The data, collected by the New York City Department of Health and Mental Hygiene, offers the first snapshot of an outbreak that infected more than 40,000 and killed more than 1,000 in the city in its first month.

The coronavirus has spread into virtually every corner of the city, and some wealthier neighborhoods have been overrun with cases, including some parts of Manhattan and Staten Island. But that may be because of the availability of testing in those areas. Nineteen of the 20 neighborhoods with the lowest percentage of positive tests have been in wealthy ZIP codes.

The patterns are even more striking when analyzing the data on people who visited the city’s 53 emergency rooms with the “flulike symptoms” that are a hallmark of the coronavirus.

Overall, nearly three times as many people with “flulike symptoms” like fever, cough or sore throat visited city emergency rooms this March when compared with the same month in previous years.

Over the last four years, there were on average 9,250 flu-related visits to emergency rooms in March; this March, the number tripled to about 30,000.

The increases in flu-related emergency room visits varied widely by neighborhood, with many of the surges occurring among residents of neighborhoods where the typical household income is less than the city median of about $60,000, the data shows.

In Corona, Queens, for example, the median household income is about $48,000, according to the U.S. Census Bureau. That neighborhood is near the Elmhurst Hospital Center, which Mayor Bill de Blasio has cited as the hardest-hit hospital in the city. Doctors in the overwhelmed emergency room there have described the conditions as “apocalyptic.”

Dr. Jessica Justman, an epidemiologist at Columbia University in Manhattan, said the numbers were most likely because many immigrants and low-income residents live with large families in small apartments and cannot isolate at home.

“I think unfortunately this is showing how devastating that can be,” Justman said.

In New York, experts said, a vast majority of people visiting emergency rooms with flulike symptoms probably have the coronavirus.

“We’ve actually stopped testing for the flu because it’s all coronavirus,” said Bruce Farber, chief of infectious disease at North Shore University Hospital, part of Northwell Health, a network of 23 hospitals throughout the state. “Almost anybody who has an influenza illness right now almost certainly has coronavirus.”

Many of the emergency rooms with the biggest increases in patients who have flulike symptoms are in Queens, the borough that has the highest number of confirmed coronavirus cases. There are about 616 confirmed cases for every 100,000 residents in Queens and 584 confirmed cases for every 100,000 residents in the Bronx. That’s far more per 100,000 than the 376 in Manhattan and 453 in Brooklyn.

With infections across all five boroughs, New York has far more confirmed cases than any other city in the United States.

The emergency room data also tracks admissions — the number of ER visitors who end up treated at a hospital. On that metric, the data shows that older visitors are far more likely to be admitted than younger visitors.

There is a simple reason for that difference, according to the hospital officials and experts: The coronavirus seems to take a bigger toll on older people, as well as those with compromised immune systems.

“I don’t think that infection rates are necessarily different between older and younger people,” said Dr. Isaac Weisfuse, the former deputy head for disease control at the city’s Department of Health. “Elderly have worse clinical outcomes than younger patients and may have more preexisting conditions.”

Overall, more than 8,500 people have been hospitalized with the coronavirus in New York City. That number is expected to soar in the coming weeks.

But officials are hopeful that the social distancing restrictions put in place by the state may have finally started to at least slow the spread of the coronavirus. They have noted that the number of hospitalizations is now doubling every six days, instead of every two or three days.

The city’s data shows a slight decline in emergency room admissions over last weekend and then continuing increases this week.

Dr. Denis Nash, an epidemiologist at the City University of New York’s School of Public Health, said it was still too soon to tell whether the social distancing restrictions were working.

“It may be too soon to say what’s really going on here,” he said. “I just hope it means something good.”

This article originally appeared in The New York Times .