LONDON >> The U.K. tightened up rules today on mask-wearing and on testing of international arrivals after finding two cases of the new potentially more contagious omicron variant of the coronavirus that was identified just a few days ago in South Africa.

Amid fears that the new variant has the potential to be more resistant to the protection offered by vaccines, there are growing concerns around the world that the pandemic and associated lockdown restrictions will persist for far longer than hoped.

Nearly two years on since the start of the pandemic that has claimed more than 5 million lives around the world, countries are on high alert. Many countries have already imposed travel restrictions on flights from southern Africa as they seek to buy time to assess whether the omicron variant is more transmissable than the current dominant delta variant.

In an attempt to slow the spread in the U.K., British Prime Minister Boris Johnson said it was necessary to take “targeted and precautionary measures” after two people tested positive for the new variant in England.

“Right now this is the responsible course of action to slow down the seeding and the spread of this new variant and to maximize our defenses,” he told a news conference.

Among the measures announced, Johnson said anyone arriving in England will be asked to take a a mandatory PCR test for COVID-19 on the second day after their arrival and must self isolate until they provide a negative test. And if someone tests positive for the omicron variant, then he said their close contacts will have to self-isolate for 10 days regardless of their vaccination status — currently close contacts are exempt from quarantine rules if they are fully-vaccinated.

He also said mask-wearing in shops and on public transport will be required and said the independent group of scientists that advises the British government on the rollout of coronavirus vaccines has been asked to accelerate the vaccination program. This could involve widening the booster program to younger age groups, reducing the time period between a second dose and a booster and allowing older children to get a second dose.

“From today we’re going to boost the booster campaign,” he said.

Britain’s Department of Health said the two cases found in the U.K. were linked and involved travel from southern Africa. One of the two new cases was in the southeastern English town of Brentwood, while the other is in the central city of Nottingham. The two confirmed cases are self-isolating with their households while contact tracing and targeted testing takes place.

The British government also added four more countries — Angola, Malawi, Mozambique and Zambia — onto the country’s travel red list from Sunday. Six others — Botswana, Eswatini (formerly Swaziland), Lesotho, Namibia, South Africa and Zimbabwe — were added Friday. That means anyone permitted to arrive from those destinations will have to quarantine.

Many countries have slapped restrictions on various southern African countries over the past couple of days including Australia, Brazil, Canada, the European Union, Iran, Japan, Thailand and the United States, in response to warnings over the transmissability of the new variant — against the advice of the World Health Organization.

Despite the banning of flights, there are mounting concerns that the variant has already been widely seeded around the world. In addition to the U.K, cases have been reported in travelers in Belgium, Israel and Hong Kong.

Germany also said it suspected three positive cases while Italy was investigating a recently returned positive case from southern Africa. Dutch authorities were also testing whether 61 people who arrived on two flights from South Africa with COVID-19 have the omicron variant. And Israel said it is tracing 800 travelers who returned recently from southern African countries.

The variant’s swift spread among young people in South Africa has alarmed health professionals even though there was no immediate indication whether the variant causes more severe disease.

A number of pharmaceutical firms, including AstraZeneca, Moderna, Novavax and Pfizer, said they have plans in place to adapt their vaccines in light of the emergence of omicron. Pfizer and its partner BioNTech said they expect to be able to tweak their vaccine in around 100 days.

Professor Andrew Pollard, the director of the Oxford Vaccine Group which developed the AstraZeneca vaccine, expressed cautious optimism that existing vaccines could be effective at preventing serious disease from the omicron variant, noting that most of the mutations appear to be in similar regions as those in other variants.

“At least from a speculative point of view we have some optimism that the vaccine should still work against a new variant for serious disease but really we need to wait several weeks to have that confirmed,” he told BBC radio.

Some experts said the variant’s emergence illustrated how rich countries’ hoarding of vaccines threatens to prolong the pandemic.

Fewer than 6% of people in Africa have been fully immunized against COVID-19, and millions of health workers and vulnerable populations have yet to receive a single dose. Those conditions can speed up spread of the virus, offering more opportunities for it to evolve into a dangerous variant.

“One of the key factors to emergence of variants may well be low vaccination rates in parts of the world, and the WHO warning that none of us is safe until all of us are safe and should be heeded,” said Peter Openshaw, a professor of experimental medicine at Imperial College London.