Underscoring increasing concerns about Omicron, scientists in South Africa said on Friday that the newest coronavirus variant appeared to spread more than twice as quickly as Delta, which had been considered the most contagious version of the virus.
Omicron’s rapid spread results from a combination of contagiousness and an ability to dodge 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 that it might even be less transmissible than Delta.”
Dr. Pearson posted the results on Twitter. The research has not yet been peer-reviewed nor published in a scientific journal.
On Thursday, researchers reported that the new variant may partly dodge immunity gained from a previous infection. It’s still unclear whether, or to what degree, Omicron may evade protection conferred by the vaccines.
But some experts said they would expect the outcome to be similar.
“It’s scary that there are so many reinfections happening, which means that vaccine-induced immunity may also be impacted in 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 on Friday morning that his administration’s newest pandemic measures, which were announced this week, should be sufficient to blunt the spread of Omicron.
The variant was first identified in South Africa on Nov. 23 and has quickly come to account for about three-quarters of new cases in that country. South Africa reported 11,535 new coronavirus cases on Thursday, a 35 percent jump from the day before, and the proportion of positive test results increased to 22.4 percent from 16.5 percent.
“It is actually really 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.
Omicron cases are doubling roughly every three days in Gauteng province, which is home to South Africa’s densely populated economic hub, according to the new estimates by the researchers.
In a mathematical analysis, they estimated the variant’s Rt — a measure of how quickly a virus spreads — and compared it with the metric for Delta. They found that Omicron’s Rt is nearly 2.5 times as high as that of Delta’s.
That figure depends on not just how contagious the variant may be, but also its ability to sidestep the body’s immune defenses once it reaches a new host.
Based on the mutations Omicron carries, some researchers had warned that the variant may turn out to be highly transmissible, and that the current vaccines may not be as effective against it as against previous variants.
In the research published on Thursday, Dr. Pulliam and her colleagues estimated the new variant’s ability to evade immunity by looking at confirmed cases in the country through late November.
They reported an uptick in reinfections among people who had tested positive for the virus at least 90 days earlier, suggesting that the immunity gained from a previous bout with the virus was no longer as protective as it had appeared. The increase in reinfections coincided with Omicron’s spread in the country.
A quirk of Omicron’s genetic code made it easy to distinguish the variant from Delta in diagnostic tests, and that helped scientists quickly spot its steep rise, Dr. Pulliam said.
“If we hadn’t had that, we probably would be several weeks behind where we are now in terms of recognizing 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 estimated that the risk of reinfection with the Omicron variant is roughly 2.4 times as much as the risk seen with the original version of the coronavirus.
Vaccines are thought to produce much higher levels of antibodies in the body, compared with the levels produced from infection with the coronavirus. But antibodies produced after an infection are capable of fending 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 is reinfecting people who have recovered from Covid, “I don’t think that there will be much of a difference” in how Omicron responds to vaccines, Dr. Krammer said. “It’s not a good sign.”
South African researchers did not have information on the severity of the first illness compared with the second. But the immune system should be able to prevent most severe symptoms in people who had a previous infection or have been immunized, Dr. Iwasaki said.
“I suspect, and I’m hopeful, that these aren’t all going to result in severe disease,” she said. “Maybe there are lots of infections, but they may be milder.”
The Gauteng province in South Africa is now the epicenter of what scientists say is the country’s fourth wave of infections. The week-on-week increase in hospital admissions is already higher than in previous waves, according to data from the National Institute for Communicable Diseases of South Africa.
“Gauteng was absolutely clobbered by the Delta wave only five months ago, so there’s no doubt that this variant is causing significant numbers of reinfections, due to that fact alone,” said Kristian Andersen, a virologist at the Scripps Research Institute in San Diego.
The percentage of children younger than 5 among total cases has also risen sharply — second only to those over 60 — but that may be because more adults are immunized now than in previous waves.
Pediatricians are also admitting more children to hospitals, but mostly as a precautionary measure, said Dr. Waasila Jassat, a public health specialist at the National Institute for Communicable Diseases.
“Later in the wave, they wouldn’t meet the criteria for admission,” Dr. Jassat said. Most hospitalized children are unvaccinated, she said, and live with parents who have also not been immunized.
Lynsey Chutel contributed reporting from South Africa.