Another 700,000 Europeans could die by March 1, the WHO said on Tuesday, in addition to the 1.5 million who have already succumbed to the virus.
It expects "high or extreme stress in intensive care units (ICUs) in 49 out of 53 countries between now and March 1, 2022".
Europe's return as the pandemic's epicentre has been blamed on sluggish vaccine uptake in some nations, the highly contagious Delta variant, colder weather moving people indoors again and the easing of restrictions.
The surge has seen Austria return to lockdown this week, while Germany and the Netherlands are poised to announce new restrictions.
In the European Union, 67.7 percent of the population is fully vaccinated.
But rates vary widely between countries, with low levels in many eastern countries. Only 24.2 percent of Bulgarians are fully vaccinated, compared with 86.7 percent in Portugal.
According to WHO data, COVID-related deaths in its European region of 53 countries increased last week to nearly 4,200 a day, doubling from 2,100 deaths a day at the end of September.
It said evidence was growing that vaccine-induced protection against infection and mild disease was declining.
Several countries, including Greece, France and Germany, are moving towards requiring a third shot for someone to be considered fully vaccinated.
Austria, meanwhile, closed shops, restaurants and festive markets on Monday, the most drastic restrictions seen in Western Europe for months.
Chancellor Angela Merkel has warned that Germany is not doing enough to curb its "highly dramatic" fourth wave of the pandemic.
With intensive care beds swiftly filling up and its weekly incidence rate at an all-time high of 399.8 new infections per 100,000 people, Germany's worst-hit regions have ordered new shutdowns, including the closure of Christmas markets.
Al Jazeera's Dominic Kane, reporting from Berlin, said there are two milestones on people's minds in Germany.
"The first one has already been exceeded," he said. "More than five million cases in this country since the pandemic began. The second one, which is perhaps psychologically much more significant is going to happen within the next two days potentially: 100,000 people dead in this country from coronavirus."
The number of COVID deaths in the country currently stands at just less than 99,000, but with 300 people dying every day from COVID, Kane said.
'Challenging winter ahead'
The WHO said a high number of unvaccinated people, as well as "reduced vaccine-induced protection", were among the factors stoking high transmission in Europe alongside the dominance of the Delta variant and the relaxation of hygiene measures.
The regional director for WHO Europe, Hans Kluge, said Europe and Central Asia "face a challenging winter ahead".
He called for a "vaccine plus" approach, consisting of a combination of vaccinations, social distancing, the use of face masks and hand washing.
The WHO said face masks reduce COVID incidence by 53 percent, according to a recent study, and "over 160,000 deaths could be prevented (by March 1) if universal mask coverage of 95 percent was achieved".
But the prospect of a winter under renewed restrictions has sparked unrest in several countries.
Belgium, the Netherlands and France's Caribbean islands of Guadeloupe and Martinique were still reeling on Tuesday from violent protests against new anti-COVID measures.
Dutch police arrested at least 21 people during a fourth night of clashes, which Prime Minister Mark Rutte labelled "pure violence" by "idiots".
Dutch coronavirus infection numbers hit a new weekly record on Tuesday, climbing 39 percent while hospital and intensive care unit admissions also rose sharply, prompting the government to make social distancing mandatory again for all adults.
The Dutch government has also introduced legislation that would clear the way to restrict access for unvaccinated people to indoor venues such as bars, restaurants and museums if infections keep rising. The bill is expected to be debated in parliament next week.
Al Jazeera's Andrew Simmons, reporting from Rotterdam, said the surge in cases was "alarming" and that there were also deep divisions over the prospect of social restrictions becoming law.
"There is still concern about restrictions here because the level of restrictions is reasonably high. Now, moves to make some of those restrictions into law are making some people really concerned. There is a division."
"The stunning fact is that 84 percent of people here are vaccinated. So that's a high vaccination rate and a high infection rate. This is a situation that's alarming to all concerned." (mr/aje)
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