Preliminary data has shown vaccine effectiveness against the Omicron Covid-19 variant "considerably increased" in the early period after a booster dose, providing around 70% to 75% protection against symptomatic infection.
That is according to the UK Health Security Agency, which also said Omicron is expected to become the dominant variant in the UK by mid-December with one million infections by the end of the month.
Analysis of 581 people with confirmed Omicron cases showed the AstraZeneca and Pfizer vaccines provided "much lower" levels of protection against the variant when compared with the Delta variant.
But vaccine effectiveness "considerably increased" in the early period after a booster dose, providing around 70% to 75% protection against symptomatic infection.
Dr Mary Ramsay, head of immunisation at the UKHSA, said: "These early estimates should be treated with caution but they indicate that a few months after the second jab, there is a greater risk of catching the Omicron variant compared to Delta strain.
"The data suggest this risk is significantly reduced following a booster vaccine, so I urge everyone to take up their booster when eligible.
"We expect the vaccines to show higher protection against the serious complications of Covid-19, so if you haven't yet had your first two doses please book an appointment straight away."
An additional 448 confirmed cases of the Omicron Covid-19 variant have been reported across the UK today, bringing the total number of confirmed Omicron cases there to 1,265.
It comes as all passengers arriving into Ireland from Britain are being advised to carry out an antigen test every day for five days after arrival.
A breakdown shows 443 of the additional cases have been reported in England, one in Scotland and four in Wales. No further cases have been reported in Northern Ireland.
Britain's Communities Secretary Michael Gove said the UK faces a "deeply concerning situation" given the fast rate the Omicron variant of coronavirus is spreading there.
Speaking to broadcasters, the Cabinet minister said: "The Cobra meeting I've just chaired with first ministers of all the devolved administrations was presented with some very challenging new information.
"We know that we have the highest number of Covid infections across the UK recorded today since 9 January.
"We know the Omicron variant is doubling every two to three days in England, and possibly even faster in Scotland.
"We know that 30% of reported cases in London are the Omicron variant and of course we only identified Omicron in this country a fortnight ago."
Stormont Health Minister Robin Swann said that new Covid restrictions are not inevitable for Northern Ireland in January.
Mr Swann told the PA news agency: "In regards to Omicron, we are watching what is happening across the rest of the world but also in England and Scotland in regards to the veracity, the ability of transmit, but also how that translates into hospitalisations as well.
"Until we see what is actually in front of us we will take each day as it comes."
He added: "I don't think they (new restrictions) are inevitable, but let's not kid ourselves that they may not be needed come January in regards to where we are."
