Omicron is spreading rapidly in countries with high levels of population immunity, but it is unclear if this is due to the virus' ability to evade immunity, its inherent increased transmissibility or a combination of both, the WHO said in an update.

The agency designated Omicron a variant of concern on Nov 26, soon after it was first detected, and much is still not known about it, including the severity of the illness it causes.

"There are still limited data on the clinical severity of Omicron," the WHO said. "More data are needed to understand the severity profile and how severity is impacted by vaccination and pre-existing immunity."

It added, "There are still limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for Omicron".

The WHO warned that with cases rising so rapidly, hospitals could be overwhelmed in some places.

"Hospitalizations in the UK and South Africa continue to rise, and given rapidly increasing case counts, it is possible that many healthcare systems may become quickly overwhelmed."