The World Health Organization urges caution over the Omicron variant, saying it has a growth advantage over the Delta variant.
Omicron still poses a "very high" risk and could overwhelm healthcare systems, the World Health Organization (WHO) has warned.
The United Nations health agency said the overall risk related to the new variant remains "very high" in its Covid-19 weekly epidemiological update on Wednesday.
"Consistent evidence shows that the Omicron variant has a growth advantage over the Delta variant with a doubling time of two to three days," it said.
The WHO said early data from Britain, South Africa, and Denmark 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.
Omicron's rapid growth "will still result in large numbers of hospitalisations, particularly amongst unvaccinated groups, warned WHO Europe's Covid Incident Manager Catherine Smallwood.
It will also "cause widespread disruption to health systems and other critical services", she added.
Case numbers have shot up 11 percent globally in the last week, forcing governments to find a difficult balance between anti-virus restrictions and the need to keep economies and societies open.
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.
The Netherlands and Switzerland said Omicron had become the dominant strain in their countries, while France, Britain, Greece and Portugal all reported record daily case numbers on Tuesday.
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.
Finland and Sweden have begun requiring negative tests for incoming non-resident travellers from Tuesday, a day after Denmark applied the same measure.
Meanwhile, 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.