Millions of healthy Brits face missing out on a Covid booster vaccine this winter because it might not "make much difference", a top expert has indicated.
Prof Adam Finn, of the Joint Committee on Vaccination and Immunisation (JCVI), said the group will "imminently" recommend a booster programme for vulnerable Brits.
But with JCVI members due to meet today, he warned "we need more evidence" before recommending a wider rollout, for instance for healthy under-50s.
The JCVI has never ruled out giving healthy under-50s a booster shot, saying in June it would be "considered at a later time when more information is available."
But Prof Finn today appeared to pour cold water on the idea of all Brits getting a third dose.
He told BBC Radio 4's Today programme: "I think we will be imminently deciding there’ll be some people who need a third dose, particularly people who we know are very unlikely to be well protected by those first two doses.
“But I think we do need more evidence before we can make a firm decision on a much broader booster programme.
"The principle all the way through this programme has been to try to use the doses of vaccines we've got as effectively as possible, to get the most bang for your buck.
"Giving booster doses is really getting a very marginal benefit in many cases."
The JCVI is waiting for the results of more clinical trials before announcing its decision on who should get a Covid booster jab.
Those trials do not report back until around the end of this month so a firm recommendation is not expected until September, the Mirror understands.
They are looking at different age groups as well as "mix and match" dosing - whether a Pfizer recipient can get AstraZeneca, for example.
Health Secretary Sajid Javid has said he "anticipates" boosters will be given from early September and England's health chiefs have drawn up plans to deliver shots between September 6 and December 17, at the same time as flu jabs.
In interim advice on June 30, the JCVI suggested there could be two stages to a booster rollout, similar to the rules for first doses between January and March.
Stage one would give boosters to immunosuppressed and extremely vulnerable over-16s; care home residents; frontline health and care workers; and all over-70s.
Stage two would give them to all over-50s, plus all over-16s in a flu or Covid risk group, and any adults who share their home with an immunosuppressed person.
The JCVI did not rule out extending booster shots to healthy under-50s but said in June it was too early to make a decision.
It comes after scientists warned Covid-19 herd immunity could be harder to achieve, due to a study suggesting the Delta variant spreads more than other strains between vaccinated people.
Oxford University analysed data from 2.6 million PCR tests conducted on almost 400,000 participants.
All jabs worked well at preventing infection and serious disease - but the findings suggest vaccinated people can still put loved ones at risk.
In a quarter of those jabbed who could still catch coronavirus , levels of the Delta variant in their body were just as high as in the unvaccinated.
Prof Finn told BBC Breakfast: "I think at this point we need to focus on individuals who are more likely, if you like, to get sick again if they've not got a booster.
"And in fact we'll be having a JCVI meeting this morning to discuss exactly that.
"So, trying to identify the people who are really at risk and really need that third dose.
"I think it's less clear really whether a third dose in a more general way, for sort of all people above a certain age, is really going to make very much difference.
"But at this point I think the main message is that the direct protective effects of these vaccines is excellent i.e. if you get the vaccination you're in a much better place in terms of getting sick.
"But the ability of the programme to actually stop the virus from circulating around in the population is less good than we'd hoped."
