The public is being urged to take part in trials to find out whether a third dose of Covid vaccine could protect against new variants.
All seven vaccines the UK has ordered will be tested on working-age people and over-75s as part of a randomised trial. Data on side-effects and immune responses will be gathered.
The findings will help vaccine advisers decide if re-vaccinating some people in the autumn is necessary.
More than 20 million people have been fully vaccinated - with two doses of a Covid vaccine - since the UK's vaccination programme started, but it's still not clear how long protection lasts. Offering some groups a third dose to boost protection from coronavirus ahead of winter has been suggested - but not confirmed.
The Cov-Boost study, which starts in June, will recruit 3,000 people of all ages who had their first dose in December or January, to test if this is worthwhile.
Professor Saul Faust, chief investigator for the trial, from the University of Southampton, said: "It could be that some age groups may not need a booster and others do," "We are not trying to say one is better than the other.
“The aim is to find out whether there should be a booster campaign and which vaccine to use." He said he was not expecting any vaccines to be detrimental" but some could cause high fevers or very sore arms, for example, which would be useful to know.
The vaccines will be trialled at 18 sites across the UK, and half doses will also be tested including: Southampton, London (University College Hospital, Guy's and St Thomas' Hospital and Northwick Park Harrow), Leicester, Bournemouth, Portsmouth, Wrexham, Bradford, Oxford, Glasgow, Leeds, Cambridge, Birmingham, Brighton, Stockport, Liverpool and Exeter.
Participants will be asked to keep diaries of any side-effects after a third dose, which could be one of seven different Covid vaccines, and researchers will test participants' immune response after one, three and 12 months. This involves testing their blood for antibodies to coronavirus - high levels are a sign that the body's defences are primed to fight off the virus.
All doses would be from current vaccines, designed to protect against the original form of the virus.
In addition to doses of AstraZeneca, Pfizer and Moderna, some people will be given doses of vaccines from Novavax, Janssen, Valneva and CureVac, which are currently being trialled in large numbers of people, but have not yet been approved by the UK regulator. There will also be a control group, who will be given a dummy vaccine.
