A past coronavirus infection appears to give little immunity to the new Omicron variant rippling across the globe, South African scientists warned on Thursday, potentially tearing away one layer of defense that humanity has won slowly and at immense cost.

Just a week after its existence was revealed to the world, the heavily mutated variant, which scientists fear could be the most contagious one yet, is already by far the dominant form of the virus in South Africa and spreading fast, according to officials there. Top European disease experts said Thursday that it could be the dominant form in Europe within months.

Scientists in South Africa have reported a sudden, sharp rise last month in coronavirus cases among people in that country who had already been infected, in a study that has not yet been reviewed and published by a scientific journal. The authors noted that there was no such upswing when the Beta and Delta variants emerged.

They did not say how many of those reinfections could be attributed to Omicron, but South Africa’s National Institute for Communicable Diseases reported on Wednesday that when it conducted a genetic analysis on a sampling of coronavirus-positive test results from November, almost three-quarters were the new variant.

“Population-level evidence suggests that the Omicron variant is associated with substantial ability to evade immunity from prior infection,” the authors of the unpublished study wrote.

In an online briefing held by the World Health Organization’s regional office for Africa, South African scientists presented a blunter version of the same conclusion, simply based on the country’s raw numbers: About 40 percent of South Africans have had the coronavirus and about 30 percent have been at least partially vaccinated (though there is no doubt some overlap), and yet the number of new cases is soaring.

“We believe that previous infection does not provide them protection from infection due to Omicron,” said Anne von Gottberg, a microbiologist at the communicable disease institute.

South Africa has the world’s fastest-growing caseload, though the figures are small compared with those in many other countries. In the first half of November, it was averaging about 260 new reported cases a day. On Tuesday, the figure was over 4,300, the highest in months. It jumped to more than 8,600 on Wednesday, and to more than 11,500 on Thursday.

Scientists say that the number and type of changes suggest that Omicron is much more transmissible than earlier forms of the virus, though solid proof of that is still lacking. As countries around the world have raced to implement new travel rules, with some barring fliers from southern Africa, experts say those measures could have a limited effect if not accompanied by other steps, including expanding vaccinations, wearing masks and social distancing.

“Border control can reduce the risk of importation and can buy time,” Dr. Takeshi Kasai, the World Health Organization’s regional director for the western Pacific told an online news conference on Friday. “But it’s not possible to completely stop the virus unless you completely close the border, which has a significant impact.”