The Omicron variant of the coronavirus has been detected in 89 countries and is spreading rapidly even in places with high levels of population immunity, the World Health Organisation (WHO) said on Saturday.

The number of Covid-19 cases involving Omicron is doubling every 1.5 to three days in countries where there is community transmission of the variant and not just people who were infected abroad, the WHO said.

Omicron’s “substantial growth advantage” over the Delta variant means it is likely to soon overtake Delta as the dominant variant in those countries, the UN health agency said.

Read: WHO flags global risk from Omicron as states tighten restrictions

It remains unclear if the rapid growth of Omicron cases is because the variant evades existing immunity, is inherently more transmissible than previous variants, or a combination of both, the WHO said.

Other major questions about Omicron remain unanswered, including how effective each of the existing Covid-19 vaccines are against it. Conclusive data also does not exist yet on how ill Omicron makes Covid-19 patients, the health agency said.

The WHO first labelled Omicron a variant of concern on November 26.