Coronavirus has mutated several times since it first appeared in China in 2019, according to epidemiologists. South Africa reported on Friday that it had identified a variable that it said was driving a new wave of infections there. Scientists said it was different from the one identified in Britain.

Most of these variants are neither large nor widespread. But as the number of people affected increases – and presumably they have developed some immunity – the pressure on the pathogen to mutate also increases. Some of the new variants could be more transmissible or lead to more serious disease than previous versions of the virus.

For months, Mr Johnson has struggled to balance his response to the virus and pressure not to further damage the economy. At times, it fell behind other European leaders in imposing restrictions. At other moments, he appeared to be resisting the advice of his government’s scientific advisers.

Britain recorded 27,052 new cases and 534 deaths on Saturday. The number of deaths in the country from the epidemic is the highest in Europe. Johnson referred to the launch of a vaccine that Britain had approved and began administering it before other Western countries as a ray of hope. He said 350,000 people in the country had received the first dose of the Pfizer-BioNTech vaccine, which is a two-dose regimen.

But as the economy weakens, the lockdowns have become politically charged. When the government placed Manchester, Liverpool and other northern cities, where infection rates were higher, into a more restrictive category, local officials complained that they were being unfairly targeted. They noted that life in London has remained largely unchanged.

Last week, amid indications that London was becoming a new hotspot, Mr Johnson placed the capital and most of the southeast at Level 3, the highest level of restrictions. But he clung to his promise to put off Christmas from December 23 to December 27, even as he pleaded with people to keep family gatherings short and small.