Participants(healthy adults aged >=18 to <=55 years)will receive Ad26.COV2.S in primary regimen or matching Placebo on Day 1 and 57,followed by booster vaccination at 8 or 14 months (that is, 6 or 12 months after completion of primary regimen)with same dose or matching Placebo.At unblinding visit,post EUA,conditional licensure,or approval for single dose regimen of Ad26.COV2.S vaccine,participants initially receiving placebo will be offered to receive single dose of Ad26.COV2.S vaccine and who are not willing to receive single dose of Ad26.COV2.S vaccine will continue to receive booster vaccination.All eligible participants who have previously received any COVID-19 vaccination(as primary regimen or additional dose)if last vaccination was >= 6 months ago,will be offered to receive single ad hoc booster dose of Ad26.COV2.S and who are not willing to receive single ad hoc booster dose of Ad26.COV2.S or are not eligible for ad hoc booster dose will continue to receive booster vaccination.

Participants (good or stable health adults aged >=65 years) will receive Ad26.COV2.S at 2 dose levels, as a single dose or 2 dose schedule with an 8-week interval or matching Placebo on Day 1 and Day 57. At unblinding visit, post EUA, conditional licensure, or approval for the single dose regimen of Ad26.COV2.S vaccine, participants initially receiving placebo will be offered to receive a single dose of Ad26.COV2.S. If they choose not to receive Ad26.COV2.S they will be asked to continue to be followed in this study. All eligible participants who have previously received any COVID-19 vaccination (as primary regimen or additional dose) if the last vaccination was >=6 months ago will be offered to receive a single ad hoc booster dose of Ad26.COV2.S. If they choose not to receive ad-hoc booster dose or if they are not eligible for ad-hoc booster dose, then they will be asked to continue to be followed in this study.

Participants will receive Placebo.

Number of participants with solicited local AEs for 7 days after vaccination 1 in Cohorts 2a and 2b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after booster vaccination 1 in Cohort 2a were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after booster vaccination 2 in Cohort 2a were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after ad hoc booster vaccination in Cohort 2a were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after Vaccination 2 in Cohort 2b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after booster vaccination 1 in Cohort 2b were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after booster Vaccination 2 in Cohort 2b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited local AEs for 7 days after ad hoc booster vaccination in Cohort 2b were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

Number of participants with solicited systemic AEs for 7 days after vaccination 1 in Cohorts 1a and 1b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after vaccination 2 in Cohorts 1a and 1b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after ad hoc booster vaccination in Cohorts 1a and 1b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post Ad Hoc Booster Vaccination (day of Ad hoc booster vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after vaccination 1 in Cohorts 2a and 2b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days post booster vaccination 1 in Cohort 2a were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after booster vaccination 2 in Cohort 2a were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after ad hoc booster vaccination in Cohort 2a were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post adhoc booster vaccination (day of ad hoc booster vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after vaccination 2 in Cohort 2b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after booster vaccination 1 in Cohort 2b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after booster vaccination 2 in Cohort 2b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited systemic AEs for 7 days after ad hoc booster vaccination in Cohort 2b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post ad hoc booster vaccination (day of ad hoc booster vaccination and the subsequent 7 days).

Number of participants with solicited local AEs for 7 days after vaccination 1 in Cohort 3 were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, were used to assess the reactogenicity of the study vaccine and were pre-defined local (injection site).

Number of participants with solicited systemic AEs for 7 days after vaccination 1 in Cohort 3 were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited local AEs for 7 days after vaccination 2 in Cohort 3 were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, were used to assess the reactogenicity of the study vaccine and were pre-defined local (injection site).

Number of participants with solicited systemic AEs for 7 days after vaccination 2 in Cohort 3 were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

Number of participants with solicited local AEs for 7 days after ad hoc booster vaccination in Cohort 3 were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, were used to assess the reactogenicity of the study vaccine and were pre-defined local (injection site).

Number of participants with solicited systemic AEs for 7 days after ad hoc booster vaccination in Cohort 3 were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post ad hoc booster vaccination (day of ad hoc booster vaccination and the subsequent 7 days).

Number of participants with unsolicited AEs after vaccination 1 in Cohorts 1a and 1b were reported. An AE was 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.

Number of participants with unsolicited AEs after vaccination 2 in Cohorts 1a and 1b were reported. An AE was 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.

Number of participants with unsolicited AEs after ad hoc booster vaccination in Cohorts 1a and 1b were reported. An AE was 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.

Number of participants with unsolicited AEs after vaccination 1 in Cohorts 2a and 2b were reported. An AE was 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.

Number of participants with unsolicited AEs after booster 1 vaccination in Cohort 2a were reported. An AE was 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.

Number of participants with unsolicited AEs after booster vaccination 2 in Cohort 2a were reported. An AE was 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.

Number of participants with unsolicited AEs after ad hoc booster vaccination in Cohort 2a were reported. An AE was 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.

Number of participants with unsolicited AEs after vaccination 1 in Cohort 2b were reported. An AE was 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.

