The University Hospital of Giessen, one of Germany’s foremost clinics for pulmonary disease, is at capacity. The number of Covid-19 patients has tripled in recent weeks. Nearly half of them are on ventilators.

And every single one is unvaccinated.

“I ask every patient: Why didn’t you get vaccinated?” said Dr. Susanne Herold, head of infectious diseases, after her daily round on the ward on Thursday. “It’s a mix of people who distrust the vaccine, distrust the state and are often difficult to reach by public information campaigns.”

Patients like hers are the main drivers of the fourth wave of Covid-19 cases in Germany that has produced tens of thousands of new daily infections — more than the country has had at any point in the pandemic.

For Germany, it is a startling turnabout. At the onset of the pandemicGermany had set an example for how to manage the virus and keep the death toll low. It was quick to put in place widespread testing and treatment, expand the number of intensive care beds, and had a trusted leader in Chancellor Angela Merkel, a trained scientist, whose government’s social distancing guidelines were widely observed

But today, a combination of factors has propelled a new surge, among them wintry temperatures, a slow rollout of booster vaccines, and an even more pronounced spike in infections in neighboring eastern European nations like the Czech Republic. The fact that Germany is in a kind of political limbo as it transitions between governments has not helped.

But virologists and pandemic experts say there is little doubt that it is unvaccinated who are contributing most to the wave of infections burdening in hospitals across the country.

“It’s our low rate of vaccination — we haven’t done what was necessary,” said Dr. Herold in Giessen. She was part of a team of scientists who modeled the impact of a fourth wave and warned in early summer that with the hyper contagious Delta variant at least 85 percent of the whole population would need to be vaccinated to avert a crisis in the health care system.