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Britons should not get their second Covid vaccine after just four weeks, one of the Government's top advisers claimed today.
Ministers are keen to halve the gap between doses — which currently stands at eight weeks — amid surging cases.
But Professor Adam Finn, of the Joint Committee on Vaccination and Immunisation (JCVI) — which advises No10 on the roll-out, is against the move.
He told BBC Radio 4's Today programme: 'I think we would need to be pretty careful about trying to change the approach right now in the middle of this third wave.'
Professor Finn warned giving second doses after just four weeks would likely offer people less protection against the disease in the future.
Studies have shown jabs made by AstraZeneca and Pfizer actually perform slightly better when spaced out for longer than a month.
This is because the longer gap leads to a better priming of the immune system to fight off the virus.
On the other hand, Professor Finn accepted that halving the gap to four weeks may 'pay off' if Britain's third wave keeps spiralling.
Professor Adam Finn, a top vaccines adviser, has said the gap between doses should not be slashed to four weeks because it could make jabs less effective
It comes as Department of Health data shows the inoculation drive is slowing amid a glut in demand for first doses
Nadhim Zahawi tried to quell a backlash from local leaders on plans to drop mandatory masks on public transport yesterday, after a poll found 50 per cent of Britons want 'Freedom Day' delayed.
The vaccines minister insisted people will still be 'expected' to wear coverings in confined spaces under new guidance being issued this week, even though the legal compulsion will go.
But he insisted that the unlocking schedule was set to go ahead as planned on July 19 - with Boris Johnson due to give more details at a press conference tonight.
'We're seeing a rise in infection rates in this country, but also in Europe and elsewhere. The difference for us is that the vaccination programme has been so successful,' Mr Zahawi said.
The comments came after Manchester Mayor Andy Burnham warned July 19 could turn into 'anxiety day', complaining that face coverings should remain a legal requirement.
Sadiq Khan is still considering a bid to force passengers to keep wearing masks on trains, Tube and buses in London.Advertisement
Ministers, including newly-appointed Health Secretary Sajid Javid, fear the UK could see upwards of 100,000 cases a day by next month.
Originally, both AstraZeneca and Pfizer's vaccines were approved to be dished out in three-week intervals because that was the gap tested in the research trials.
But No10's scientists pushed the regimen back to 12 weeks to get wider protection in winter, when the second wave started to take off.
They said the decision would allow more people to get some immunity against the disease in the shortest period of time possible.
Last week the JCVI recommended cutting the gap to eight weeks for everyone, in a bid to protect more people.
Health chiefs already fear the UK may be close to maximum vaccine uptake, with young people having been eligible for appointments for almost a month.
The roll-out has ground to fewer than 100,000 first doses a day, with the drive currently centered on ensuring millions get fully inoculated.
Professor Finn, from the University of Bristol, said: 'There is of course an advantage in giving the second dose early, in the current circumstances with all the cases that we are seeing because you get a further boost,
'But the downside to that is the size of that boost is smaller and probably that will mean that the duration of protection you get from that second dose will be shorter.
'So there’s a sweet spot, and at the moment the advice we have given is we should not reduce the interval less than eight weeks.'
He added that ramping up dosing would not delay Britain's rampant second wave because it takes jabs several weeks to kick in.
Professor Finn said: 'Vaccination is not really a very good tool for dealing with fire in the house once the fire is really going, it’s much better as a way of stopping the house from being inflammable.
'Trying now to deal with the problem we have got with vaccination is really very difficult because it takes time to give those doses and after you have given them it takes time for them to take effect.'
The JCVI has been asked to issue urgent advice on whether the pros and cons of slashing the gap between doses, the Sunday Times claimed. An announcement is expected within days.
Experts fear leaving young people only partially protected for longer amid a second wave could drive up rates of 'long Covid'.
Public Health England evidence shows jabs cut the risk of infection by between 55 and 70 per cent after one dose, but that this rises to 65 to 90 per cent after two.
UK health officials say the longer gap has made jabs more effective.
But they were approved for use in the country based on clinical trials which had a three-week delay between doses.
Some vaccine centres are dishing out second doses early amid a glut in demand for first jabs.
But NHS England has wrapped many centres on the knuckles for doing this, insisting they must stick to the eight-week delay.
Many countries never widened the gap between jabs, saying they had to be lead by the science and had no evidence to support doing so.
Department of Health data shows that over the last seven days the average number of first doses dished out has tumbled to 87,175, half the number from just over a week ago.
But the number of second doses given out has remained steady at just below 200,000 a day.
Young people — the last group to be inoculated — are less likely to get the jab than others because they see themselves as not at threat from the virus.
Ministers are hoping, however, that a double-jab requirement for holidays and to avoid self-isolation will bolster uptake.
It comes as the Prime Minister is expected to confirm the easing of most of the remaining restrictions today — although he will likely sound a more cautious tone.
No10 sources say Mr Johnson plans to put his trust in the 'innate good sense of the British people' not to embrace the new liberties 'recklessly'.
Top experts including the Academy of Medical Royal Colleges (AMRC) warned last week that the rising case numbers would put the NHS under heavy pressure.