Omicron still poses "very high" risk and could overwhelm healthcare systems, the WHO warned on Wednesday, as the highly transmissible coronavirus variant fuelled record outbreaks in many countries.
Case numbers have shot up 11 percent globally in the last week, forcing governments from China to Germany and France to find a difficult balance between anti-virus restrictions and the need to keep economies and societies open.
The Netherlands and Switzerland said Omicron had become the dominant strain in their countries, and while some studies suggested it causes milder Covid-19, the World Health Organization urged caution.
"The overall risk related to the new variant of concern Omicron remains very high," the UN health agency said in its Covid-19 weekly epidemiological update.
"Consistent evidence shows that the Omicron variant has a growth advantage over the Delta variant with a doubling time of two to three days."
The WHO said early data from Britain, South Africa, and Denmark -- which currently has the world's highest rate of infection per person -- suggested there was a reduced risk of hospitalisation for Omicron compared with Delta.
But it added that further data was needed to understand Omicron's severity.
And despite those studies, Omicron's rapid growth "will still result in large numbers of hospitalisations, particularly amongst unvaccinated groups, and cause widespread disruption to health systems and other critical services", warned WHO Europe's Covid Incident Manager Catherine Smallwood.
- Painful restrictions -
Europe was again one of the hotspots for the pandemic, which is known to have claimed more than 5.4 million lives around the world.
France, Britain, Greece and Portugal all reported record daily case numbers on Tuesday. France reported almost 180,000 infections over 24 hours.
To hold back the tide, many nations on the continent have brought back curbs with heavy economic and social consequences.
Contact restrictions were in place in Germany for the second year in a row heading into the New Year, as Europe's biggest economy shut nightclubs and forced sports competitions behind closed doors.
It also limited private gatherings to 10 vaccinated people -- or two households where any unvaccinated people are present.
Finland on Tuesday said it would bar unvaccinated foreign travellers from entering. Only residents, essential workers or diplomats will be exempt.
The Nordic country, like Sweden, had begun requiring negative tests for incoming non-resident travellers from Tuesday, a day after Denmark applied the same measure.
But the Belgian government's plans to introduce further restrictions were thwarted as a court suspended an order closing entertainment venues.
Prime Minister Alexander De Croo had announced the original measure on December 22 as Belgium saw a sharp increase in the percentage of tests showing the Omicron variant.
- Travel chaos -
The Covid spikes around the world have caused severe disruptions to travel over the holidays, with thousands of flights cancelled worldwide.
They are also threatening sports events again.
England's top football league is grappling with record cases, while China has imposed strict lockdowns on millions of people to control the spread of the Delta variant ahead of the Beijing Winter Olympics.
Mexico City's mayor on Tuesday cancelled the capital's massive New Year's Eve celebrations as a preventative measure after a rise in Covid-19 cases.
The top American sports leagues have also been hit, as the United States battles a surge fuelled by Omicron, as well as large pockets of unvaccinated people and a lack of access to quick and easy testing.
The US Centers for Disease Control has halved the isolation period for asymptomatic cases to try and limit disruptions and mass labour shortages.
President Joe Biden said the United States was generally well prepared, though some hospitals could be "overrun".
As New Yorkers marked "Good Riddance Day" at Times Square -- burning pieces of paper representing their worst memories of 2021 -- many just wanted to say goodbye to Covid-19.
"I want to get rid of Covid and quarantines," said teacher Robin Myers on Tuesday.
"So that my kids can be normal again and see their friends and be out and be social."