Number of Participants With Adverse Events (AEs) Post Each Dose of Study Intervention;Number of Participants With Binary Response;Number of Participants With Local and Systemic Solicited AEs in the Substudy Only;Number of Participants With Serious Adverse Events (SAE), Medically Attended Adverse Events (MAAE), and Adverse Event of Special Interest (AESI) Prior to Non-study COVID-19 Vaccination

Number of Participants With Adverse Events (AEs) Post Each Dose of Study Intervention;Number of Participants With Binary Response;Number of Participants With Local and Systemic Solicited AEs in the Substudy Only;Number of Participants With Serious Adverse Events (SAE), Medically Attended Adverse Events (MAAE), and Adverse Event of Special Interest (AESI) Prior to Non-study COVID-19 Vaccination

Number of Participants Achieving Binary Response;Number of Participants With Adverse Events (AEs) Occurring Post Each Dose of Study Intervention;Number of Participants With Local and Systemic Solicited AEs in the Substudy Only;Number of Participants With Serious Adverse Events (SAE), Medically Attended Adverse Events (MAAE), and Adverse Event of Special Interest (AESI) Occurring Throughout the Study

Number of Participants Achieving Binary Response;Number of Participants With Adverse Events (AEs) Occurring Post Each Dose of Study Intervention;Number of Participants With Local and Systemic Solicited AEs in the Substudy Only;Number of Participants With Serious Adverse Events (SAE), Medically Attended Adverse Events (MAAE), and Adverse Event of Special Interest (AESI) Occurring Throughout the Study

The efficacy of 2 IM doses of AZD1222 compared to saline placebo for the prevention of COVID-19;The reactogenicity of 2 IM doses of AZD1222 compared to saline placebo (Substudy only);The safety and tolerability of 2 IM doses of AZD1222 compared to saline placebo

The efficacy of 2 IM doses of AZD1222 compared to saline placebo for the prevention of COVID-19;The reactogenicity of 2 IM doses of AZD1222 compared to saline placebo (Substudy only);The safety and tolerability of 2 IM doses of AZD1222 compared to saline placebo

The efficacy of 2 IM doses of AZD1222 compared to placebo for the prevention of COVID-19 in adults ≥ 18 years of age;The safety and tolerability of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age;The reactogenicity of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age (Substudy only)

The efficacy of 2 IM doses of AZD1222 compared to placebo for the prevention of COVID-19 in adults ≥ 18 years of age;The safety and tolerability of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age;The reactogenicity of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age (Substudy only)

To assess the safety and tolerability of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age;To estimate the efficacy of 2 IM doses of AZD1222 compared to placebo for the prevention of COVID-19 in adults ≥ 18 years of age;To assess the reactogenicity of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age (Substudy only)

To assess the safety and tolerability of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age;To estimate the efficacy of 2 IM doses of AZD1222 compared to placebo for the prevention of COVID-19 in adults ≥ 18 years of age;To assess the reactogenicity of 2 IM doses of AZD1222 compared to placebo in adults ≥ 18 years of age (Substudy only)