Covid booster jabs are to be offered to all over-18s in the UK to help stop a potential wave of infections driven by the new variant Omicron.
The Joint Committee on Vaccination and Immunisation made a series of recommendations to expand the booster rollout.
It said the minimum gap between the second vaccine dose and booster should be reduced from six to three months.
The experts also said children aged 12-15 should be invited for a second jab.
Early evidence suggests Omicron has a higher re-infection risk, but scientists say it will take about three weeks before it is known how the variant impacts on the effectiveness of vaccines.
The government has announced new rules on self-isolation and that face masks will be compulsory in shops and on public transport in England from 04:00 GMT on Tuesday in an effort to contain the spread of the variant. Eleven cases of Omicron have now been detected in the UK.
England's deputy chief medical officer Prof Jonathan Van-Tam told a Downing Street briefing that Omicron was the "new kid on the block" and it has "always been the case that, at some point, we are going to get a variant that gives us heightened concern".
He said: "If vaccine effectiveness is reduced - as seems pretty likely, to some extent - the biggest effects are likely to be in preventing infections and, hopefully, there will be smaller effects in preventing severe disease.
"Vaccine boosting is the thing we can do most easily while we wait for that science mist to clear."
Prof Wei Shen Lim, chair of the JCVI, said he was not predicting the new variant would take hold in the UK but the experts wanted to be in the best possible position if there was an increase in infection.
He said: "With any vaccine during a pandemic, we get the greatest benefit for individuals and society if the vaccine is deployed before the wave starts. We want to provide boosters early enough.... before any possible wave".
Director general of the World Health Organization, Tedros Adhanom, said the emergence of Omicron "underlines just how perilous and precarious our situation is".
The aim of this announcement is to get ahead of a potential wave driven by the new variant Omicron.
If that happens - and it is an "if" at this stage because so much is unknown about the new variant - giving more people a booster jab will certainly help.
While Omicron may make the vaccines less effective, the booster dose has been shown to significantly increase the immune response, which will help counter any advantages this variant may have.
But that will only happen if the vaccine is in people's arms. The NHS is doing about 2.5 million boosters a week on average at the moment. At that rate it will take three months to vaccinate all those eligible.
Increasing that will not be easy. About 50 mass vaccination centres have closed since the spring. Many GPs have also stepped back to concentrate on flu jabs and their day-to-day work.
It means the solution is likely to lie in extending the opening hours of current vaccination clinics late into the evenings and bringing in extra staff and volunteers to help.
In its advice, the JCVI also recommended severely immunosuppressed people should be offered a fourth dose of the vaccine as a booster. They are currently receiving three jabs.
The committee said the booster rollout should take account of vulnerability. Boosters - either the Moderna or Pfizer-BioNTech vaccines - should be offered in order of descending age groups, with priority given to those in a Covid at-risk group, it said.
NHS England said people would be contacted when it was time for them to book their booster.
A spokesperson said: "The NHS will shortly set out how staff will expand the booster programme - this will include how booster jabs will be given in priority order... while also increasing capacity to vaccinate millions more people in a shorter space of time."
Booster jabs were originally offered to the over-50s, front-line medical staff and people with health conditions that put them at greater risk. Earlier this month the rollout was widened to include the over-40s.
The JCVI's latest advice has been given to ministers in all parts of the UK. It only gives recommendations and the final decision on measures to combat Covid lies with the politicians.
So far, ministers in England, Scotland and Wales have said they would follow the recommendations, and Northern Ireland is also expected to accept them.
The JCVI said it would continue to review data on the potential benefits and risks of offering the Covid vaccine to children aged five to 11.
Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency, said a review found it was safe for those aged 12 to 15 to have a second dose of the Pfizer-BioNTech vaccine. This should be offered at a minimum of 12 weeks from their first dose.
The latest cases of the Omicron variant were detected in the Camden and Wandsworth areas of London, the UK Health Security Agency said. Like the other three cases in England, they have been linked to travel in southern Africa, where infections have been circulating.
Ms Sturgeon said some of the six cases identified in Scotland had no travel history. This suggested a degree of community transmission, she said, but there was currently no suggestion this was widespread.
'A fine Christmas'
UK Health Secretary Sajid Javid was asked in the Commons about any impact Omicron could have on Christmas.
He said: "The reason we've taken measures is to protect the progress we have made, and if we can do that then we can look forward to a fine Christmas."
Scotland's First Minister Nicola Sturgeon said she was not asking anyone to put plans on hold as "these are protections, not restrictions".
Meanwhile, Wales' First Minister Mark Drakeford said people should approach Christmas "in the way that we've advised all the way through" - that it should be different and better this year.
"That doesn't mean that people should neglect those simple things that we can do in our own lives, that still help to protect ourselves and others," he said.
Mr Javid also told MPs they would be given a chance to debate and vote on the face masks and isolation rule changes.
He said: "If it emerges that this variant is no more dangerous than the Delta variant, then we won't keep measures in place for a day longer than necessary.
"Our experience of fighting this virus has shown us it's best to act decisively and swiftly when we see a potential threat. Which is why we're building our defences and putting these measures in place without delay."
The average number of daily confirmed Covid cases in the UK began rising again in early November. A further 42,583 confirmed cases were announced on Monday.