UK announces booster drive to fight 'tidal wave' of Omicron

"No-one should be in any doubt: there is a tidal wave of Omicron coming," he said in a televised address, after the country's medical advisers raised the Covid Alert Level due to a "rapid increase" in infection from the variant.

Johnson, under pressure because of claims he and staff flouted Covid restrictions last Christmas, called the spread of the mutation "an emergency", as Omicron was doubling every two to three days.

"We know from bitter experience how these exponential curves develop," he added.

The increase in the five-stage Covid Alert Level from three to four comes after a further 1,239 confirmed cases of the variant were recorded on Sunday.

That brings the total number of UK cases of Omicron to 3,137 -- a 65 percent increase from Saturday's total of 1,898.

Britain began easing coronavirus restrictions in June and the alert level was at stage three, which means the epidemic is in general circulation.

Level four means "transmission is high and pressure on healthcare services is widespread and substantial or rising".

The four chief medical officers for England, Wales, Scotland and Northern Ireland said the move was prompted by advice from the UK Health Security Agency, a public health body.

"Transmission of COVID-19 is already high in the community, mainly still driven by Delta, but the emergence of Omicron adds additional and rapidly increasing risk to the public and healthcare services," they said.

"Early evidence shows that Omicron is spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced.

"Data on severity will become clearer over the coming weeks but hospitalisations from Omicron are already occurring and these are likely to increase rapidly."

The officers said boosters were vital given that vaccine protection was reduced with Omicron, and both third jabs used -- Pfizer and Moderna -- increased immune response and showed "good effectiveness".

The move is designed to reduce pressure on the state-run National Health Service (NHS) which is dealing with seasonal respiratory infections such as flu.