“The number of patients who had chronic comorbidities surprised us,” said Karina Davidson, a senior vice president at the Feinstein Institutes for Medical Research, who was the paper’s senior author.

The paper, published in the Journal of the American Medical Association, analyzed data about 5,700 Covid-19 patients admitted between March 1 and April 4 to a dozen hospitals in New York City, Long Island and Westchester County that are part of the Northwell Health system. Scientists at the Feinstein Institutes, the research arm of Northwell, used electronic health records and other demographic information to analyze the characteristics of the patients.

Dozens of children and teenagers got sick but survived, the researchers found. Women had a clear edge: Fewer were hospitalized to begin with, and they were more likely to survive.

One in five of the hospital stays ended with the patient dying, but more than half of the patients studied were still hospitalized when the study ended. In total, 553 of the patients died.

About a fifth of the patients — 1,151 — were put on ventilators, and most of those — 831 — were still on the machines when the study ended.

Of the other 320 intubated patients, 282 died and 38 were discharged from the hospital. Their mortality rate, 88 percent, is higher than some other early case reports, which found death rates for coronavirus patients on ventilators ranging from 50 percent to close to 70 percent. Given that the length of hospital stay for these Northwell cases was relatively short, four days on average, it’s possible that those who died were mainly patients who were so ill that they were unlikely to be helped by any treatment.

While the study provided a valuable granular look at the characteristics of an early onslaught of patients in the United States, Dr. Davidson cautioned that it was observational in nature, and that there was no comparison group with which to contrast frailties or outcomes.

“We’re simply describing the patients who came in and required hospitalization,” she said. “We are not comparing them to those who were positive and stayed out of the hospital, or who didn’t get infected, or to patients with any other disease.”