But it could take weeks for scientists to fully understand the variant's mutations and whether existing vaccines and treatments are effective against it. Omicron is the fifth variant of concern designated by the WHO.
Those worries pummelled financial markets, especially stocks of airlines and others in the travel sector, and oil, which tumbled by about US$10 a barrel. In Geneva the WHO - whose experts on Friday discussed the risks that the variant, called B.1.1.529, presents - had earlier warned against travel curbs for now.
"This is why we talked about the risk of vaccine apartheid. This virus can evolve in the absence of adequate levels of vaccination," he told Reuters. "Most likely this virus is already in other places. And so if we shut the door now, it's going to be probably too late," said Ben Cowling of the University of Hong Kong.