Omicron variant up to 3 times more infectious than Delta, CDC says

The Omicron variant of the coronavirus is up to three times more infectious than the Delta variant, the US Centers for Disease Control and Prevention said Tuesday.

The CDC included the estimate as part of an update to its guidelines on isolating after testing positive for the virus or quarantining after exposure to it.

“Preliminary data suggest that the Omicron variant is up to three times more infectious than the Delta variant,” the CDC says in the update.

“With the recommended shorter isolation and quarantine periods, it is critical that people continue to wear well-fitting masks and take additional precautions for 5 days after leaving isolation or quarantine,” it adds.

“Modeling data from the United Kingdom reinforce the importance of mask use; after the 5th day after a positive test, an estimated 31% of persons remain infectious,” the CDC said. It cites a pre-print study posted online on Dec. 24.

“Mask use and layered prevention strategies, such as receiving all recommended vaccination and booster doses, physical distancing, screening testing, and improved ventilation, are key to preventing COVID-19 and decreasing transmission,” the CDC continued.

For the estimate of transmissibility, the CDC cites a study published in the Journal of Medical Virology. Kimihito Ito of Japan’s Hokkaido University and colleagues used data including 758 cases of Omicron variant infection confirmed in Denmark. They developed a method to calculate the infectiousness of a variant based on how many times it is detected over a certain period of time.

“We have estimated the effective (instantaneous) reproduction number of Omicron as 3.19 times greater than that of Delta under the same epidemiological conditions,” they wrote in the report.

“Theoretically, the enormous advantage of transmission can stem from two independent factors, (i) intrinsically greater transmissibility of Omicron compared to that of Delta, and (ii) substantial capacity of Omicron variant to escape from existing population-level immunity conferred either naturally or by vaccination,” they added.

The UK study, posted on the online server MedRxiv, was done by the UK Health Security Agency and was a modeling study done to support earlier release from isolation.

One table in that paper shows estimates that 31% of people would still be infectious on day five after a test. By day six it’s 22% and by day seven it’s 16%. “The current 10-day isolation period results in the release of 5% of the infected population being released from isolation whilst still being infectious,” the researchers wrote. They advocated for a seven day isolation period.



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