A new study of thousands of people who were hospitalized in New York City after contracting the coronavirus found that more than nine in 10 had at least one chronic health condition and that most had at least two.

The findings were included in a paper published in the Journal of the American Medical Association that describes the characteristics of thousands of Covid-19 patients admitted from March 1 to April 4 at a dozen hospitals in New York City and Westchester County and on Long Island that are operated by Northwell Health.

The paper was written by scientists at the Feinstein Institutes for Medical Research, Northwell’s research arm. The senior author cautioned that the study was observational in nature, and that there was no comparison group with which to contrast frailties or outcomes.

The researchers found that dozens of children and teenagers were hospitalized with the virus, but survived it, and that women had a clear edge in beating the virus. Fewer of them were hospitalized to begin with, and they were more likely to survive.

One in five hospital stays ended in death. The mortality rate for those who were placed on ventilators and were no longer in the hospital was 88 percent. That was higher than some other early case reports, which found death rates of 50 percent to almost 70 percent, have shown.

Given that the length of hospital stays in the Northwell cases was relatively short, four days on average, it is possible that those who died were mainly patients who were so ill that any treatment was unlikely to help them.

Like several other reports on smaller patient groups at area hospitals, the Northwell research indicated that obesity, high blood pressure and diabetes were common risk factors for severe Covid-19 disease requiring hospitalization. One of the most striking findings: only 6 percent of hospitalized patients had no underlying health conditions at all.