The European Commission urged EU members states Thursday to better explain and enforce social distancing and hygiene rules to halt a dangerous new wave of coronavirus infections.
Health commissioner Stella Kyriakides said: "In some member states, the situation is now even worse than during the peak in March. This is a real cause for concern.
"All member states need to roll out measures immediately and at the right time at the very first sign of potential new outbreaks."
Europe has now had more than five million coronavirus cases, and several countries have begun reimposing local lockdown rules to head off a return to uncontrolled spread.
The death rate has not returned to the levels seen earlier this year, but cases of new infections are soaring once again in many areas.
Eight major cities in France including Paris will see new restrictions, including limitations on public gatherings to 10 people and earlier closing hours for bars.
The Madrid region, the heart of an explosion of infections in Spain, has already locked down roughly 850,000 people.
On Thursday, Germany reported 2,143 new cases and 19 more deaths, according to the disease control agency Robert Koch Institute.
As a result, Munich -- Germany's third-largest city -- has made face masks compulsory in some busy outdoor spaces and is considering a targeted alcohol ban.
Germany also this week added a slew of popular destinations, including Copenhagen, Dublin and the United Arab Emirates, to its list of foreign locations that require travellers to self-isolate or get tested when returning to Germany.
But Kyriakides, whose office has sought to coordinate the response across the 27 member states, warned that some areas had clearly begun to lift restrictions too soon.
"What this means, to be very clear, is that the control measures taken have simply not being effective enough or not being enforced or followed as they should have been," she said.
"We cannot lower our guard. This crisis is not behind us. Moreover, winter is the time of the year for more respiratory illnesses, including seasonal influenza."
And she warned: "Today we're here to call on everyone to act decisively... it might be our last chance to prevent a repeat of last spring."
Kyriakides said that member state governments would have to fight misinformation about the virus and reach out to young people to head off protests and conspiracy theories.
And she warned they will have to act quickly to prevent a need for a return to generalised lockdowns, which she said would hurt the economy, education and the mental health of the public.