New York's density made it a breeding ground for the coronavirus. Researchers have a few hunches as to how the outbreak grew so rapidly.

• New York is the epicenter of the US's coronavirus outbreak.
• The majority of the state's cases are in New York City, where density fueled the virus' spread.
• An MIT economist recently posited that the city's subway system was the main trigger for the outbreak, but schools and crowded housing complexes could also be to blame.
• Visit Business Insider's homepage for more stories.

New York state alone has more coronavirus cases than any country outside the US.

More than half of those cases are concentrated in New York City, where the case count has surpassed China's reported total.

To some extent, these high figures are a reflection of the state's testing capacity. New York has administered nearly 650,000 tests as of Tuesday — more than twice the number of tests administered in California and more than three times that in Texas.

But New York's case count is also a product of its density.

Crowded cities create more opportunities for people to transmit a virus. Since the average coronavirus patient is expected to infect at least two others, a single case can have a domino effect as more people come in contact.

"Why New York? Why are we seeing this level of infection?" Gov. Andrew Cuomo said at a press briefing on April 13. "It's very simple. It's about density. It's about the number of people in a small geographic location allowing that virus to spread."

While it's possible that the virus may have spread in tourist hotspots like Times Square, which receives nearly 40 million visitors each year, research from Imperial College London suggests that people in such hotspots don't often come in contact long enough to infect one another.

Instead, the researchers estimated that around one-third of coronavirus transmission occurs in households, one-third takes place in schools and workplaces, and one-third happens in the community (at churches, bars, restaurants, grocery stores, playgrounds, etc.). In New York City, however, mass transit could represent a more significant mode of transmission than in other places.

New York had a far more difficult task than sparser cities

New York's density has made it more difficult to contain the outbreak, even after social distancing measures were in place.

New York City reported its first coronavirus case on March 2. San Francisco reported its first two cases three days later, but the city was quicker to shut down schools and enact stay-at-home measures.

San Francisco schools closed on March 12 — three days before New York City issued the same mandate. The Bay Area was also three days ahead of New York City in implementing a shelter-in-place order.

Some health experts believe these moves helped contain local outbreaks in California. But others point to density as the reason New York's cases skyrocketed.

"We know the further away you are from people, the less likely you are to get infected," John Swartzberg, a clinical professor emeritus at UC Berkeley's School of Public Health, told Business Insider.

New York City is one of the densest cities in the world, with more than 28,000 people per square mile. As of April 2019, the city was expected to acquire another 400,000 residents by 2050. San Francisco, by contrast, has around 18,000 people per square mile, despite being the second densest city in the US.

That means reducing crowds was a far more gargantuan task in New York City than anywhere else in the country.

Meanwhile, Seattle (a city with around 8,000 people per square mile) reported the first coronavirus case in the US, but is already close to containing its outbreak.

"We brought [cases] down by basically making our population seem much sparser than it really is," Dr. Elizabeth Halloran, a biostatistician at Fred Hutchinson Cancer Research Center and the University of Washington, told Business Insider. "The virus doesn't stand a chance to get transmitted around here."

In New York City, cases have plateaued, but deaths continue to climb.

Subways and schools could have fueled the spread

Any gathering creates an opportunity for viruses to spread, but there's some debate over what specifically fueled New York City's outbreak.

MIT economist Jeffrey Harris argued in a recent paper that the city's subway system was primarily to blame.

Before the pandemic hit, more than 5 million people in New York City took the subway each day. Harris's paper, which has not been peer-reviewed, found that the coronavirus had a slower rate of transmission in Manhattan compared to other boroughs at the end of March. That was around the same time that Manhattan's subway ridership decreased by more than 90% — the sharpest drop of any borough.

NYU mathematician Alon Levy poked holes in Harris' conclusion, however, by identifying high rates of transmission in neighborhoods with relatively low transit ridership like Eastern Queens and Staten Island. He also pointed to confounding factors like more people working from home, which could mean the virus was no longer spreading among coworkers.

"It doesn't appear to be a full-blown study," Mayor Bill de Blasio said at a press briefing last week. "Any place people gather is a place of concern."

New York City also has the largest school district in the US in terms of enrollment, with nearly 985,000 students. A recent report from the US Centers for Disease Control and Prevention found that children have helped fuel the spread of the coronavirus in the US.

"Closing schools is likely to dwarf the effect of stopping one-off 500 person gatherings," Paul Bieniasz, a virology professor at The Rockefeller University, told Forbes in March. "Can you imagine a more effective way to spread a respiratory virus than sending one or two family members (children) off to mix with hundreds of others, having them return to their families in the evening, and repeating that process every day?"

Infection rates are high in neighborhoods with crowded homes

Cramped apartments could be another key mode of transmission — particularly while lockdowns are in effect.

Research from the Shenzhen Center for Disease Control and Prevention suggests that people living with a coronavirus patient have a 15% chance of getting ill themselves. Another study of households in Guangzhou, which is still awaiting peer review, estimated an even higher rate: at least 19%.

The researchers added that the chances of transmission could rise if there are more elderly people in a household. The study also determined that the new coronavirus is more transmissible within households than SARS or MERS.

In New York City, the borough with the most coronavirus infections — Queens — has the highest number of people in an average household: at least three. The Queens neighborhood with the most reported infections — Corona — is also the densest neighborhood in the city, according to 2010 Census data, with around 216,000 residents per square mile.

"It's not just a dense city or a dense community. It's any person in a dense environment," Cuomo said earlier this month. "You bring people together and this virus has a feeding frenzy."

Holly Secon contributed reporting.

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