Hospital admission rates have also doubled, although intensive care admissions remain low.
In a statement on Tuesday, the WHO’s Europe director Dr Hans Kluge described Covid-19 as “a nasty and potentially deadly illness” that people should not underestimate.
He said super-infectious relatives of the Omicron variant were driving new waves of disease across the continent and repeat infections could potentially lead to long Covid.
“With rising cases we’re also seeing a rise in hospitalisations, which are only set to increase further in the autumn and winter months,” Dr Kluge said.
“This forecast presents a huge challenge to the health workforce in country after country, already under enormous pressure dealing with unrelenting crises since 2020.”
Earlier this week, editors of two British medical journals said that at no other time have so many parts of the NHS been so close to collapse.
Kamran Abbasi of the BMJ and Alastair McLellan of the Health Service Journal wrote in a joint editorial that the UK Government was failing to address persistent problems worsened by Covid, including the queuing of ambulances outside hospitals too overloaded to accept new patients.
In the WHO’s autumn strategy for Covid-19 released on Tuesday, the UN agency called for measures including a second booster dose for anyone aged five and over with weak immune systems, promoting mask-wearing indoors and on public transport and better ventilation in places including schools and offices.
He said southern hemisphere countries were experiencing a very active flu season that, combined with Covid, was straining health systems.
“We are likely to see a similar scenario in the northern hemisphere,” Dr Kluge said, warning that increased pressure could lead to business, travel and school chaos.
He urged people to make their own decisions, even in countries where authorities have largely abandoned coronavirus restrictions.
“We’re all aware of the tools we have to keep ourselves safe, assess our level of risk and take the necessary steps to protect others if we get infected,” Dr Kluge said. “Just because a mask isn’t mandated doesn’t mean it’s prohibited.”