Experts want to know if a third dose could protect people against Covid-19 and its variants.
Seven existing vaccines - including the Oxford/Astrazeneca one - are to be tested in the Cov-Boost trial to see which jabs could be used in any forthcoming autumn vaccination programme.
Some 2,886 people aged 30 and older are being recruited at 18 NHS sites from London to Glasgow, with the Oxford Vaccine Group recruiting people from across the Thames Valley.
The first booster jabs will be administered in early June.
Scientists want people who received their first dose of either Pfizer/BioNTech or Oxford/AstraZeneca in December or January to sign up, and hope people aged 75 and over will also come forward.
Experts believe that all seven vaccines will boost immunity, and lab studies will check their response to variants circulating in the UK, including those from India, Kent and South Africa.
The £19.3 million clinical trial will test the Oxford jab and Pfizer jab, alongside those from Moderna, Novavax, Janssen from Johnson & Johnson, Valneva and CureVac.
Three of the vaccines will also be tested at a half dose, with experts expecting an adequate immune response at this level.
The half doses will inform whether side-effects are reduced at a lower dose, and could offer useful information to countries where vaccine supply may be more scarce.
All of the information will be fed to to the Joint Committee on Vaccination and Immunisation (JCVI) at the end of August or early September, and the data will be stored by Oxford University.
The JCVI will then guide the Government on whether people should be boosted with a third dose and which vaccines should be used, depending on supply.
Among the information gathered will be any data on side-effects, including among people whose third booster jab is a different type to that used for their first two shots.
Professor Saul Faust, director of the National Institute for Health Research Southampton clinical research facility and lead investigator for the trial, said the “hope of a booster is that we raise the antibody level enough to be able to cover existing and variant strains of coronavirus.”
He added: “We’re hoping the immune responses will be high enough to protect people against all the strains circulating in the UK, including we’ll be testing in the lab against the Indian variant, the South African variant, the Kent variant as well as the original strain.”
Any other variants that England’s chief medical officer, Professor Chris Whitty, wants adding to the mix can be tested as part of the trial over the summer.
Experts believe booster shots of existing vaccines could be enough to provide protection against all variants, with some scientists suggesting that developing new vaccines against variant strains may actually impair people’s immune responses.
Dr Matthew Snape, associate professor of paediatrics and vaccinology at the University of Oxford, told a briefing that changing the vaccine to, for example, one that targets the South African variant, could actually leave the body trying to respond to the original Wuhan strain of coronavirus that an earlier vaccine protects against.
He said more research was needed, but added: “In some situations you never forget your first love…you’re still trying to respond to that first vaccine.”
The researchers stressed that the aim of the new study is not to pit the vaccines against one another, but to check whether they all increase antibodies and to look for potential side-effects.
All participants in the trial will have bloods taken to measure their immune responses at days 28, 84, 308 and 365 of the trial – with a small number having blood tests at other times.
People can sign up for the new trial at covboost.org.uk