The Omicron variant has now been identified in 89 countries and is spreading significantly faster than the Delta variant in places where community transmission is high, with a doubling time between 1.5 to 3 days, the WHO has said.

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In its 'Enhancing Readiness for Omicron (B.1.1.529): Technical Brief and Priority Actions for Member States' report on 17 December, the World Health Organisation said that given the current available data, it is likely that Omicron will outpace Delta where community transmission occurs.

"Omicron is spreading rapidly in countries with high levels of population immunity and it remains uncertain to what extent the observed rapid growth rate can be attributed to immune evasion, intrinsic increased transmissibility or a combination of both," it said. "As of 16 December 2021, the Omicron variant has been identified in 89 countries across all six WHO regions. Current understanding of the Omicron variant will continue to evolve as more data becomes available," it said.

There is consistent evidence that Omicron has a substantial growth advantage over Delta. It is spreading significantly faster than the Delta variant in countries with documented community transmission, with a doubling time between 1.5 3 days, the global health body said.

The WHO designated Omicron, or B.1.1.529 variant, as a variant of concern (VOC) on November 26 after it was first detected in South Africa. There is still limited data on the clinical severity of Omicron. More data are needed to understand the severity profile and how severity is impacted by vaccination and pre-existing immunity, the WHO said.

The overall threat posed by Omicron largely depends on how transmissible the variant is; how well vaccines and prior infection protect against infection, transmission, clinical disease and death; how virulent the variant is compared to other variants; and how populations understand these dynamics, perceive risk and follow control measures, including public health and social measures, it said.

There is still limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for Omicron, the WHO added.