The UK authorities have decided to carry out trials to find out whether a third dose of Covid vaccine boosts immunity and protects against new variants, according to a BBC report.

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, and people are being urged to participate, the report said. Data on immune responses and side-effects will be gathered.

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.

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.

"It could be that some age groups may not need a booster and others do," said Prof Saul Faust, chief investigator for the trial, from the University of Southampton. "We are not trying to say one is better than the other,” BBC quoted him as saying. 

Prof Faust 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. 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.

Even though the study is using current vaccines, rather than vaccines which have been updated to target new variants, scientists say they expect a boost in antibodies which could be enough to protect against all circulating forms of the virus.