Substudy A  Inclusion Criteria:  Male or female participants ≥16 years of age at Visit 1 (Day 1) who participated in C4591001. Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. Healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. Capable of giving signed informed consent. Participants who have received 2 prior doses of 30 µg BNT162b2 19-42 days apart, with the second dose being at least 175 days before Visit 1 (Day 1).  Exclusion Criteria:  Other medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (eg, anaphylaxis) to any component of the study intervention. Previous clinical (based on COVID-19 symptoms/signs alone, if a SARS-CoV-2 NAAT result was not available) or microbiological (based on COVID-19 symptoms/signs and a positive SARS-CoV-2 NAAT result) diagnosis of COVID-19. Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination. Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection. Women who are pregnant or breastfeeding. Individuals who receive treatment with radiotherapy or immunosuppressive therapy, including cytotoxic agents or systemic corticosteroids, eg, for cancer or an autoimmune disease, or planned receipt throughout the study. Receipt of blood/plasma products, immunoglobulin, or monoclonal antibodies, from 60 days before study intervention administration, or receipt of any passive antibody therapy specific to COVID-19, from 90 days before study intervention administration, or planned receipt throughout the study. Prior receipt of any COVID-19 vaccine other than BNT162b2. Investigator site staff or Pfizer/BioNTech employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members. Receipt of medications intended to prevent COVID-19. Prior receipt of more than 2 doses of BNT162b2 30 µg. Participation in other studies involving study intervention within 28 days prior to study entry, other than C4591001, and/or within 28 days of confirmed receipt of BNT162b2 within the study.  Substudy B  Inclusion Criteria:  Male or female participants:  18 to 30 years of age, inclusive, who have received 2 prior doses of 30 µg BNT162b2 19 to 60 days apart, with the second dose being at least 175 days before Visit 1 (Day 1) or 12 to 17 years of age, inclusive, who have received 2 prior doses of 30 µg BNT162b2 19 to 60 days apart, with the second dose being at least 365 days before Visit 1 (Day 1). Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. Healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. Capable of giving signed informed consent.  Exclusion Criteria:  Other medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (eg, anaphylaxis) to any component of the study intervention. Previous clinical (based on COVID-19 symptoms/signs alone, if a SARS-CoV-2 NAAT result was not available) or microbiological (based on COVID-19 symptoms/signs and a positive SARS-CoV-2 NAAT result) diagnosis of COVID-19. Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination. Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection. Women who are pregnant or breastfeeding. Individuals who receive treatment with radiotherapy or immunosuppressive therapy, including cytotoxic agents or systemic corticosteroids, eg, for cancer or an autoimmune disease, or planned receipt throughout the study. Receipt of blood/plasma products, immunoglobulin, or monoclonal antibodies, from 60 days before study intervention administration, or receipt of any passive antibody therapy specific to COVID-19, from 90 days before study intervention administration, or planned receipt throughout the study. Prior receipt of any COVID-19 vaccine other than BNT162b2. Investigator site staff or Pfizer/BioNTech employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members. Receipt of medications intended to prevent COVID-19. Prior receipt of more than 2 doses of BNT162b2 30 µg. Participation in other studies involving study intervention within 28 days prior to study entry through and including 28 days after the last dose of study intervention, with the exception of non-Pfizer interventional studies for prevention of COVID-19, which are prohibited throughout study participation.  Substudy C  Inclusion Criteria:  Male or female participants:  ≥18 years of age, inclusive, who have received 2 prior doses of 30 µg BNT162b2 19 to 60 days apart, with the second dose being at least 175 days before Visit 301 (Day 1) or 12 to 17 years of age, inclusive, who have received 2 prior doses of 30 µg BNT162b2 19 to 60 days apart in Study C4591001, with the second dose being at least 365 days before Visit 301 (Day 1). Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures. Healthy participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion in the study. Capable of giving signed informed consent.  Exclusion Criteria:  Other medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. History of severe adverse reaction associated with a vaccine and/or severe allergic reaction (eg, anaphylaxis) to any component of the study intervention. Previous clinical (based on COVID-19 symptoms/signs alone, if a SARS-CoV-2 NAAT result was not available) or microbiological (based on COVID-19 symptoms/signs and a positive SARS-CoV-2 NAAT result) diagnosis of COVID-19. Immunocompromised individuals with known or suspected immunodeficiency, as determined by history and/or laboratory/physical examination. Bleedin doses of 30 µg BNT162b2 19-42 days apart, with the second dose being at least 175 days before Visit 1 (Day 1). g diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection. Women who are pregnant or breastfeeding. Individuals who receive treatment with radiotherapy or immunosuppressive therapy, including cytotoxic agents or systemic corticosteroids, eg, for cancer or an autoimmune disease, or planned receipt throughout the study. Receipt of blood/plasma products, immunoglobulin, or monoclonal antibodies, from 60 days before study intervention administration, or receipt of any passive antibody therapy specific to COVID-19, from 90 days before study intervention administration, or planned receipt throughout the study. Prior receipt of any COVID-19 vaccine other than BNT162b2. Investigator site staff or Pfizer/BioNTech employees directly involved in the conduct of the study, site staff otherwise supervised by the investigator, and their respective family members. Receipt of medications intended to prevent COVID-19. Prior receipt of more than 2 doses of BNT162b2 30 µg. Participation in other studies involving study intervention within 28 days prior to study entry through and including 28 days after the last dose of study intervention, with the exception of non-Pfizer interventional studies for prevention of COVID-19, which are prohibited throughout study participation..