Number of participants with unsolicited AEs 28 days after booster vaccination 1 in Cohort 2b were reported. An AE was 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.

Number of participants with unsolicited AEs 28 days after booster vaccination 2 in Cohort 2b were reported. An AE was 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.

Number of participants with unsolicited AEs 28 days after ad hoc booster vaccination in Cohort 2b were reported. An AE was 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.

Number of participants with unsolicited AEs 28 days after vaccination 1 in Cohort 3 were reported. An AE was 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.

Number of participants with unsolicited AEs 28 days after vaccination 2 in Cohort 3 were reported. An AE was 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.

Number of participants with unsolicited AEs 28 days after ad hoc booster vaccination in Cohort 3 were reported. An AE was 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.

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. An SAE is an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly/birth defect; suspected transmission of any infectious agent via a medicinal product or medically important.

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. An SAE is an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly/birth defect; suspected transmission of any infectious agent via a medicinal product or medically important.

Number of participants with AESIs was reported. AESIs were significant AEs that were 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, was considered to be an AESI in this study. A suspected TTS case was defined as: Thrombotic events: suspected deep vessel venous or arterial thrombotic events; Thrombocytopenia, defined as platelet count below 150,000/microliter.

Number of participants with AESIs was reported. AESIs were significant AEs that were 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, was considered to be an AESI in this study. A suspected TTS case was defined as: Thrombotic events: suspected deep vessel venous or arterial thrombotic events; Thrombocytopenia, defined as platelet count below 150,000/microliter.

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

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

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

GMTs of SARS-CoV-2 neutralizing antibodies to the Wild-type VNA were reported.

GMTs of SARS-CoV-2 neutralizing antibodies to the Wild-type VNA were reported.

GMTs of SARS-CoV-2 neutralizing antibodies to the Wild-type VNA were reported.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell Responses for IFNg+ or IL2+ not Helper cell type 2 (TH2) was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell Responses for IFNg+ or IL2+ not Helper cell type 2 (TH2) was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell Responses for IFNg+ or IL2+ not Helper cell type 2 (TH2) was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell Responses for IFNg+ or IL2+ not Helper cell type 2 (TH2) was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell responses for IL4+ and/or (IL5+/IL13+) and CD40L+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell responses for IL4+ and/or (IL5+/IL13+) and CD40L+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell responses for IL4+ and/or (IL5+/IL13+) and CD40L+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD4+ T-cell responses for IL4+ and/or (IL5+/IL13+) and CD40L+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD8+ T-cell Responses for IFNg+ or IL2+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD8+ T-cell Responses for IFNg+ or IL2+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD8+ T-cell Responses for IFNg+ or IL2+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with SARS-Cov2 S Specific CD8+ T-cell Responses for IFNg+ or IL2+ was reported. Cellular immunogenicity was measured by intracellular cytokine staining (ICS), allowing characterization of individual CD4 and CD8 T cell immune responses to vaccination.

Percentage of participants with Th1 (IFN-g OR IL2 NOT TH2) /Th2 (IL4 OR IL5 OR IL13 AND CD40L) ratio >=1 and <1 was reported.

Percentage of participants with Th1 (IFN-g OR IL2 NOT TH2) /Th2 (IL4 OR IL5 OR IL13 AND CD40L) ratio >=1 and <1 was reported.

Percentage of participants with Th1 (IFN-g OR IL2 NOT TH2) /Th2 (IL4 OR IL5 OR IL13 AND CD40L) ratio >=1 and <1 was reported.

Percentage of participant with Th1 (IFN-g OR IL2 NOT TH2) /Th2 (IL4 OR IL5 OR IL13 AND CD40L) ratio >=1 and <1 was reported.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (vaccination1) and 57 (Vaccination2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and a matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (vaccination1) and 57 (Vaccination2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Group 1 and Group 4 healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) followed by matching placebo at 6 and 12 months as matching Booster (B) 1 and Booster 2 vaccination. As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S. 5*10^10 vp >=6 months after Vaccination 2.

Group 2 healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) followed by first booster vaccination with single IM injection of Ad26.COV2.S 5*10^10 vp booster 1 at 6 months and a matching placebo at 12 months to match second Booster vaccination.

Group 3 healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) followed by a matching placebo booster 1 injection after 6 months and Ad26.COV2.S 5*10^10 vp after 12 months as Booster 2.

Group 5 healthy adult participants received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (Vaccination 1), 6 months (Booster 1) and 12 months (Booster 2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Group 1 and 4 healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2) and matching placebo at 6 and 12 months after vaccination 2 as Booster 1 and Booster 2 vaccines. As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad hoc booster dose of 5*10^10 vp Ad26.COV2.S. >=6 months after Vaccination 2.

Group 2 healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1), Day 57 (Vaccination 2) and 6 months after Vaccination 2(Booster 1). At 12 months after vaccination 2, participants received placebo matching to Ad26.COV2.S vaccine (Booster 2).

