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Underscoring growing concerns about Omicron, scientists in South Africa said Friday that the newest coronavirus variant is spreading twice as fast as Delta, which is considered the most contagious version of the virus.

The rapid spread of Omicron was due to a combination of contagion and its ability to circumvent the body’s immune defenses, the researchers said. However, the contribution of each factor is not yet certain.

“We’re not sure what that mix is,” said Carl Pearson, a mathematical modeler at the London School of Hygiene and Tropical Medicine, who led the analysis. “It may be even less contagious than Delta.”

Dr. Pearson posted the results on Twitter. The research has not yet undergone peer-reviewed or published in a scientific journal.

On Thursday, researchers found that the new variant partially bypassing immunity acquired from a previous infection. It is still unclear whether or to what extent Omicron will escape the protection provided by vaccines.

But some experts said they would expect the result to be similar.

“It’s scary that so many reinfections occur, which means that vaccine-induced immunity may be similarly affected,” said Akiko Iwasaki, an immunologist at Yale.

The Omicron variant appeared in about two dozen countries. The United States has identified at least 10 cases in six states. President Biden reiterated Friday morning that his administration’s newest pandemic measures announced this week must be sufficient to curb the spread of Omicron.

The variant was first identified on 23 November in South Africa and quickly accounted for about three-quarters of new cases in that country. South Africa reported 11,535 new cases of coronavirus on Thursday, a 35 percent increase from the previous day and the rate of positive test results rose to 22.4 percent from 16.5 percent.

“It’s actually really striking that it’s taken over so quickly,” said Juliet Pulliam, director of an epidemiological modeling center at the University of Stellenbosch in South Africa and leading earlier research on immunity.

Omicron cases are roughly doubling every three days in Gauteng province, home to South Africa’s densely populated economic hub, according to new estimates by researchers.

In a mathematical analysis, they estimated the variant’s Rt (a measure of how quickly a virus spreads) and compared it to the Delta metric. They found that Omicron’s Rt was about 2.5 times higher than Delta’s.

This figure depends not only on how contagious the variant can be, but also on its ability to circumvent the body’s immune defenses when it reaches a new host.

Based on the mutations that Omicron carries, some researchers have warned that the variant might exist. highly contagiousand that current vaccines may not be as effective against it as they are against previous variants.

In research published Thursday, Dr. Pulliam and colleagues estimated the new variant’s ability to evade immunity by looking at confirmed cases in the country by the end of November.

They reported an increase in cases of re-infection in people who tested positive for the virus at least 90 days ago, suggesting that immunity gained from a previous encounter with the virus is no longer as protective as it seems. The increase in reinfections coincided with the spread of Omicron in the country.

Dr. A quirk of Omicron’s genetic code made it easy to distinguish the variant from Delta in diagnostic tests, which helped scientists quickly identify its rapid rise, Pulliam said.

“If we didn’t have this, we would probably be a few weeks behind where we are now in terms of recognizing it as a new variant,” he added.

The team did not confirm that the reinfections they observed were due to the new variant, but said it was a reasonable assumption. The scientists noted that a similar increase did not occur when the Beta and Delta variants were dominant.

Dr. Pulliam and colleagues estimate that the risk of re-infection with the Omicron variant is roughly 2.4 times the risk seen in the original version of the coronavirus.

Vaccines are thought to produce much higher levels of antibodies in the body compared to the levels produced from coronavirus infection. But antibodies produced after an infection have the ability to fend off variants with a wider range of mutations, said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai, New York.

Dr. If the new variant is reinfecting people who have recovered from Covid, “I don’t think there will be much difference” in how Omicron responds to vaccines, Krammer said. “This is not a good sign.”

South African researchers did not know about the severity of the first disease compared to the second. Dr. However, Iwasaki said the immune system should be able to prevent the most severe symptoms in people who have previously been infected or vaccinated.

“I suspect and am hopeful that not all of these will lead to serious illness,” he said. “Maybe there are multiple infections, but they can be milder.”

Gauteng province in South Africa is now the epicenter of what scientists say is the country’s fourth wave of infections. The weekly increase in hospital admissions is higher than in previous waves, according to data from the South African National Institute of Infectious Diseases.

“Gauteng was definitely blocked by the Delta wave just five months ago, so there’s no doubt that this variant is causing a significant number of reinfections, just because of this fact,” said Kristian Andersen, a virologist at the Scripps Research Institute in San Diego.

The percentage of children younger than 5 years of age among total cases also increased sharply – second only to those over 60 – but this may be because more adults are now vaccinated than in previous waves.

Public health expert at the National Institute of Infectious Diseases, Dr. Waasila Jassat said pediatricians are also admitting more children to hospitals, but mostly as a precaution.

Dr. “They were not going to meet the admission criteria later in the wave,” Jassat said. Most hospitalized children are unvaccinated, he said, and live with unvaccinated parents.

Lynsey Chutel Contributed to reporting from South Africa.