Early analysis by the UK Health Security Agency (UKHSA) found that a third dose prevents about 75% of people from getting any symptoms.
The findings have led to calls for all of those eligible to make sure they get their booster jab.
What did the findings show about the Omicron variant?
However, the data - which looked at 581 people with confirmed Omicron - also suggested effectiveness seemed to “increase considerably” in the early period after a booster dose.
A third jab was found to give around 70 to 75% protection against symptomatic infection.
How many Covid and Omicron cases are there in the UK?
Daily Covid case numbers have reached their highest level since January.
There were, as of 9am on Friday, a further 58,194 lab-confirmed Covid-19 cases in the UK.
The last time a higher daily figure was reported was on January 9, when 59,937 cases were recorded.
An additional 448 confirmed cases of the Omicron variant have been reported across the UK, bringing the total number to 1,265.
The UKHSA has predicted that, if current trends continue, the UK will exceed one million infections by the end of the month.
Will new Covid restrictions be introduced across the UK?
Cabinet minister Michael Gove said it was a “deeply concerning situation” after attending a Cobra meeting on Friday (10 December) to discuss the latest data.
He said that the Omicron variant was doubling every two or three days in England “and possibly even faster in Scotland”.
Mr Gove warned that evidence suggests Omicron is “more likely” than past Covid variants to “potentially” lead to hospital admissions among the fully vaccinated.
The Guardian has also reported that Health Secretary Sajid Javid had been given a presentation from UKHSA which warned that even if Omicron leads to less serious disease than Delta, it still risks overwhelming the NHS with 5,000 people admitted to hospital a day.
The leaked report called for “stringent action” on or before December 18 if the variant’s doubling time stays at 2.5 days.
However, Number 10 maintained that there were “no plans” to go further with measures in England - despite reports that proposals are being drawn up for a Plan C.
In Scotland, First Minister Nicola Sturgeon has warned of the possibility of a “tsunami of infections” from the new variant.
She refused to rule out more restrictions in the country.
What Covid restrictions are currently in place?
In England, the legal requirement to wear masks was extended to more indoor venues - including museums, galleries and community centres.
From Monday (13 December), there will also be a return to working from home guidance.
Mandatory Covid passports for large venues will be implemented from Wednesday (15 December).
New guidance expected to also come into force from Wednesday, will see care home residents allowed only three visitors and one essential care worker
The new regulations will be put to a debate and vote in the House of Commons next week.
Despite the prospect of a large Conservative revolt, the measures are also backed by Labour and are certain to be approved.
Scotland’s First Minister announced the return of a 10-day isolation period for all contacts of a positive Covid case.
What is Plan C?
Officially, there is no Plan C and the Prime Minister’s spokesperson said there were "no plans to go beyond what we’ve set out already".
There have, however, been reports suggesting that the Government could turn to new restrictions that go beyond Plan B in order to combat the spread of the Omicron variant.
Plan C could see people having to once again check in with the NHS Covid app or provide their personal details in writing when going to pubs and restaurants.
These measures would also extend rules on mask wearing in public indoor spaces and lead to a wider rollout of vaccine passports.
A reduction of household mixing could be another measure that the Government turns to, as well as another lockdown.
How many daily Covid hospital admissions could there be in the UK?
Professor Neil Ferguson, from Imperial College London, told the Guardian that Omicron could “very substantially overwhelm the NHS, getting up to peak levels of admissions of 10,000 people per day”.
He said such a figure could be reached “sometime in January”.
However, he added that it was based on assumptions around the variant’s ability to get around existing protection, and the premise that it is similar to Delta in terms of the severity of disease it causes – something that is not yet known.
He said: “Even the best-case scenarios involve several-fold more admissions per day than we’re getting at the moment – we are at about 700 right now.”
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