JOHANNESBURG—South Africa’s government is considering new public-health restrictions to contain a fast-spreading new variant of the coronavirus that scientists say has a high number of mutations that may make it more transmissible and allow it to evade some of the immune responses triggered by previous infection or vaccination.

The warning from the South African scientists and the Health Ministry, issued in a hastily called news briefing Thursday, prompted the World Health Organization to call a meeting of experts for Friday...

JOHANNESBURG—South Africa’s government is considering new public-health restrictions to contain a fast-spreading new variant of the coronavirus that scientists say has a high number of mutations that may make it more transmissible and allow it to evade some of the immune responses triggered by previous infection or vaccination.

The warning from the South African scientists and the Health Ministry, issued in a hastily called news briefing Thursday, prompted the World Health Organization to call a meeting of experts for Friday to discuss whether to declare the new strain a “variant of concern.”

The WHO uses this label for virus strains that have been proven to be more contagious, lead to more serious illness or decrease the effectiveness of public-health measures, tests, treatments or vaccines. Other variants of concern include the Delta variant that is now dominant world-wide and the Alpha variant that drove a deadly wave of infections across Europe and the U.S. last winter and spring.

While the scientists said they were still studying the exact combination of mutations of the new variant—currently dubbed B.1.1.529—and how they affect the virus, its discovery underlines how changes to the virus’s genome continue to pose a risk to the world’s emergence from the Covid-19 pandemic.

“It just again reinforces the fact that this invisible enemy we are dealing with is very unpredictable,” said South African Health Minister Joe Phaahla. He said the government would hold discussions over the weekend on whether new restrictions on social gatherings and other activities such as travel were necessary to stem the spread of the new variant.

“This is going to present a major challenge,” he said, urging all South Africans who haven’t gotten vaccinated against Covid-19 to do so now. Only around 24% of South Africa’s 60 million citizens are fully vaccinated.

Researchers first detected B.1.1.529 on Nov. 12, said Tulio de Oliveira, who heads the Centre for Epidemic Response & Innovation at South Africa’s Stellenbosch University and is a member of the WHO working group monitoring new coronavirus variants. Since then, the variant has driven an exponential rise in Covid-19 infections in the country, albeit from a very low level, with the number of new infections topping 1,200 on Wednesday, about four times as many as two weeks ago.

As the Delta variant sweeps the globe, scientists are learning more about why new versions of the coronavirus spread faster, and what this could mean for vaccine efforts. The spike protein, which gives the virus its unmistakable shape, may hold the key. Illustration: Nick Collingwood/WSJ The Wall Street Journal Interactive Edition

B.1.1.529 is now responsible for around 90% of cases in South Africa’s most populous province, Gauteng, home to the political and economic capitals of Pretoria and Johannesburg, quickly crowding out the Delta variant. It has also been detected in neighboring Botswana and in a South African traveler in Hong Kong, said Prof. de Oliveira.

Prof. de Oliveira said he expected the WHO working group to give B.1.1.529 a new name taken from the Greek alphabet, which suggests that it would either be declared a variant of concern or a “variant of interest.” Variants of interest have genetic changes known to affect how the virus works, but still require further study, according to the WHO.

The new variant has more than 50 mutations compared with the coronavirus that was first detected in Wuhan, China, in 2019. More than 30 of those mutations are in the spike protein, through which the virus attaches to human cells and which is the main target of the current crop of Covid-19 vaccines.

While many of these mutations appear to be new, several are known to scientists from other variants of concern, where they appeared to make the virus more contagious or allowed it to evade parts of the immune response prompted by vaccination or a previous Covid-19 infection.

“All these things give us some concern that this variant might have not just enhanced transmissibility, so spread more efficiently, but might also be able to get around parts of the immune system and the protection we have in our immune system,” Richard Lessells, an infectious-disease specialist at the KwaZulu-Natal Research Innovation and Sequencing Platform, said during the briefing.

Dr. Lessells said he and other colleagues were still studying whether B.1.1.529 was mostly infecting unvaccinated people and whether it leads to more serious cases of Covid-19. “The mutation profile gives us concerns, but we now need to do the work to really understand the significance of this variant and what it means for the response to this pandemic,” he said.

Covid-19’s Delta variant is proliferating world-wide threatening unvaccinated populations and economic recovery. WSJ breaks down events in key countries to explain why Delta spreads faster than previously detected strains. Composite: Sharon Shi The Wall Street Journal Interactive Edition

Botswana’s government said Thursday that all four infections with B.1.1.529 it had detected were in people who were fully vaccinated and got tested ahead of planned travel.

In contrast with most other prominent variants, which can be detected only through time-consuming and costly genome sequencing, the presence of B.1.1.529 gets flagged in some of the commonly used polymerase chain reaction, or PCR, tests, Prof. de Oliveira said. That makes it easier to track and is one of the reasons why South African scientists were able to pick up on its fast spread in the country so quickly.

Maria Van Kerkhove, who leads the WHO’s technical work on Covid-19, said the agency’s experts were keeping a close eye on the new variant because of the high number of mutations.

“This is one to watch. I would say we have concern, but I think you would want us to have concern,” she said during a question-and-answer session on social media. “We have people who are on this.”

She added that the best way to avoid the emergence of new variants was to contain the spread of the virus. “Everybody that’s out there needs to understand that the more this virus circulates, the more opportunities the virus has to change, the more mutations we will see,” she said.

Write to Gabriele Steinhauser at gabriele.steinhauser@wsj.com