Group 3 healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2) and matching placebo at 6 months after vaccination 2 as Booster 1 vaccination and Ad26.COV2.S 5*10^10 vp at 12 months after vaccination 2 as Booster 2 vaccination.

Group 5 healthy adult participants aged >=18 to <=55 received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (Vaccination 1), Day 57 (Vaccination 2), 8 Month (Booster 1) and 14 months (Booster 2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Adult participants (with good or stable health) aged >=65 years received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad hoc booster dose of 5*10^10 vp Ad26.COV2.S. >=6 months after Vaccination 2.

Adult participants (with good or stable health) aged >=65 years received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad hoc booster dose of 5*10^10 vp Ad26.COV2.S. >=6 months after Vaccination 2.

Adult participants (with good or stable health) aged >=65 years received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad hoc booster dose of 5*10^10 vp Ad26.COV2.S. >=6 months after Vaccination 2.

Adult participants (with good or stable health) aged >=65 years received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad hoc booster dose of 5*10^10 vp Ad26.COV2.S. >=6 months after Vaccination 2.

Adult participants (with good or stable health) aged >=65 years received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (vaccination1) and 57 (Vaccination2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Total of all reporting groups

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single ad AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (vaccination1) and 57 (Vaccination2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 5*10^10 vp on Day 1 (Vaccination 1) and a matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of Ad26.COV2.S 1*10^11 vp on Day 1 (Vaccination 1) and matching placebo on Day 57 (Vaccination 2). As per protocol amendment 15, all eligible participants who had previously received 1 or more doses of any COVID-19 vaccine were offered a single AHBV of Ad26.COV2.S at the dose of 5*10^10 vp >=6 months after Vaccination 2.

Healthy adult participants aged >=18 to <=55 received a single IM injection of placebo matching to Ad26.COV2.S vaccine on Day 1 (vaccination1) and 57 (Vaccination2). As per protocol amendment 10, after unblinding, enrolled participants who had initially received placebo were offered a single dose of 5*10^10 vp Ad26.COV2.S.

Number of participants with solicited local AEs for 7 days after vaccination 2 in Cohorts 1a and 1b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure.

Number of participants with solicited local AEs for 7 days after ad hoc booster vaccination in Cohorts 1a and 1b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure. Here, 0 in the "Overall Number of Participants Analyzed" field signifies that no participants were available for the analysis because none of the participants received ad hoc booster vaccination in the respective arms.

Number of participants with solicited local AEs for 7 days after vaccination 1 in Cohorts 2a and 2b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented.

Number of participants with solicited local AEs for 7 days after booster vaccination 1 in Cohort 2a were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure.

Number of participants with solicited local AEs for 7 days after booster vaccination 2 in Cohort 2a were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure. Here, 0 in the "Overall Number of Participants Analyzed" field signifies that no participants were available for the analysis because none of the participants received booster vaccination 2 in the respective arm.

Number of participants with solicited local AEs for 7 days after ad hoc booster vaccination in Cohort 2a were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who received Ad Hoc booster vaccination and were evaluable for this outcome measure. Here, 0 in the "Overall Number of Participants Analyzed" field signifies that no participants were available for the analysis because none of the participants received ad hoc booster vaccination in the respective arms.

Number of participants with solicited local AEs for 7 days after Vaccination 2 in Cohort 2b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure.

Number of participants with solicited local AEs for 7 days after booster vaccination 1 in Cohort 2b were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure. Here, 0 in the "Overall Number of Participants Analyzed" field signifies that no participants were available for the analysis because none of the participants received booster vaccination 1 in the respective arm.

Number of participants with solicited local AEs for 7 days after booster Vaccination 2 in Cohort 2b were reported. 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure. Here, 0 in the "Overall Number of Participants Analyzed" field signifies that no participants were available for the analysis because none of the participants received booster vaccination 2 in the respective arms.

Number of participants with solicited local AEs for 7 days after ad hoc booster vaccination in Cohort 2b were reported. An AE was 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. Solicited local (injection site) AEs that included injection site pain/tenderness, erythema and swelling at the study vaccine injection site, are used to assess the reactogenicity of the study vaccine and are pre-defined local (injection site).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who received Ad Hoc booster vaccination and were evaluable for this outcome measure. Here, 0 in the "Overall Number of Participants Analyzed" field signifies that no participants were available for the analysis because none of the participants received ad hoc booster vaccination in the respective arms.

Number of participants with solicited systemic AEs for 7 days after vaccination 1 in Cohorts 1a and 1b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

FAS included all participants with at least one vaccine administration documented.

Number of participants with solicited systemic AEs for 7 days after vaccination 2 in Cohorts 1a and 1b were reported. An AE was 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. Systemic events included events such as fatigue, headache, nausea, and myalgia, for which participants were specifically questioned and which will be noted by participants in their participant diary for 7 days post-vaccination (day of vaccination and the subsequent 7 days).

FAS included all participants with at least one vaccine administration documented. Here, 'N' (Overall number of participants analyzed) signifies participants who were evaluable for this outcome measure.