The results from third doses of seven possible vaccine candidates are expected in September – when a large-scale booster programme is pencilled to start.
“The data from this world-first clinical trial will help shape the plans for our booster programme later this year,” said Matt Hancock, the health secretary.
“I urge everyone who has had both doses of a Covid-19 vaccine, and is eligible, to sign up for this study and play a part in protecting the most vulnerable people in this country and around the world for months and years to come.”
Backed by £19.3m of government funding, the trial “will be the first in the world to provide vital data on the impact of a third dose on patients’ immune responses”, health chiefs said.
The announcement comes after Mr Hancock revealed that almost 3,000 cases of the Indian coronavirus “variant of concern” have been identified – an increase of 600 since Monday.
Surge testing and extra vaccine supplies are being deployed to hotspot areas to control the spread of the highly-transmissible strain, MPs were told.
“The race between the virus and the vaccine has got a whole lot closer,” the health secretary said – revealing that 2,967 cases of the B1617.2 variant had been identified, up from 2,323.
Bolton, Blackburn with Darwen and Bedford have been the main areas where worrying numbers of cases had been identified.
At a Downing Street press conference, Mr Hancock also announced surge testing and vaccinations in Burnley, Leicester, Kirklees, North Tyneside, and Hounslow in west London.
He also announced “two further tools” in the fight against new variants, the first being travel data to identify “where the virus is at risk of spreading”.
The booster study will take place at 16 sites across England, as well as at Health and Care Research Wales and NHS Research Scotland sites.
All participants will be monitored for any side effects and will have blood taken to measure their immune responses at 28 days, then 84, 308 and after a full year.
Electronic diaries will allow them to send alerts in real time if needed and they will be given a 24-hour emergency phone to contact a doctor for further clinical advice if necessary.