratory tract sample (example, nasal swab sample, sputum sample, throat swab sample, saliva sample) or other sample. Moderate included one sign or symptom such as respiratory rate >= 20 breaths per minute and symptoms such as shortness of breath or two signs or symptoms such as heart rate >= 90 beats per minute (beats/minute) and symptoms such as cough from a list of signs and symptoms and severe/critical included one of the following signs and symptoms: respiratory rate >=30 breaths/minute, heart rate >=125 beats/minute,SpO2 <=93% on room air at sea level respiratory failure, evidence of shock, significant acute renal, hepatic, or neurologic dysfunction, admission to the ICU, death defined as per FDA guidance. Seronegative is defined as N-serology seronegative at the time of boosting or at the Year 1 visit if not boosted.

FAS included all randomized participants with a documented double-blind study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment.

An AE is any untoward medical occurrence in a participant participating in a clinical study that does not necessarily have a causal relationship with the pharmaceutical/biological agent under study. SAE is any untoward medical occurrence that at any dose may result in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, is a suspected transmission of any infectious agent via a medicinal product.

FAS included all randomized participants with a documented double-blind study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure.

Number of participants with AESIs were reported. AESIs are significant AEs that are judged to be of special interest because of clinical importance, known or suspected class effects, or based on nonclinical signals. Thrombosis with Thrombocytopenia Syndrome (TTS), a syndrome characterized by a combination of both a thrombotic event and thrombocytopenia, is considered to be an AESI in this study. A suspected TTS case is defined as: Thrombotic events: suspected deep vessel venous or arterial thrombotic events; Thrombocytopenia, defined as platelet count below 150,000/micro liter.

FAS included all randomized participants with a documented double-blind study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure (OM).

An AE is any untoward medical occurrence in a participant participating in a clinical study that does not necessarily have a causal relationship with the pharmaceutical/biological agent under study. MAAEs were defined as AEs with medically-attended visits including hospital, emergency room, urgent care clinic, or other visits to or from medical personnel for any reason.

FAS included all randomized participants with a documented double-blind study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure.

MAAEs were defined as AEs with medically-attended visits including hospital, emergency room, urgent care clinic, or other visits to or from medical personnel for any reason. Routine study visits were not considered medically-attended visits. New onset of chronic diseases was collected as part of the MAAEs.

FAS included all randomized participants with a documented double-blind study vaccine administration, regardless of the occurrence of protocol deviations and serostatus at enrollment. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure.

An AE is any untoward medical occurrence in a participant participating in a clinical study that does not necessarily have a causal relationship with the pharmaceutical/biological agent under study. Participants who were enrolled in safety subset were asked to note in the e-Diary occurrences of injection site pain/tenderness, erythema, and swelling at the study vaccine injection site daily for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Safety population is a subset of FAS for the analysis of solicited local adverse event. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure.

An AE is any untoward medical occurrence in a participant participating in a clinical study that does not necessarily have a causal relationship with pharmaceutical/biological agent under study. Participants who were enrolled in safety subset were instructed on how to record daily temperature using a thermometer provided for home use. Participants recorded the temperature in the e-Diary in the evening of the day of vaccination, and then daily for the next 7 days approximately at the same time each day. If more than 1 measurement was made on any given day, the highest temperature of that day was recorded in the e-Diary. Fever was defined as endogenous elevation of body temperature >= 38.0 degree Celsius or >=100.4-degree Fahrenheit, as recorded in at least 1 measurement. Participants also noted the signs and symptoms in the e-Diary on a daily basis for 7 days post vaccination (day of vaccination and the subsequent 7 days), for the following events: fatigue, headache, nausea, myalgia.

Safety population is a subset of FAS for the analysis of solicited systemic adverse event. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure.

An AE is any untoward medical occurrence in a participant participating in a clinical study that does not necessarily have a causal relationship with the pharmaceutical/biological agent under study. Unsolicited AEs were all AEs for which the participant is not specifically questioned in the participant diary.

Safety population is a subset of FAS for the analysis of unsolicited adverse event. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure.

Binding antibodies to SARS-CoV-2 S protein as assessed by enzyme-linked immunosorbent assay (ELISA) to measure humoral immune response was reported. The lower limit of quantification (LLOQ) and upper limit of quantification (ULOQ) were 50.3 EU/mL and 58,158.10 EU/mL, respectively. A sample was considered positive if the value was strictly greater than the LLOQ (>LLOQ).

PPI set included all randomized and vaccinated participants, including those who were part of the immunogenicity subset and for whom immunogenicity data were available, excluding participants with major protocol deviations expected to impact the immunogenicity outcomes. Here N (Overall number of participants analyzed) signifies participants evaluated for this outcome measure and 'n' (number analyzed) signifies number of participants evaluable at specified time points.

Number of participants with antibody titers to Ad26.COV2.S to measure immune response were reported.

PPI booster analysis set included all participants who received an Ad26.COV2.S booster dose (Year 1(Week 52)/Booster Visit) and who had been selected for inclusion in homologous or heterologous booster subset as of protocol amendment 6. This OM was planned to be analyzed based on homologous and heterologous booster groups. Here, "0" participants in "overall number of participants analyzed" field indicated that data for this OM was not collected and analyzed due to change in planned analysis.

Number of participants with binding antibodies to SARS- CoV-2S protein as measured by ELISA was reported.

Per protocol booster immunogenicity analysis set included all participants who received an Ad26.COV2.S booster dose (Year 1 [Week 52]/Booster Visit) and who had been selected for inclusion in the homologous or heterologous booster subset as of protocol amendment 6. Here, 'N' (number of participants analyzed) signifies participants evaluable for this outcome measure. As planned, this outcome measure was analyzed based on homologous and heterologous booster groups.