Vaccinating everyone on the planet against Covid-19 regularly is not sustainable or affordable, a UK vaccine scientist has said.
Prof Sir Andrew Pollard, who helped develop the Oxford-AstraZeneca vaccine, said the most at risk should be identified and prioritised instead.
He said the vaccine rollout had gone "extremely well" in the UK but other parts of the world were falling behind.
Booster jabs have been offered to all eligible adults in the UK.
There has been a surge of Omicron cases in the UK, with a record 218,724 cases reported on Tuesday.
However, this figure includes a backlog of two days worth of cases from Wales and four days of cases from Northern Ireland, due to the holiday weekend.
But Prime Minister Boris Johnson's spokesman said the government "doesn't see any data to suggest that further restrictions would be the right approach" in England.
He said the public should be "in no doubt" it would be a difficult time for the NHS but there were mitigations in place to help them through a "challenging winter".
A number of hospital trusts have declared critical incidents, with coronavirus cases leading to staff shortages and increased pressure on services.
Plan B measures currently in place - including mask wearing in some indoor settings and guidance to work from home where possible - are "the correct course", the spokesman added.
Health Secretary Sajid Javid said the government was not looking at cutting the self-isolation period for those who test positive to five days.
Speaking after visiting a vaccination centre in south London, he said the current rules allowing people to leave their 10 days isolation early if they test negative on days six and seven was the "right, balanced, proportionate approach".
The prime minister will host a Downing Street news conference later. He will be joined by chief medical officer for England Prof Sir Chris Whitty and chief scientific adviser Sir Patrick Vallance.
New variants may change view
Prof Pollard told BBC Radio 4's Today programme: "It really is not affordable, sustainable or probably even needed to vaccinate everyone on the planet every four to six months.
"We haven't even managed to vaccinate everyone in Africa with one dose so we're certainly not going to get to a point where fourth doses for everyone is manageable."
There is not "full certainty" on whether another booster might be needed in the UK, added Prof Pollard, director of the Oxford Vaccine Group.
He said the UK would be in a good position if variants continued to lead to milder disease, as has been the case with Omicron.
"We may well need to have boosters for the vulnerable in the population but I think it's highly unlikely that we'll have programmes going forwards regularly of boosting everyone over the age of 12," he added.
Prof Pollard said those who would need further boosters were likely to be older adults or those with health conditions.
"There will be new variants after Omicron," he added. "We don't yet know how they're going to behave - and that may completely change the view on what the right thing to do is."
Prof Pollard is chair of the Joint Committee on Vaccination and Immunisation (JCVI), which advises the government on vaccines, but he has no involvement in decision-making on Covid-19 vaccinations in the UK.
Maggie Throup, minister for vaccines and public health, told the BBC the government would take advice from the JCVI about a fourth Covid vaccine dose programme and then "decide whether it's appropriate".
She said it was important for people to have their boosters now, or first or second doses if they had not yet done so.
Infectious disease expert Prof Neil Ferguson said he was "cautiously optimistic" that Covid cases were starting to plateau in London in the 18-50 age group, which had been seeing especially high numbers.
The epidemiologist said case numbers should start to fall in the next week in the English capital, and in other regions from a week to three weeks' time.
He also said current case data was not giving the full picture, with test kits in short supply over Christmas and re-infections not being counted in the official figures - some 10-15% of Omicron cases are re-infections, he added.
But it is too early to tell what happens next, especially because of "current mixing trends" and the "effect of open schools" - with pupils returning to classrooms from Tuesday.
Covid-related staff and pupil absences are expected this term with teaching unions saying it is likely some will be sent home to learn remotely at times. Face-to-face teaching will remain the norm, says England's Education Secretary Nadhim Zahawi.
Prof Ferguson said it was likely there would be "quite high infection levels" in school-aged children - albeit with mild illness.
The fact Omicron is less severe than previous variants is "good news" and vaccines are "holding up against severe disease and against severe outcomes well", he added.
But "that doesn't mean it's not going to be, as the prime minister said, a difficult few weeks for the NHS".