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Throughout March, as the coronavirus pandemic gained momentum in the United States, much of the preparations focused on the breathing machines that were supposed to save lives.

New York Gov. Andrew M. Cuomo (D) and President Trump sparred over how many ventilators the state was short. DIYers brainstormed ways to make modifications to treat more patients. And ethicists agonized over how to allocate them fairly if they ran out.

Now five weeks into the crisis, a paper published in the journal JAMA about New York state’s largest health system suggests a reality that, like so much else about the novel coronavirus, confounds our early expectations.

Researchers found that 20 percent of all those hospitalized died — a finding that’s similar to the percentage who perish in normal times among those who are admitted for respiratory distress.

But the numbers diverge more for the critically ill put on ventilators.

A total of 1,151 patients required mechanical ventilators. Of the 320 for whom final outcomes are known (either death or discharge), 88 percent died. That compares with about 80 percent of patients who died on ventilators before the pandemic, according to previous studies — and with the death rate of about 50 percent that some critical-care doctors had optimistically hoped for when the first cases were diagnosed.

“For those who have a severe enough course to require hospitalization through the emergency department, it is a sad number,” said Karina W. Davidson, the study’s lead author and a professor at the Feinstein Institutes for Medical Research at Northwell Health.

At the end of the study period, 831 — or 72 percent — remained hospitalized, so the mortality rate may be higher or lower depending on how well those patients do in the coming days.

In a clarification issued on April 24, JAMA said that if the still-hospitalized patients are included, 3.3 percent of the total number who were on ventilators were discharged and 24.5 percent died.

The analysis is the largest and most comprehensive study of outcomes in the United States to be published. Researchers looked at the electronic medical records of 5,700 patients infected with covid-19 between March 1 and April 4 who were treated at Northwell Health’s 12 hospitals in New York City, Long Island and Westchester County — all heavily hit by the outbreak. Sixty percent were male, 40 percent female and the average age was 63.

“It’s important to look to American data, as we have different resources in our health-care system and different demographics in our populations,” Davidson said.

The paper also found that of those who were hospitalized, 57 percent had hypertension, 41 percent were obese and 34 percent had diabetes, which is consistent with risk factors listed by the Centers for Disease for Control and Prevention. Noticeably absent from the top of the list was asthma. As doctors and researchers have learned more about covid-19, the less it seems that asthma plays a dominant role in outcomes.

One other surprising finding from the study was that 70 percent of the patients sick enough to be admitted to the hospital did not have a fever. Fever is listed as the top symptom of covid-19 by the CDC, and for weeks, many testing centers for the virus turned away patients if they did not have one.

Davidson said that as a result of that finding, Northwell is encouraging people with underlying health conditions, such as hypertension and diabetes, who are potentially exposed to the virus and who may not have a fever to consult with a doctor sooner rather than later.

Correction: A previous version of this story misstated that 57 percent of patients in the study who died had hypertension, 41 percent were obese and 34 percent had diabetes. The numbers refer to hospitalized patients.

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