The Omicron variant of the coronavirus is spreading significantly faster than the Delta strain in countries with documented community transmission, with the number of cases doubling in 1.5 to 3 days, the World Health Organization (WHO) has said.

In an update on Saturday, the United Nations’ health agency said Omicron has been reported in 89 countries.

The highly mutated variant is spreading rapidly in countries with high levels of population immunity, the WHO said, 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 agency designated Omicron a variant of concern on November 26, soon after it was first detected by scientists in South Africa, and much is still not known about it. Scientists remain uncertain how dangerous it is, but early data suggests it can be more resistant to vaccines and more transmissible than the Delta variant.

“There are still limited data on the clinical severity of Omicron,” the WHO said in its update. “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.”