Youngsters aged 12-16 are to be offered a mix of coronavirus vaccines in a new trial to determine whether children need a second jab and if so which type would be most effective.
It comes after health leaders approved first doses of the Pfizer vaccine for over-12s on Monday.
Researchers from the University of Oxford are carrying out the trial which will analyse how the participants respond to various combinations.
Those involved in the study will all receive a full dose of the Pfizer jab initially.
It is the second one that differs. Some children will remain with Pfizer and get either another full dose or a half measure.
Others will receive a half dose of the Moderna vaccine or a full one of Novavax.
There will be an eight-week interval between injections.
The randomised trials, in which the children will not know which second vaccine they are getting, will happen at sites in London, Bristol, Southampton and Oxford.
Funded by the Vaccines Taskforce and the National Institute for Health Research (NIHR), the Com-COV 3 trial is looking for 360 volunteers.
Professor Matthew Snape, its chief investigator, said it would provide "vital information on the range of options for immunising teenagers".
The resulting data will be used to assess which combinations "can be used to generate the best and most durable immune response, in as safe a manner as possible", he added.
"This will provide the Joint Committee on Vaccination and Immunisation (JCVI) with information crucial to informing their advice about immunising teenagers in the UK," Prof Snape said.
It is hoped that initial results will be reported by December.
Giving COVID-19 vaccines to this age group has proved controversial, with different sets of experts disagreeing about whether to go ahead.
The UK's four chief medical officers have recommended that those aged 12-15 should be offered a first dose.
But the JCVI did not recommend the move, saying the virus poses a very low risk to healthy children and inoculating them would provide only a marginal benefit.