A preliminary study in South Africa shows a sudden, sharp rise in reinfections as Omicron spreads.

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A previous coronavirus infection seems to confer little immunity to the new Omicron variant that is surging around the world, South African scientists warned on Thursday, potentially stripping away a layer of defense that humanity has been slowly and at a tremendous cost.

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

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

They didn’t say how many of those reinfections could be attributed to Omicron, but the South African National Institute of Communicable Diseases reported on Wednesday that when it conducted a genetic analysis of a sample of coronavirus-positive test results from November, nearly three-quarters of the new variant.

“Population-level evidence suggests that the Omicron variant is associated with a significant ability to evade immunity to previous infection,” the authors of the unpublished study wrote.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, based simply on the country’s raw numbers: About 40 percent of South Africans have the coronavirus. and about 30 percent have had it. are at least partially vaccinated (although there is undoubtedly some overlap), and yet the number of new cases is rising.

“We believe that a previous infection does not protect them against infection by Omicron,” said Anne von Gottberg, a microbiologist at the Institute of Infectious Diseases.

South Africa has the world’s fastest growing caseload, although the numbers are small compared to many other countries. In the first half of November, there were an average of about 260 new reported cases per day. On Tuesday, the figure was above 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 the number and type of changes indicate that Omicron is much more transmissible than previous forms of the virus, although solid evidence for this is still lacking. As countries around the world rush to introduce new travel regulations, with some fliers from southern Africa, experts say those measures could have limited effect if not accompanied by other steps, including expanding vaccinations, wearing masks and social distancing.

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

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