To Evaluate the Safety, Tolerability, Efficacy and Immunogenicity of BNT162b2 Boosting Strategies Against COVID-19 in Participants ≥12 Years of Age.

Substudy A: The study will evaluate the safety, tolerability, and efficacy of a booster dose of BNT162b2 when administered to participants having previously received 2 doses of BNT162b2 at least 6 months prior to randomization. The study is designed to describe vaccine efficacy of a booster dose of BNT162b2 over time against COVID-19 * At a dose of 30µg (as studied in the Phase 2/3 study C4591001) * In healthy adults 16 years of age and older * The duration of the study for each participant will be up to approximately 12 months. * The study will be conducted in the United States, Brazil and South Africa Substudy B: The study will assess the safety and tolerability of a single dose of BNT162b2 as compared to placebo control, through the potential analysis of serum troponin levels, in participants ≥12 and ≤30 years of age who have received 2 or 3 prior doses of BNT162b2 (30-µg doses) with their last dose at least 4 months (120 days) prior to randomization. * Blood samples will be collected for troponin testing * The duration of the study for each participant will be up to approximately 2 months. * The study will be conducted in the United States, Germany, Poland and South Africa Substudy C: The study will assess the safety, tolerability, and immunogenicity of a booster (third) dose of BNT162b2 at doses of 10 µg or 30 µg in participants who have completed a 2-dose primary series of BNT162b2 (30 µg doses) at least 5 months (150 days) prior to randomization. * In healthy adults 12 years of age and older * The duration of the study for each participant will be up to approximately 12 months. * The study will be conducted in the United States, Germany and South Africa Substudy D: The study will assess the safety, tolerability, and immunogenicity of a 2-dose primary series of BNT162b2 OMI, and as a booster (third, fourth or fifth) dose * Participants in Cohort 1 will have completed a 2-dose primary series of BNT162b2 (30-µg doses), with their last dose 90 to 240 days prior to enrolment * Participants in Cohort 2 will be enrolled from Study C4591001 and C4591031 Substudy A and will have completed a 2-dose primary series and received a single booster (third) dose of BNT162b2, with their last dose 90 to 180 days prior to randomization * Participants in Cohort 3 who are COVID-19 vaccine-naïve and have not experienced COVID-19 will be enrolled to receive 2 doses (primary series) of BNT162b2 OMI, 3 weeks apart, with a dose of BNT162b2 approximately 5 months (150 days) later. If participants do not consent to receive BNT162b2 as a third dose, they will not receive a third dose. No participants should receive BNT162b2 OMI as a third dose. * In healthy adults 18 to 55 years of age * The duration of the study for each participant will be up to approximately 12 months. * The study will be conducted in the United States and South Africa Substudy E: This study will assess the safety, tolerability, and immunogenicity of high-dose BNT162b2 (60 µg), high-dose BNT162b2 OMI (60 µg), and a high-dose combination of BNT162b2 and BNT162b2 OMI at 60 µg (30 µg each), given as a single dose * In healthy adults 18 years of age and older who have received 3 prior doses of BNT162b2 (30 µg) with the most recent dose being 5 to 12 months (150 to 360 days) prior to randomization * The duration of the study for each participant will be approximately 6 months. * The study will be conducted in the United States Substudy F: This study will assess the safety, tolerability, and immunogenicity of high-dose BNT162b2 (60 µg), high-dose BNT162b2 OMI (60 µg), and a high-dose combination of BNT162b2 and BNT162b2 OMI at 60 µg (30 µg each), given as a single dose. * In healthy adults 60 years of age and older who have received 3 prior doses of BNT162b2 (30 µg) with the most recent dose being ≥4 months prior to randomization * The duration of the study for each participant will be approximately 6 months. * The study will be conducted in Israel