Europe reported its first case of the rapidly spreading new coronavirus variant, and dozens of countries restricted travel to and from southern Africa, where it was first detected, amid fears that it could be more contagious than previous strains and render Covid-19 vaccines ineffective.

Belgian authorities confirmed one case of the variant in the country on Friday. According to Health Minister Frank Vandenbroucke, the person had traveled from abroad, was unvaccinated, and had never been infected with Covid-19 before.

The World Health Organization’s experts were meeting Friday afternoon to decide whether to classify the new strain, currently known as B.1.1.529, as a “variant of concern.” The World Health Organization assigns this label to virus strains that have been shown to be more contagious, cause more serious illness, or reduce the effectiveness of public-health measures, tests, treatments, or vaccines.

Other variants of concern include the Delta variant, which is now dominant globally, and the Alpha variant, which was responsible for a deadly wave of infections in Europe and the United States last winter and spring.

While scientists say they are still figuring out the exact effects of the new variant’s more than 50 mutations, it has been driving a sharp increase in infections in South Africa over the last two weeks, albeit from very low levels. It has also been found in Botswana and in two Hong Kong visitors. Israel, one of the countries that has restricted travel from southern Africa, announced on Friday that it had identified the B.1.1.529 variant in a Malawian traveler and was investigating whether it had also infected two others.

The discovery of the variant highlights the dangers of an evolving virus to the world’s recovery from a pandemic. According to experts, high levels of transmission allow the virus to develop mutations that make it more dangerous, including persistent infection in patients with a suppressed immune system.

BioNTech SE of Germany, which developed one of the most widely used Covid-19 vaccines with Pfizer Inc., said it would take about two weeks to determine whether the new variant makes its shot less effective. A BioNTech spokeswoman said that if necessary, the companies could create a new vaccine tailored to any variant in six weeks and ship initial batches in 100 days.

During the same time period, Covid-19 cases in the 60-million-person country increased from around 300 just over two weeks ago to 2,465 on Thursday. Although this is a much lower count than what is currently seen in many European countries, scientists and South Africa’s Health Ministry have expressed concern about the new variant’s rapid spread as well as its unusually high number of mutations.

The European Union announced on Friday that it would propose activating a “emergency brake” to halt air travel between the 27-country bloc and the southern African region, citing B.1.1.529 concerns. Several member states have already moved to impose restrictions, following the lead of the United Kingdom and several Asian countries, including Singapore.

While many of these mutations appear to be novel, scientists are familiar with several of them from other variants of concern, where they appeared to make the virus more contagious or allowed it to evade parts of the immune response induced by vaccination or a previous Covid-19 infection. Botswana’s government said Thursday that all four B.1.1.529 infections it had discovered were in people who had been fully vaccinated and tested prior to planned travel.

Unlike most other prominent variants, which can only be identified through time-consuming and expensive genome sequencing, the presence of B.1.1.529 is detected by some commonly used polymerase chain reaction, or PCR, tests. This makes it easier to track, which is why South African scientists were able to detect its rapid spread in the country so quickly.

The international travel restrictions, which come just before the holiday season, as well as the prospect of new domestic public-health measures to combat the spread of the new variant, are a painful setback for South Africa, whose economy and population have been severely impacted by the coronavirus.