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The World Health Organization has warned that half of Europe will have been infected with the Omicron variant of Covid-19 within six to eight weeks.
Dr Hans Kluge said a "west-to-east tidal wave" of Omicron was sweeping across the region, on top of the surge of the Delta variant already present.
The projection was based on the seven million new cases reported across Europe in the first week of 2022.
The number of infections has more than doubled in a two-week period.
"Today the Omicron variant represents a new west-to-east tidal wave, sweeping across the region on top of the Delta surge that all countries were managing until late 2021," Dr Kluge told a news conference.
He said European and Central Asian countries remained under "intense pressure" as the virus spread from western countries into the Balkans.
"How each country now responds must be informed by its epidemiological situation, available resources, vaccination uptake status and socio-economic context", he added.
Recent studies suggest that Omicron is less likely to make people seriously ill than previous Covid variants. But Omicron is still highly contagious and can infect people even if they are fully vaccinated.
The record number of people catching it has left health systems under severe strain.
On Monday, the UK reported a further 142,224 confirmed cases of the virus and 77 deaths. A number of hospitals have declared "critical" incidents due to staff absences and rising pressures caused by Covid.
Elsewhere, hospital numbers are also rising. France's Health Minister Olivier Veran warned last week that January would be tough for hospitals.
He added that Omicron patients were taking up "conventional" beds in hospitals while Delta was putting a strain on ICU departments.
In eastern Europe, Poland reported that 100,000 people had died from the virus in the country since the start of the pandemic. Poland now has the sixth-highest mortality rate in the world from Covid-19, and almost 40% of its population remains unvaccinated.
On Monday, the pharmaceutical firm Pfizer said it would be able to launch a version of its vaccine that offers special protection against Omicron, to be rolled out in March. Health experts say it is not yet clear whether this is needed.