GENEVA — The Omicron variant of COVID-19 has been detected in 89 countries and is spreading rapidly even in places with high levels of population immunity, the World Health Organization said Saturday.

Omicron cases double every 1.5 to 3 days in countries with community transmission of the variant — meaning, not being passed on just by people who were infected abroad, 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.

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.

Other major questions about Omicron remain unanswered, including how effective each of the existing COVID-19 vaccines might be against it.

Nations across Europe are moving to reimpose tougher measures to stem a new wave of infections spurred by Omicron, triggering calls for protests from Paris to Barcelona.

As case numbers escalated, France and Austria tightened travel restrictions, while Paris canceled its New Year's Eve fireworks.

Other countries that have imposed new measures due to Omicron include Denmark, which shut down public venues such as theatres and concert halls, and Ireland, where pubs and bars can only work until 8 p.m.

WHO first labeled Omicron a variant of concern on Nov. 26. — Euronews