The Omicron variant has been detected in 38 countries but no deaths have yet been reported, the WHO said on Friday, as authorities worldwide rushed to stem the heavily mutated Covid-19 strain's spread amid warnings that it could damage the global economic recovery.
The United States and Australia became the latest countries to confirm locally transmitted cases of the variant, as Omicron infections pushed South Africa's total cases past three million.
The World Health Organization has warned it could take weeks to determine how infectious the variant is, whether it causes more severe illness and how effective treatments and vaccines are against it.
"We're going to get the answers that everybody out there needs," WHO emergencies director Michael Ryan said.
The WHO said on Friday it had still not seen any reports of deaths related to Omicron, but the new variant's spread has led to warnings that it could cause more than half of Europe's Covid cases in the next few months.
The new variant could also slow global economic recovery, just as the Delta strain did, International Monetary Fund chief Kristalina Georgieva said on Friday.
"Even before the arrival of this new variant, we were concerned that the recovery, while it continues, is losing somewhat momentum," she said.
"A new variant that may spread very rapidly can dent confidence."
A preliminary study by researchers in South Africa, where the variant was first reported on November 24, suggests it is three times more likely to cause reinfections compared to the Delta or Beta strains.
The emergence of Omicron was the "ultimate evidence" of the danger of unequal global vaccination rates, Red Cross head Francesca Rocca said.
South African doctors said there had been a spike in children under five admitted to hospital since Omicron emerged, but stressed it was too early to know if young children were particularly susceptible.
"The incidence in those under-fives is now second-highest, and second only to the incidence in those over 60," said Wassila Jassat from the National Institute for Communicable Diseases.
In the US, two cases involved residents with no recent international travel history -- showing Omicron is already circulating inside the country.
"This is a case of community spread," the Hawaii Health Department said.
US President Joe Biden on Thursday unveiled his plans to battle Covid-19 during the winter, with new testing requirements for travellers and a surge in vaccination efforts.
'Kick in the nuts'
All incoming travellers will need to test negative within a day of their flights, and rapid tests that cost $25 will be covered by insurance and distributed free to the uninsured.
Australia on Friday reported three students in Sydney had tested positive for the variant, despite a ban on non-citizens entering the country and restrictions on flights from southern Africa, with multiple countries rushing to limit travel from the region in the past week.
"It's quite a kick in the nuts," said Sabine Stam, who runs a South African tour company and whose customers are demanding refunds. "Everyone is too scared to set a new travel date," she said.
In Norway, officials said at least 13 people who contracted Covid-19 after an office Christmas party in Oslo last week had the Omicron variant -- though so far they have only had mild symptoms, city health official Tine Ravlo told AFP.
But the government ushered in restrictions in greater Oslo after fears of the cluster surfaced.
On Friday, Malaysia also reported a first Omicron infection in a foreign student arriving from South Africa on November 19. Sri Lanka also announced its first case, a citizen returning from South Africa.
Russia's federal statistics agency Rosstat, meanwhile, said that nearly 75,000 people died of coronavirus in the country in October, making it the deadliest month of the pandemic.
The new variant poses a major challenge to ending the pandemic.
Rising Delta cases had already forced European governments to reintroduce mandatory mask-wearing, social distancing, curfews or lockdowns, leaving businesses fearing another grim Christmas.
Belgian authorities said on Friday that primary schools would close a week early for the Christmas holidays.
Germany's regional leaders agreed new measures including a ban on fireworks at new year parties to discourage large gatherings.
Ireland said it will close nightclubs and reintroduce social distancing in some settings over Christmas and the New Year.
In the UK, ministers have been expressing divergent opinions, not only on the idea of hosting parties, but also on the kind of conduct deemed acceptable.
"For what it's worth, I don't think there should be much snogging under the mistletoe," said one minister, Therese Coffey.