Health officials have renewed their call for everyone eligible to come forward for a Covid booster jab.
It comes after early analysis by the UK Health Security Agency showed a third vaccine dose prevents about 75% of people from getting any Covid symptoms.
On Friday, the UK reported 58,194 new Covid cases, including 448 of Omicron.
Cabinet minister Michael Gove said the UK faced a “deeply concerning situation” and that anti-infection measures were being kept under review.
On Friday face coverings became compulsory in most indoor venues in England, including theatres, cinemas, places of worship, museums and indoor sport stadium, under measures to tackle the Omicron variant.
And it was announced that in England, care home residents will only be allowed three visitors and one essential care worker from Wednesday.
The Department of Health and Social Care (DHSC) said the move was “in order to balance the current Covid-19 risk and the need to keep people safe in line with clinical advice”.
“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.”
The DHSC added that testing would also increase for fully vaccinated residents making visits outside and for staff, and that its vaccination teams were being expanded to increase the take-up of the booster jab.
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In Scotland, household contacts of people who have tested positive for Covid are now being asked to isolate for 10 days – regardless of their vaccination status or if they have had a negative PCR result.
In Wales, the Welsh government is urging members of the public to take a lateral flow test before going shopping or to Christmas parties.
Officials in Northern Ireland are not planning to extend restrictions over the festive period but Northern Ireland’s chief medical officer, Sir Michael McBride, has warned further restrictions may be reintroduced in the new year.
Levelling Up Secretary Michael Gove, who chaired a meeting with first ministers of the devolved nations on Friday, said measures taken so far in England were “proportionate”, but ministers were “absolutely” keeping everything under review.
“Action is absolutely required, and as new data comes in we will consider what action we do require to take in the face of that data,” he added.
However, more Conservative MPs have suggested publicly that they will vote against the government’s latest Covid restrictions in England when they are put before parliament on Tuesday.
The BBC estimates that, from their public pronouncements, some 57 Conservative MPs might vote against the measures. They are still likely to pass because Labour has said it will support them.
Friday’s increase in Covid cases was the biggest since 9 January and it is estimated that the number of Omicron cases is doubling every two to three days.
The UK Health Security Agency (UKHSA) estimates that more than half of all cases in the country will be Omicron by mid-December and that if growth continues unabated there will be more than 100,000 cases a day by the end of the month.
It analysed data from 581 Omicron cases and thousands of Delta cases to calculate how effective the vaccines were against the new variant.
The analysis is based on limited data, but showed a dramatic drop in effectiveness for the Oxford-AstraZeneca vaccine and a significant drop off for two doses of Pfizer.
The 75% protection against Covid symptoms after a booster is not as high as against previous variants.
Dr Mary Ramsay, head of immunisation at the agency, 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.”
She added: “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.”