Public health officials in the U.S. could take heart at the end of the summer. Even as the new coronavirus continued to spread, fewer people were winding up in the hospital because of COVID-19, and fewer were dying.

Now, as the seasons turn, there are signs suggesting there will be more deaths and serious illness ahead.

Data collected by the COVID Tracking Project shows that the number of people hospitalized has plateaued at about 30,000 in the past week, after a decline from nearly 60,000 that began in late July. Deaths, meanwhile, averaged about 750 over the seven days through Sunday, higher than the roughly 600 deaths a day in the first week of July.

Scientists had hoped that a warm-weather reprieve could soften an expected reemergence of the coronavirus in the colder months. Instead, the contagion continued to spread across the country after Memorial Day, with early-summer outbreaks in Sun Belt states followed by the recent surge of new infections in the Upper Midwest and on college campuses nationwide.

Any indication hospitals are attending to more coronavirus patients is likely to reignite concerns that the health care system could be overwhelmed by new cases as the weather cools and more activities, including school and holiday socializing, move indoors.

History and science suggest the second winter with coronavirus is likely to be worse than the first. The pathogen is more entrenched and most respiratory viruses circulate primarily in the winter months.

“We haven’t had exposure to COVID throughout an entire winter, when more people are indoors and close together for prolonged periods,” said William Schaffner, an infectious disease professor at Vanderbilt University in Nashville, Tennessee. “We are certainly concerned that COVID could spread even more readily in the winter than it has so far.”

The Trump administration has pointed to the increasing availability of coronavirus tests as the reason the number of new cases in the U.S. remains high. Diagnostics manufacturers are now shipping more than 1.2 million tests nationwide each day, up from 600,000 at the start of May, according to AdvaMed, a trade group for the medical-technology industry.

Increased testing has also made it possible to catch coronavirus cases earlier. That, combined with improved hospital care and medicines such as Gilead Sciences Inc.’s remdesivir and generic steroid dexamethasone, allowed more patients to survive their infections this summer.

However, a weeklong plateau in new cases, hospitalizations and deaths are an early warning that things could be about to get worse. Along with the resumption of school, more states are easing curbs on restaurants and bars, giving the virus more chances to find vulnerable people to infect. Last week, Florida Gov. Ron DeSantis lifted capacity limits on restaurants and other businesses.

“It’s not complicated to explain. All of that opening up, so many people taking off their masks, gathering together in bars and parties, going back to the old normal,” Schaffner said. “We should not be surprised that we are seeing an increase in COVID again. COVID loves that environment.”

States that had been doing well, including New York, which was wracked early on by the virus, are seeing a new surge. New York Gov. Andrew Cuomo said there were 868 new cases in the state on Sunday, an 18% increase from two weeks earlier. A higher percentage of those getting tested are now coming back positive, suggesting the amount of virus in the community is on the rise.

Similar increases are happening among the nation’s children, as more than 56 million returned to school this month. More than a quarter of a million children were infected with coronavirus from March through Sept. 19, according to the U.S. Centers for Disease Control and Prevention. While those hardest hit were more likely to have other conditions, about three of every four who were hospitalized, needed intensive care or died had no other health concerns, the CDC said.

The number of cases among children has increased dramatically since the start of September, when many went back to school in person at least part time.

Coronavirus cases in those 19 and younger have increased threefold since May, according to the CDC, suggesting they may play an increasingly important role in community transmission even if their individual risk of serious illness is low.

“School studies suggest that in-person learning can be safe in communities with low SARS-CoV-2 transmission rates, but might increase transmission risk in communities where transmission is already high,” the agency said in its Morbidity and Mortality Weekly Report.