Evidence from South Africa, where the Omicron variant already dominates, shows a high rate of reinfection of people who have already had the coronavirus.

By Lynsey Chutel and Richard Pérez-Peña

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 a few months.

By Thursday, Omicron had been detected in 25 countries on six continents, and experts say it will soon be in every populated corner on earth. That could mean that a world already battered by two years of pandemic and — until recently — harboring hopes for recovery is instead headed for another wave of cases.

Scientists have known since early in the pandemic that the immunity gained from a coronavirus infection is not total, and probably not permanent, and that some people are reinfected. Even so, with a huge number of people already infected and recovered — about 260 million worldwide that have been detected, and in reality far more, experts say — whatever protection they had looked like an important layer in the world’s defenses.

The new variant calls that into question.

Scientists in South Africa have reported a sudden, sharp rise in November 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the Omicron variant is associated with substantial ability to evade immunity from prior infection,” the authors of the unpublished study wrote.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 the population has had the coronavirus and about 30 percent has been at least partially vaccinated (though there is no doubt some overlap), and yet the number of new cases is soaring.

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