Britain’s Prime Minister Boris Johnson attends a virtual press conference inside 10 Downing Street in central London on Nov. 26, 2020.

England’s top medical officer on Saturday announced that the U.K. has identified a new variant of the coronavirus that “can spread more quickly” than prior strains of the virus, leading Prime Minister Boris Johnson to impose fresh restrictions on parts of the nation to control its spread.

“We’re learning about it as we go, but we already know enough, more than enough, to be sure that we must act now,” Johnson said during a press briefing on Saturday where he laid out fresh restrictions on London and other parts of England ahead of the Christmas holiday.

“When the virus changes its method of attack, we must change our method of defense,” Johnson said.

The U.K. government announced the new coronavirus strain on Monday following an increase in cases in the southern and eastern parts of England. Just over 1,100 Covid-19 cases with the new variant had been identified as of Sunday, according to a statement from Public Health England.

Now, it’s thought that the new strain could be up to 70% more transmissible than the original strain of the disease, Johnson said on Saturday, adding that it appears to be driving the rapid spread of infections. Johnson called on residents to refrain from traveling and “stay local” to prevent the new strain from moving around the country and abroad.

The United Kingdom is reporting roughly 24,061 new Covid-19 cases every day, based on a weekly average, which is a more than 40% increase compared with a week ago, according to a CNBC analysis of data compiled by Johns Hopkins University.

“This is early data, and it’s subject to review, but it’s the best that we have at the moment and we have to act on information as we have it because this is now spreading very fast,” Johnson said.

Professor Chris Whitty, England’s chief medical officer, said at the press briefing that “viruses mutate all the time.” Seasonal influenza mutates every year, and there have already been other new variants of the coronavirus identified in countries like Spain, according to Public Health England.

What needs to be answered is whether the new strain transmits more easily, makes people sicker and whether it changes the way someone’s immune system responds to the virus if they were already infected or vaccinated, Whitty said.

So far, a collection of evidence from genetic, frequency and laboratory studies suggests the new strain “has a significant, substantial increase in transmissibility,” Whitty said. However, there’s no evidence so far to suggest that the new strain causes a higher death rate.

Health officials believe the new variant first appeared in mid-September in London or Kent, and by the middle of November it’s thought to have caused roughly 28% of cases in London and other parts of southeast England, Whitty said.

Now those figures are much higher, he said. In London, data over the past week suggests the new variant has accounted for more than 60% of new cases, Whitty said.

“So what this tells us is that this new variant not only moves fast, it is increased in its ability to transmit, but it’s becoming the dominant variant. It is beating all the others in terms of transmission,” he said.

Yet there’s “no evidence” it causes a more severe disease, more hospitalizations or “more trouble than the other virus,” Whitty said. While there are reasons to suspect the new variant might alter someone’s immune response to the disease, there’s nothing to indicate that’s the case so far, he said.

“Our working assumption at the moment, from all of the scientists, is that the vaccine response should be adequate for this virus,” he said. “That obviously needs to be looked at going forward, and we need to keep vigilant about this.”

The U.K. has alerted the World Health Organization and will continue to analyze data on the new strain.