Current participant exclusion criteria as of 25/05/2022: For Cohort A: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 For Cohort B: 1. Previous receipt of more than two doses of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 30 µg 2. Participants who received their second dose of COVID-19 vaccine in Cohort A of this study are not eligible to enrol in Cohort B For Cohorts A and B: 1. Belonging to a 'high- risk' cohort advised to receive additional doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and 'Green Book' guidelines current on 28/02/2022). 2. First-degree relative of a study site staff member 3. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 4. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG). 5. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post study vaccination (for post-menarcheal females only) 6. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent 7. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 8. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 9. Any serious chronic illness requiring hospital specialist supervision 10. Congenital cardiovascular conditions 11. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well-controlled comorbidities are allowed) 12. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 13. Significant renal or hepatic impairment 14. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 15. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 16. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 17. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 18. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion f

Current participant exclusion criteria as of 25/05/2022: For Cohort A: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 For Cohort B: 1. Previous receipt of more than two doses of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 30 µg 2. Participants who received their second dose of COVID-19 vaccine in Cohort A of this study are not eligible to enrol in Cohort B For Cohorts A and B: 1. Belonging to a 'high- risk' cohort advised to receive additional doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and 'Green Book' guidelines current on 28/02/2022). 2. First-degree relative of a study site staff member 3. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 4. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG). 5. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post study vaccination (for post-menarcheal females only) 6. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent 7. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 8. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 9. Any serious chronic illness requiring hospital specialist supervision 10. Congenital cardiovascular conditions 11. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well-controlled comorbidities are allowed) 12. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 13. Significant renal or hepatic impairment 14. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 15. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 16. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 17. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 18. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion f

Current participant exclusion criteria as of 19/11/2021: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a ‘high risk’ cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book' guidelines current as of 13th September 2021). 3. First degree relative of a study site staff member 4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG) 6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only 7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 10. Any serious chronic illness requiring hospital specialist supervision 11. Congenital cardiovascular conditions 12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed) 13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 14. Significant renal or hepatic impairment 15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the study Previous participant exclusion criteria: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a cohort advised to receive 2 doses of a COVID-19 vacc

Current participant exclusion criteria as of 19/11/2021: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a ‘high risk’ cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book' guidelines current as of 13th September 2021). 3. First degree relative of a study site staff member 4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG) 6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only 7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 10. Any serious chronic illness requiring hospital specialist supervision 11. Congenital cardiovascular conditions 12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed) 13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 14. Significant renal or hepatic impairment 15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the study Previous participant exclusion criteria: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a cohort advised to receive 2 doses of a COVID-19 vacc

Current participant exclusion criteria as of 19/11/2021:1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b22. Belonging to a ‘high risk’ cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book' guidelines current as of 13th September 2021).3. First degree relative of a study site staff member4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days)5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG)6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban)10. Any serious chronic illness requiring hospital specialist supervision11. Congenital cardiovascular conditions12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed)13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis)14. Significant renal or hepatic impairment15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the studyPrevious participant exclusion criteria:1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b22. Belonging to a cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book guidelines).3. First degree relative of a study site staff member4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months,

Current participant exclusion criteria as of 19/11/2021:1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b22. Belonging to a ‘high risk’ cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book' guidelines current as of 13th September 2021).3. First degree relative of a study site staff member4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days)5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG)6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban)10. Any serious chronic illness requiring hospital specialist supervision11. Congenital cardiovascular conditions12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed)13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis)14. Significant renal or hepatic impairment15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the studyPrevious participant exclusion criteria:1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b22. Belonging to a cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book guidelines).3. First degree relative of a study site staff member4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months,

Current participant exclusion criteria as of 19/11/2021: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a ‘high risk’ cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book' guidelines current as of 13th September 2021). 3. First degree relative of a study site staff member 4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG) 6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only 7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 10. Any serious chronic illness requiring hospital specialist supervision 11. Congenital cardiovascular conditions 12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed) 13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 14. Significant renal or hepatic impairment 15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the study Previous participant exclusion criteria: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a cohort advised to receive 2 doses of a COVID-19 vacc

Current participant exclusion criteria as of 19/11/2021: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a ‘high risk’ cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book' guidelines current as of 13th September 2021). 3. First degree relative of a study site staff member 4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG) 6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only 7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 10. Any serious chronic illness requiring hospital specialist supervision 11. Congenital cardiovascular conditions 12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed) 13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 14. Significant renal or hepatic impairment 15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the study Previous participant exclusion criteria: 1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a cohort advised to receive 2 doses of a COVID-19 vacc

1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book guidelines). 3. First degree relative of a study site staff member 4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG) 6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only 7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 10. Any serious chronic illness requiring hospital specialist supervision 11. Congenital cardiovascular conditions 12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed) 13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 14. Significant renal or hepatic impairment 15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the study

1. Previous receipt of more than one dose of a COVID-19 vaccine, or a COVID-19 vaccine other than BNT162b2 2. Belonging to a cohort advised to receive 2 doses of a COVID-19 vaccine (participants at increased risk of COVID-19, or household contacts of immunocompromised, based on JCVI and ‘Green Book guidelines). 3. First degree relative of a study site staff member 4. Any confirmed or suspected immunosuppressive or immunodeficient state, asplenia, recurrent severe infections and use of immunosuppressant medication within the past 6 months, except topical steroids or short-term oral steroids (course lasting =14 days) 5. History of anaphylaxis, allergic disease or reactions likely to be exacerbated by any component of study vaccines (e.g. hypersensitivity to the active substance or any of the SmPC/IB-listed ingredients of any study vaccine). This includes latex and polyethylene glycol/macrogol (PEG) 6. Pregnancy, lactation or unwillingness to practice effective contraception from enrolment to 3 months post booster vaccination, for post-menarcheal females only 7. Malignancy requiring receipt of immunosuppressive chemotherapy or radiotherapy for treatment of solid organ cancer/haematological malignancy within the 6 months prior to consent. 8. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following IM injections or venepuncture 9. Continuous use of anticoagulants, such as coumarins and related anticoagulants (i.e. warfarin) or novel oral anticoagulants (i.e. apixaban, rivaroxaban, dabigatran and edoxaban) 10. Any serious chronic illness requiring hospital specialist supervision 11. Congenital cardiovascular conditions 12. Severe and/or uncontrolled respiratory disease, gastrointestinal disease, liver disease, renal disease, rheumatological disease, and neurological illness (mild/moderate well controlled comorbidities are allowed) 13. History of active or previous auto-immune neurological disorders (e.g. multiple sclerosis, Guillain-Barre syndrome, transverse myelitis) 14. Significant renal or hepatic impairment 15. Scheduled elective surgery requiring overnight admission and/or general anaesthetic during the trial 16. Insufficient level of English language to undertake all study requirements in opinion of the Investigators 17. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines 18. Participants who have participated in another research trial involving an investigational product in the past 12 weeks 19. Note that a prior history of confirmed or suspected COVID-19 is NOT an exclusion criterion for this study, provided the participant otherwise satisfies the health screening criteria for the study