The European Commission president has warned countries not to “make the same mistakes again” by relaxing coronavirus lockdowns too quickly.
Ursula von der Leyen made the plea after some European countries announced restrictions would be eased in the run-up to Christmas.
"We must learn from the summer and not repeat the same mistakes," Von der Leyen told the European Parliament on Wednesday.
"Relaxing too fast and too much is a risk for a third wave after Christmas."
Watch: PM outlines UK Christmas agreement
The lockdown in England ends next week and the four nations of the UK have agreed to relax restrictions for Christmas to allow up to three households to meet at home for five days.
Dr Susan Hopkins, member of the government’s Scientific Advisory Group for Emergencies, said last week that every day of eased restrictions over Christmas will need to be counterbalanced with five days of tighter rules.
French president Emmanuel Macron said the country will start easing its COVID-19 lockdown this weekend so that by Christmas, shops, theatres and cinemas will reopen and people can spend the holiday with their families.
New infections and the number of people in hospital with the virus in France dropped sharply as a national lockdown went into its fourth week.
Macron added the most vulnerable people could start being vaccinated by the end of the year.
Germany on Wednesday reported a record 410 COVID-19 deaths in the last 24 hours, before its 16 federal state leaders and chancellor Angela Merkel were due to discuss an extension of restrictions into December and for Christmas and New Year.
Italy reported 853 deaths related to COVID-19 on Tuesday, soaring from 630 the day before and the highest daily toll since 28 March.
Many regions are under partial lockdown, with restrictions due to stay in place until at least 3 December.
Prime minister Giuseppe Conte has warned Italians not to ski during the Christmas holidays to help curb a second wave of the pandemic that has killed more than 50,000 people in the country.
Hopes for a successful vaccine, boosted by Pfizer, AstraZeneca and Moderna, have given world stock markets a lift.
But an approved vaccine is unlikely to be widely available for months, with scientists insisting on the continued need for vigilance as global infections approached 60 million on Wednesday.
Watch: How to remove a face covering correctly