The UK is imposing new blockades to counter the new Covid-19 strain

The new blockade means many families will not be allowed to gather for Christmas. Mr Johnson, who noted just a few days ago that it would be “inhumane” to put up borders at Christmas, said: “I must tell you with a very heavy heart that we cannot continue Christmas as we have planned.”

The new restrictions cover the capital, as well as much of the south-east and east of England. In London, the new strain of the virus accounted for 62% of new cases identified in the week to Wednesday.

Similar measures have been taken across Europe to remove the growing number of cases. In Italy, Prime Minister Giuseppe Conte announced on Friday a total blockade across the country for the first time in May, which will take effect at Christmas, from December 24 to January 6.

In and around London, non-essential stores must close and people must stay home from midnight on Saturday, with limited exceptions for some essential work and shopping, Mr Johnson said. Places of worship will remain open.

Households are prohibited from mixing Christmas, and people are denied access to other regions in an effort to stop the strain sown in another part of the country.

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

In the rest of England, Scotland and Wales, households will only be allowed to mingle on Christmas Day, instead of a five-day period previously announced by the government.

The nation reported 27,052 new infections daily on Saturday, bringing the total number of known cases in the UK to over 2 million. There are currently nearly 19,000 people in hospital with Covid-19, and the country has reported an average of more than 400 deaths a day in the past seven days.

The UK became the first Western country to start vaccinating people with a clinically approved vaccine earlier this month. Mr Johnson said 350,000 people had now been shot. That’s compared to over 137,000 on December 15th.

The country reported the discovery of the new strain on Monday and has already announced the World Health Organization.

Scientists have identified about 23 differences in its genetic material compared to established strains. These include changes in areas known to be associated with how the virus binds and enters cells, which could explain why it spreads faster, said Patrick Vallance, the British government’s chief scientific adviser.

He said it had already become the dominant strain in parts of England, replacing older variants. “This virus is taking off, moving fast and inevitably leading to a sudden increase in hospitalizations,” he said.

The scientists also said that the increased transmissibility of the strain could increase the R number – or the number of people, on average, each person with the infection continues to infect – by 0.4 or more. The most recent estimate of R in the UK is 1.1 to 1.2.

British officials said it was unclear where the variant came from – although it was possible in the south-east London country of Kent – or whether it was present abroad. They said its virulence could explain why restrictions that have been effective in other parts of the country in reducing case rates have not done so in Kent.

They said precautions should be taken to slow the spread of the virus, such as keeping people two meters away, wearing a face mask and washing hands frequently.

Viruses reproduce quickly, and mutations are common due to tiny errors in copying genetic material, which can accumulate over time, giving rise to new variants that can behave slightly differently from older strains.

Scientists around the world have been tracking these genetic changes since the beginning of the pandemic. More than 1,000 variants of the virus that causes Covid-19 have been detected so far.

The new variant is not the first time a more transmissible strain has appeared. Scientists in July described a variant that over time shifted an older strain of coronavirus to become the dominant strain in the global pandemic. The experiments showed that the variant, known as G614, reproduced faster, but appeared to be just as sensitive to antibodies to the previous strain and was not associated with a more severe disease.

Meanwhile, much of Europe’s new wave was followed by outbreaks among agricultural workers living in cramped conditions in the Catalan and Aragonese regions of Spain. Researchers in Switzerland and Spain have identified a variant known as 20A.EU1, which has since spread to nearby cities and across Europe through returning tourists.

In Italy, where the government reported 674 deaths on Friday due to Covid-19, the most affected areas are the northern regions of Lombardy, Veneto and Piedmont. But the southern region of the Campaign, which includes the city of Naples, is now one of the five most affected regions.

A maximum of two guests will be allowed in each house in Italy during this period, but the limit will not apply to children under 14 years of age. Bars, restaurants and non-essential businesses will be closed. A commissioning already in force between 22:00 and 05:00 will continue to apply.

Unlike in the spring of Italy, churches will remain open, albeit with limited capacity, subject to social distancing.

Elsewhere in Europe, Germany and the Netherlands have imposed blockades until Christmas, and Austria said on Friday that non-essential business would be closed from December 26.

—Francis X. Rocca of Rome contributed to this article.

Write to Jason Douglas the [email protected]