What Made New York So Hospitable for Coronavirus?

Perhaps it was inevitable that New York City and its suburbs would become an epicenter of the coronavirus epidemic in the United States. The population density, reliance on public transportation and constant influx of tourists — all would seem to make the metropolitan area a target.

But to stop the virus, scientists have to figure out which factors played a greater role than others. As it turns out, that is not so simple.

“We have more speculation than facts,” said Dr. Arnold Monto, professor of epidemiology and global health at the University of Michigan.

More than 20,000 New York City residents have tested positive for the virus so far — nearly a third of all confirmed cases in the nation. The prevalence in New York and the areas close to it is far greater than anywhere else in the country.

The problem is that so little is known about the spread of this new virus that it is hard to draw conclusions about whether New York is unique or a harbinger of things to come elsewhere.

There was almost certainly an early and undetected introduction of the virus into the city, probably in mid- to late January, said Benjamin Cowling, professor of infectious disease epidemiology at the University of Hong Kong.

“Many of the cases being detected in the past week are the result of that slow process that has built up over two months,” he said.

With many viral diseases, scientists can get a sense of the exact timing by comparing the mutations in the virus from samples taken at various times. But human coronaviruses are surprisingly stable, said Dr. Paul Offit, an infectious disease expert at the University of Pennsylvania and Children’s Hospital of Philadelphia.

Once in the area, New York’s density played some role in its rapid spread but was probably not the whole story, epidemiologists said. Many New Yorkers live in high rises. Sidewalks are crowded. Subways can be jammed. The city has 27,000 residents per square mile, far surpassing the second densest city, San Francisco, with 18,000 per mile.

Clusters also can contribute to what looks like random rapid spread of the virus.

Suppose 100 infected New Yorkers fly to other cities. Clusters can start in the cities where they landed. Other cities will escape because the infected people did not go there.

Dr. Monto pointed to unexpected aspects of the virus’s spread.

It took off in a region of Italy with small cities, not in the larger more crowded areas. And when those regions were closed off, many people fled to the south. Yet even though the coronavirus undoubtedly was introduced to southern Italy, so far there have been relatively few deaths. “It is nothing like what you see in the north,” Dr. Monto said.

The west coast of the United States also is puzzling, he added. Los Angeles and San Francisco “must have had multiple introductions” of the virus, he said. But the California cases have been concentrated in the Silicon Valley area and are nowhere near as numerous as the cases in Seattle.

“There are no answers,” Dr. Monto said.

“That is what scares people.”

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