Britain became the latest country Saturday to report cases of the new potentially more contagious omicron variant of the coronavirus as world governments sought to shore up their defenses by slapping restrictions on travel from nations in southern Africa.

U.K. Health Secretary Sajid Javid confirmed that two people have tested positive with the omicron variant in the southeastern English town of Chelmsford and in the central city of Nottingham. He said the cases were linked and related to travel from southern Africa.

Javid said the two confirmed cases are self-isolating alongside their households while contact tracing and targeted testing takes place. He also said arrivals from Angola, Malawi, Mozambique and Zambia will have to quarantine from Sunday and stressed the importance of booster jabs.

“This is a real reminder that this pandemic is far from over,” he said. “If we need to take further action, we will.”

Prime Minister Boris Johnson, along with his top advisers, will be holding a media briefing later Saturday.

Many countries have slapped restrictions on various 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 transmissibility of the new variant — against the advice of the World Health Organization. Pharmaceutical companies expressed optimism that they could finesse their vaccines to deal with the new variant though that would clearly take some time.

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 travellers in Belgium, Israel and Hong Kong. Germany also said it suspected a positive case and Dutch authorities were testing whether 61 people who arrived on two flights from South Africa with COVID-19 have the omicron variant.

The planes arrived in the Netherlands from Johannesburg and Cape Town shortly after the Dutch government imposed a ban on flights from southern African nations. The 539 travellers who tested negative were allowed to return home or continue their journeys to other countries. Under government regulations, those who live in the Netherlands and are allowed to return home must self-isolate for at least five days.

Meanwhile, a German official said that there’s a “very high probability” that the omicron variant has already arrived in the country.

Kai Klose, the health minister for Hesse state, which includes Frankfurt, said in a tweet that “several mutations typical of omicron” were found Friday night in a traveller returning from South Africa, who was isolated at home. Sequencing of the test had yet to be completed.

The global health body has named the new variant omicron, labelling it a variant of concern because of its high number of mutations and some early evidence that it carries a higher degree of infection than other variants. That means people who contracted COVID-19 and recovered could be subject to catching it again. It could take weeks to know if current vaccines are less effective against it.

With so much uncertainty about the omicron variant and scientists unlikely to flesh out their findings for a few weeks, countries around the world have been taking a safety-first approach, in the knowledge that previous outbreaks of the pandemic have been partly fueled by lax border policies.

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.

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. In just two weeks, omicron has turned a period of low transmission in the country into one of rapid growth.

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.

He said most of the mutations appear to be in similar regions as those in other variants.

“That tells you that despite those mutations existing in other variants the vaccines have continued to prevent serious disease as we’ve moved through alpha, beta, gamma and delta,” he told BBC radio. “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 added that it is “extremely unlikely that a reboot of a pandemic in a vaccinated population like we saw last year is going to happen.”

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.

“Global vaccine rollout is vital,” he added.