1. Participant is willing and able to give written informed consent for participation in the trial 2. Male or female, aged 50 years or above and in good health as determined by a trial clinician. Participants may have well-controlled or mild-moderate comorbidity 3. Has received one dose of the prime/boost schedules being studied via the UK COVID-19 vaccination programme at a timing to allow boost dose given in the trial to fall between days 56-84 post-prime. Evidence of this will be gathered from medical history and/or medical records. 4. Female participants of childbearing potential must be willing to ensure that they or their partner use effective contraception from 1 month prior to first immunisation continuously until 3 months after boost immunisation 5. In the Investigator’s opinion, is able and willing to comply with all trial requirements 6. Willing to allow their General Practitioner and consultant, if appropriate, to be notified of participation in the trial 7. Willing to allow investigators to discuss the volunteer’s medical history with their General Practitioner and access all medical records when relevant to study procedures 8. Agreement to refrain from blood donation during the course of the study

1. Participant is willing and able to give written informed consent for participation in the trial 2. Male or female, aged 50 years or above and in good health as determined by a trial clinician. Participants may have well-controlled or mild-moderate comorbidity 3. Has received one dose of the prime/boost schedules being studied via the UK COVID-19 vaccination programme at a timing to allow boost dose given in the trial to fall between days 56-84 post-prime. Evidence of this will be gathered from medical history and/or medical records. 4. Female participants of childbearing potential must be willing to ensure that they or their partner use effective contraception from 1 month prior to first immunisation continuously until 3 months after boost immunisation 5. In the Investigator’s opinion, is able and willing to comply with all trial requirements 6. Willing to allow their General Practitioner and consultant, if appropriate, to be notified of participation in the trial 7. Willing to allow investigators to discuss the volunteer’s medical history with their General Practitioner and access all medical records when relevant to study procedures 8. Agreement to refrain from blood donation during the course of the study