The World Health Organisation has said it is "alarmed" about the surge in COVID-19 infections across Europe as the continent battles a fresh wave of the virus.
Cases are rising again in much of Europe - sharply in the case of countries like Germany, the Netherlands and Austria, with the latter going back into a full lockdown as a result.
Robb Butler, executive director for WHO Europe, told Kay Burley on Sky News half a million more deaths could be recorded by early 2022 if measures are not taken to stem the spread of the deadly coronavirus.
"We're very alarmed," he said.
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"We've just surpassed, very sadly, the 1.5 million deaths mark last week.
"If we continue on the current course we are projecting a further 500,000 deaths by spring next year.
"That's really worrisome."
Mr Butler said only 54% of the population of the 53 WHO Europe region member states are vaccinated, "so there is so much work to be done, still. We are not there yet".
He added: "It looks like we are going to have a worrisome season ahead."
Asked what was driving the resurgence in mainland Europe, he said it was partly due to there being too many vulnerable populations to infection, but also the "waning immunity" of vaccines, particularly after 30 weeks.
"The majority of people in intensive care units are the unvaccinated," he said.
Other compounding factors include the more transmissible Delta variant - which accounts for 99% of cases - the winter season, people moving indoors and the need for face masks and more ventilation.
Mr Butler pointed to a study last week in the BMJ which suggested 53% of transmission was prevented by mask use.
He said only 48% of the population on the continent was using masks.
"If we see this go up, we will see a reduction in cases and deaths," he said. "If we saw 95% universal mask use we can project we could save about 160,000 lives (in Europe)."
Mr Butler said: "What we need is every member of society vaccinated."
Asked about his thoughts on making jabs compulsory, he replied: "Mandatory vaccination can, but doesn't always increase uptake.
"There are lessons of history here where mandates have come at the expense of trust, social inclusion. So it is very delicate, but we believe it is time to have that conversation, from an individual and population-based perspective.
"It's a healthy debate to have."
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Thousands of protesters rallied in cities across Europe over the weekend against leaders looking to tighten COVID-19 restrictions to curb the latest wave of infections.
Also asked about mandatory jabs, Deputy PM Dominic Raab told Kay Burley on Sky News: "Maybe in some countries but I don't think in the UK, given the extraordinary success we've had in rolling out the vaccine.
"The rollout scheme of boosters is working very effectively. It shows we've got the right balance in the UK and we should stick to our guns."
He added: "I don't think we need to divert from the course we're on right now."
