While the common misperception has been that only the old and frail can contract the coronavirus, early testing data in the city has shown that young New Yorkers have also been vulnerable. People ranging in ages from 18 to 44 have accounted for 46 percent of positive tests, according to city data as of Monday.

“Part of it is because we are testing more people as tests become more available and also because folks in this age group were out and about and weren’t necessarily engaging in social distancing like other age groups,” said Dr. Danielle Ompad, associate professor of epidemiology at N.Y.U.’s School of Global Public Health.

So far in New York, three people in the under-44 category have died after contracting the virus, but a report issued last week by the Centers for Disease Control and Prevention showed that across the United States, 38 percent of those hospitalized were between the ages of 20 and 54.

Nationally, 12 percent of the intensive care patients were between the ages of 20 and 44. Some experts suggested vaping might make young people more vulnerable to the virus.