Britain said on Monday that the Omicron coronavirus variant was spreading at a "phenomenal rate" and now accounted for about 40 percent of infections in London, so people should get a booster shot because the double-vaccinated are still vulnerable.

Since the first Omicron cases were detected on November 27 in the United Kingdom, Prime Minister Boris Johnson has imposed tougher restrictions and told the nation on Sunday that a "tidal wave" of Omicron was coming.

Britain says that unless action is taken there could be a million people infected with Omicron by the end of the month.

"It's spreading at a phenomenal rate, something that we've never seen before, it's doubling every two to three days in infections," Health Secretary Sajid Javid told Sky News.

"That means we're facing a tidal wave of infection, we're once again in a race between the vaccine and the virus."

The pound fell 0.4 percent to $1.3225, while it was broadly steady against the euro at 85.29 pence.

Johnson, who is grappling with a rebellion in his party over measures to curb Omicron and an outcry over alleged parties at his Downing Street office during last year's lockdowns, said people should rush to get booster vaccines to protect "our freedoms and our way of life".

After Covid-19 was first detected in China in late 2019, he faced criticism for initially resisting lockdown.

He has also faced criticism for overseeing mistakes in transferring patients into care homes, and for building a costly test-and-trace system that failed to stop a deadly second wave.

Across the world, Covid has killed 5.3 million people, wiped out trillions of dollars in economic output and turned normal life upside down for many.

Two vaccines not enough

Data released on Friday showed that vaccine efficacy against symptomatic infection was substantially reduced against Omicron with just two doses, but a third shot boosted protection up to over 70 percent.

Javid said there had been no deaths yet confirmed in England and just 10 people hospitalised in England with the variant, but Omicron was probably behind around 40 percent of infections in London.

He said that while symptoms might be milder, the variant's swift spread meant that unless the government acted then the health service could be overwhelmed.

"Even when a virus is mild, a small percentage of people from a very large number still can equal a high number of hospitalizations," Javid said.

"Two doses are not enough, but three doses still provide excellent protection against symptomatic infection."

The government wants to offer all adults a booster by New Year, an ambitious target given the Christmas holiday and that vaccinating 1 million people per day is around double the current 530,000 per day.

New vaccination sites will be set up to work seven days, the military will help, and some routine health appointments will have to be postponed.