Germany kept Covid-19 infection rates relatively low this past summer—a feat experts say might be driving a record surge in infections in the country that has prompted fears that hospitalizations and deaths could spiral in the colder months ahead.

Infections are rising again in Europe, as colder temperatures and the fading of vaccine-induced immunity drive renewed case loads.

Even so, Germany and some of its smaller Central and Eastern European neighbors stand out with a far steeper rise in infections than neighboring France, Italy and Spain. Germany registered over 37,000 new cases on Friday, the highest daily number on record, according to government figures, as the seven-day incidence of coronavirus rose to over 200 in 100,000 people.

Experts blame Germany’s figures on the German population’s relatively low exposure to the virus after the country dodged a summer swell of infections that afflicted its Western and Southern neighbors. That resulted in low natural immunity among the population.

“The figures are frightening,” said Prof. Lothar Wieler, head of the Robert Koch Institute, Germany’s center for disease control, told reporters last week, saying that prevention measures such as social distancing and mask-wearing were no longer being sufficiently adhered to.

Over 67% of the German population have been fully vaccinated, and experts say this is helping to keep hospitalizations and deaths below their level during previous case surges. The seven-day rolling average of deaths linked to Covid-19 rose to 118 on Sunday, much lower than the record 894 in January but significantly more than the average in recent months.

Germany, the European Union’s most populous country with 83.2 million inhabitants, had more than 2,500 Covid-19 patients in intensive care on Sunday, compared with over 5,000 in late April, at the peak of the previous wave of infections.

Yet Germany is struggling to raise the number of vaccinated further, with vaccinations plateauing recently. Unlike some other European countries, German authorities don’t mandate vaccinations for workers because of concern that such mandates could be overturned in court or cause a popular backlash.