Current participant inclusion criteria as of 25/05/2022: For Cohort A: 1. Aged 12 to 16 years (inclusive) at enrolment For Cohort B: 1. Aged 12 to 15.5 years (inclusive) at enrolment 2. Already received two doses of 30 µg BNT162b2, the second dose received at least 91 days prior to randomisation For Cohorts A and B: 1. In good health as determined by a trial clinician. Participants may have well-controlled or mild to moderate comorbidity, as long as this would not render them considered as belonging to a 'high-risk' cohort at particular need of additional doses of COVID-19 2. Able and willing (in the Investigator's opinion) to comply with all study requirements 3. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures 4. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial. Parent/legal guardian to provide informed consent for participants under the age of 16. Participants aged 16 years will be assumed to be able to provide consent for themselves, however parental support will be encouraged and investigators will reserve the right to refuse enrolment if concerns arise. _____ Previous participant inclusion criteria as of 19/11/2021: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them considered as belonging to a ‘high -risk’ cohort at particular need of 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures _____ Previous participant inclusion criteria: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them eligible for 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures

Current participant inclusion criteria as of 25/05/2022: For Cohort A: 1. Aged 12 to 16 years (inclusive) at enrolment For Cohort B: 1. Aged 12 to 15.5 years (inclusive) at enrolment 2. Already received two doses of 30 µg BNT162b2, the second dose received at least 91 days prior to randomisation For Cohorts A and B: 1. In good health as determined by a trial clinician. Participants may have well-controlled or mild to moderate comorbidity, as long as this would not render them considered as belonging to a 'high-risk' cohort at particular need of additional doses of COVID-19 2. Able and willing (in the Investigator's opinion) to comply with all study requirements 3. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures 4. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial. Parent/legal guardian to provide informed consent for participants under the age of 16. Participants aged 16 years will be assumed to be able to provide consent for themselves, however parental support will be encouraged and investigators will reserve the right to refuse enrolment if concerns arise. _____ Previous participant inclusion criteria as of 19/11/2021: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them considered as belonging to a ‘high -risk’ cohort at particular need of 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures _____ Previous participant inclusion criteria: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them eligible for 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures

Current participant inclusion criteria as of 19/11/2021: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them considered as belonging to a ‘high -risk’ cohort at particular need of 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures Previous participant inclusion criteria: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them eligible for 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures

Current participant inclusion criteria as of 19/11/2021: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them considered as belonging to a ‘high -risk’ cohort at particular need of 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures Previous participant inclusion criteria: 1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them eligible for 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures

1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them eligible for 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures

1. Parent/legal guardian/participant is willing and able to give written informed consent for participation in the trial 2. Aged 12 to 16 years inclusive at enrolment 3. In good health as determined by a trial clinician. Participants may have well controlled or mild-moderate comorbidity, as long as this would not render them eligible for 2 doses of COVID-19 vaccine (see exclusion criteria below) as part of the national roll out 4. Able and willing (in the Investigator’s opinion) to comply with all study requirements 5. Registered with a GP, and willing to allow the investigators to discuss the participant’s medical history with their General Practitioner and access all medical records when relevant to study procedures