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Omicron spreading more than twice as quickly as Delta variant in South Africa, scientists report

Researchers sort coronavirus test samples for genetic sequencing at Duke University in Durham, N.C., Feb. 3, 2021.Pete Kiehart/NYT

Underscoring growing concerns about omicron, scientists in South Africa said Friday that the newest coronavirus variant appears to spread more than twice as quickly as delta, until now 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 & Tropical Medicine who contributed to the analysis. “It’s possible that it might even be less transmissible than delta.”


Some of the same researchers announced Thursday that the new variant can partly dodge immunity gained from a previous infection. It’s still unclear whether or to what degree omicron may evade protection conferred by the current vaccines.

The new research was posted on Twitter, and has not yet been peer-reviewed nor published in a scientific journal.

The omicron variant has appeared in nearly two dozen countries. The United States has identified at least 10 cases in six states, and health officials say that community spread of the virus is inevitable. President Joe Biden reiterated Friday morning that his administration’s newest pandemic measures should be sufficient to deal with the spread of the new variant.

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 the country. The country reported 11,535 new coronavirus cases Thursday, a 35% jump from the day before, and the proportion of positive test results increased to 22.4% from 16.5%.

Omicron cases are doubling roughly every three days, the researchers said Friday.

“It is actually really striking how quickly it seems to have taken over,” said Juliet Pulliam, director of an epidemiological modeling center at the University of Stellenbosch in South Africa, who led the earlier research on immunity.


Looking at confirmed cases in the country through late November, Pulliam and her colleagues saw an uptick in reinfections among people who had tested positive for the virus at least 90 days earlier. The increase coincided with omicron’s spread in the country.

Pulliam and her colleagues estimated that the risk of reinfection with the omicron variant is roughly 2.4 times greater than the risk seen with the original version of the coronavirus.

Omicron may spread quickly if it is confirmed to be more contagious and less vulnerable to the body’s immune defenses, experts said. The Gauteng province, home to South Africa’s densely populated economic hub, is now the epicenter of the country’s fourth wave of infections.

The week-over-week increase in hospital admissions is already higher than it was in previous waves, according to data from South Africa’s National Institute for Communicable Diseases. But it is not yet clear whether omicron causes more severe disease than other versions of the coronavirus.

The percentage of new cases found in children younger than 5 has also risen sharply in the country, but that may be because more adults are now immunized.