1. Aged =50 years 2. In good health or have well-controlled or mild-moderate comorbidity, as determined by a trial clinician 3. Willing and able to give written informed consent for participation in the trial 4. Those 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. Able and willing to comply with all trial requirements, in the Investigator's opinion 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. Aged =50 years 2. In good health or have well-controlled or mild-moderate comorbidity, as determined by a trial clinician 3. Willing and able to give written informed consent for participation in the trial 4. Those 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. Able and willing to comply with all trial requirements, in the Investigator's opinion 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