LONDON — In a bid to halt the spread of the omicron variant of the coronavirus, British Prime Minister Boris Johnson announced new measures on Saturday evening, including mandatory self-quarantine for anyone arriving in the United Kingdom starting Tuesday and tougher rules on mask-wearing.

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       The initiatives, including significant entry-rule changes made in response to the new variant, are a sign of how countries are reintroducing rules that many had thought were left behind.

       Earlier in the day, Britain announced that two cases of the new variant, first identified in South Africa, had been detected in the U.K. The cases are linked and connected with travel to southern Africa.

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       Speaking at an evening news conference at 10 Downing Street, Johnson said that anyone entering the country will be asked to take a PCR test by the end of their second day and that they must self-quarantine until they provide a negative coronavirus test result. He also said that those who do come into contact with someone suspected of testing positive for omicron will have to quarantine for 10 days, regardless of their vaccination status.

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       Face coverings on public transport and in shops will be mandatory in England after they were scrapped in July. They have remained mandatory on public transport and in many indoor spaces in Scotland, Wales and Northern Ireland.

       Chris Whitty, England’s chief medical officer, said the main reason the U.K. is moving quickly is because of the risk that the available vaccines aren’t as effective against this new variant. “The risk here is different to delta,” Whitty said, adding that in that case the concern was how rapidly it spread. With omicron, however, there are “at least strong theoretical reasons for thinking that some degree of vaccine escape is likely,” he said.

       The prime minister said the new rules, which will be reviewed in three weeks, will help to “buy time” for scientists to better understand the variant.

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       The U.K. also added four new African countries to its travel “red list” on Saturday, meaning that travel from a total of 10 African countries is now restricted: Malawi, Mozambique, Zambia, Angola, Botswana, Eswatini, Lesotho, Namibia, South Africa and Zimbabwe. Travelers from those countries will be denied entry into the U.K. unless they are British or Irish citizens or residents, in which case they will have to quarantine at a government-approved hotel for 10 days.

       Downing Street’s announcement Saturday, however, was short on details and left many people shortly traveling to the U.K. unclear about the new rules.

       British media reported Sunday that the new PCR requirement would go into effect at 4 a.m. Tuesday and that the test had to be taken within two days of arrival.

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       But it was clear that Johnson was placing faith in the current vaccination program as he urged a faster rollout of booster shots. “We don’t yet, exactly, know how effective our vaccines will be against omicron, but we have good reasons for believing they will provide at least some measure of protection,” he said.

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       He was asked at the news conference whether Britons should consider rearranging their plans for Christmas.

       Johnson said he was “confident” that Christmas would be “considerably better than last Christmas.” Given the severe restrictions on many Britons during Christmas last year, that’s not a particularly high bar.

       Read more:

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