Underlining growing concerns about Omicron, scientists in South Africa said Friday that the latest coronavirus variant appeared to be spreading more than twice as fast as Delta, which was believed to be the most contagious version of the virus.
Omicron’s rapid spread is due to a combination of infectivity and its ability to evade the body’s immune defenses, the researchers said. But the contribution of each factor is not yet certain.
“We’re not sure what that mixture is,” said Carl Pearson, a mathematical modeler at the London School of Hygiene and Tropical Medicine who led the analysis. “It’s possible it’s even less transferable than Delta.”
dr. pearson posted the results on Twitter. The research has not yet been peer-reviewed and has not been published in a scientific journal.
On Thursday, researchers reported that the new variant can partially evade immunity gained from a previous infection. It is still unclear whether, and to what extent, Omicron can circumvent the vaccines’ protection.
But some experts said they would expect the outcome to be similar.
“It’s frightening that so many reinfections are happening, which means vaccine-induced immunity could also be affected in a similar way,” said Akiko Iwasaki, an immunologist at Yale.
The Omicron variant has appeared in nearly two dozen countries. The United States has identified at least 10 cases in six states. President Biden reiterated Friday morning that his administration’s latest pandemic measures, announced this week, should be enough to slow the spread of Omicron.
The variant was first identified in South Africa on November 23 and is quickly responsible for about three quarters of new cases in that country. South Africa reported 11,535 new coronavirus cases on Thursday, a 35 percent increase from the day before, and the share of positive test results increased to 22.4 percent from 16.5 percent.
“It’s actually quite striking how quickly it seems to have taken over,” said Juliet Pulliam, the director of an epidemiological modeling center at Stellenbosch University in South Africa, who led the earlier research on immunity.
According to the researchers’ new estimates, Omicron cases double about every three days in Gauteng province, home to South Africa’s densely populated economic center.
In a mathematical analysis, they estimated the variant’s Rt — a measure of how quickly a virus spreads — and compared it to the statistic for Delta. They found that Omicron’s Rt is nearly 2.5 times that of Delta.
That figure depends not only on how contagious the variant can be, but also on its ability to evade the body’s immune defenses once it reaches a new host.
Based on the mutations Omicron carries, some researchers had warned that the variant could prove to be highly transmissible and that current vaccines may not be as effective against it as against previous variants.
In the study published Thursday, Dr. Pulliam and her colleagues assessed the new variant’s ability to evade immunity by looking at confirmed cases in the country through late November.
They reported an increase in reinfections in people who had tested positive for the virus at least 90 days earlier, suggesting that the immunity gained from a previous attack with the virus was no longer as protective as it seemed. The increase in reinfections coincided with the spread of Omicron in the country.
A quirk of Omicron’s genetic code made it easy to distinguish Delta’s variant in diagnostic tests, and that helped scientists quickly spot the precipitous rise, said Dr. pulliam.
“Had we not had that, we would probably be several weeks behind where we are now if we recognized that there was a new variant,” she added.
The team did not confirm that the reinfections they observed were caused by the new variant, but said it was a reasonable assumption. A similar spike did not occur when the beta and delta variants were dominant, the scientists noted.
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dr. Pulliam and her colleagues estimate that the risk of reinfection with the Omicron variant is about 2.4 times greater than the risk with the original version of the coronavirus.
Vaccines are thought to produce much higher levels of antibodies in the body, compared to the protection produced against infection with the coronavirus. But antibodies produced after infection can ward off variants with a wider range of mutations, noted Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.
If the new variant reinfects people who have recovered from Covid, “I don’t think there will be much difference” in how Omicron responds to vaccines, said Dr. Krammer. “It’s not a good sign.”
South African researchers had no information on the severity of the first disease compared to the second. But the immune system should be able to prevent the most severe symptoms in people who have had a previous infection or have been immunized, said Dr. Iwasaki.
“I suspect, and I’m hopeful, that not all of these will lead to serious illness,” she said. “Maybe there are many infections, but they can be milder.”
South Africa’s Gauteng province is now the epicenter of what scientists say is the fourth wave of infections in the country. According to data from the National Institute for Communicable Diseases of South Africa, the weekly increase in hospital admissions is already higher than in the previous waves.
“Gauteng was absolutely caught off guard by the Delta wave just five months ago, so there’s no question that this variant causes a significant number of reinfections for that reason alone,” said Kristian Andersen, a virologist at the Scripps Research Institute in San Diego.
The percentage of children under 5 among the total number of cases has also risen sharply — second only to those over 60 — but that may be because more adults are being immunized now than in previous waves.
Pediatricians are also allowing more children into hospitals, but mostly as a precautionary measure, said Dr Waasila Jassat, a public health specialist with the National Institute of Communicable Diseases.
“Later in the wave, they wouldn’t meet the eligibility criteria,” said Dr. Jassat. Most hospitalized children aren’t vaccinated, she said, and live with parents who haven’t been vaccinated either.
Lynsey Chutel contributed reporting from South Africa.