The coronavirus pandemic has engulfed the nation, and the battle-lines of the war against the disease shift daily. But over the past week, one thing has become abundantly clear: New Jersey and New York are at the heart of the outbreak in the United States.

As of Friday, New Jersey had the second-highest number of confirmed COVID-19 cases in the nation, with 8,825. The Garden State was only behind neighboring New York — which had reported 44,635 cases as of Friday.

Other American cities have been hit hard by the coronavirus — Seattle was at the heart of the virus’s first outbreak in the U.S.; Boston has wrangled with exposures stemming from a medical conference — but the numbers in the New York City / New Jersey metro area have escalated more quickly than anywhere else in country thus far.

Dr. Deborah Birx, who is leading the federal response to the COVID-19 pandemic, said on Monday that the New York City area has an “attack rate” of one in 1,000, which is five times what the rest of the nation is experiencing.

Public health experts interviewed by NJ Advance Media said that a number of factors play into the region’s skyrocketing numbers, including population density, reliance on public transportation and rapidly expanding testing.

But those experts also cautioned what has become a common refrain in the coronavirus battle: There is still so much we don’t know about the disease or its spread.

A crowded space

New York City is the most densely-populated city in the nation, and New Jersey is the most densely-populated state in the nation. The sheer number of people living so closely to each other makes it easier for an infectious disease like COVID-19 to spread, said Dr. David Cennimo, an infectious disease expert and professor at Rutgers.

“I mean, we’re all over each other," Cennimo said. "I think anything that is mitigated be being distanced is going to have a hard time being controlled in any city.”

Meanwhile, the reliance on public transit in New York City and New Jersey, particularly North Jersey, makes it easier for the coronavirus to spread, and it’s not just the subway in New York, according to experts. Dr. Stephanie Silvera, an epidemiologist and professor at Montclair State University, said that reliance on public transit in North Jersey cities like Newark and Jersey City brings the same risk to those areas.

In New Jersey, Cennimo said he’s heard anecdotally from colleagues in Bergen County that towns with NJ Transit stations seem to have higher numbers of COVID-19 cases.

“It’s pejorative to say that that’s what brought it in, but I think there was more exposure that way,” Cennimo said.

Public transportation is still considered an essential service both in New Jersey, though ridership has fallen in recent days as Garden Staters have been ordered to stay home. NJ Transit has reduced service for its trains, light rails and buses in recent days.

The Metropolitan Transportation Authority, which oversees New York City’s subways, buses and two commuter rail systems, slashed its service on Tuesday.

The fact that the region is home to a trio of international airports could’ve also contributed to COVID-19′s spread, Silvera said.

“It’s possible that there were more people that arrived here that are now being tested,” Silvera said.

The data questions

Cennimo and Silvera note that New York has greatly ramped up its testing for the coronavirus, and New Jersey is following suit. That means that the two states likely have a greater grasp on the spread of COVID-19 in their populations than other states have.

“You wonder if other places were testing as much what their numbers would truly look like,” Cennimo said.

But the testing data that is available isn’t necessarily enough for experts to fully understand the patterns in the coronavirus spread.

Silvera said that New York City has released borough-by-borough statistics for COVID-19 cases, which has illustrated which groups of people are most threatened by the disease. Lower income people that have a more difficult time accessing healthcare resources in the city have shown to be most at-risk, Silvera said.

“I suspect that we’re going to start seeing those disparities exacerbated by this crisis,” Silvera said.

But, Silvera noted, New Jersey’s decentralized public health system — which is made up of the New Jersey Department of Health and 94 local health departments overseeing 565 municipalities — struggles to efficiently make municipal-level data available. And without local-level numbers, its difficult to determine if those same patterns in New York City are playing out in the Garden State.

And even as New Jersey and New York seem to be bearing the brunt of the crisis so far, other parts of the country still need to remain hyper vigilant, experts emphasize.

On Thursday, federal officials warned that Detroit and Chicago are emerging as new hotspots for COVID-19. Cennimo said he’s particularly worried about what will happen when the coronavirus begins to spread through rural areas, where residents have less access to hospitals and other healthcare infrastructure.

Silvera noted that we’re only about a week into serious social distancing measures, and she cautioned that we have yet to see the worst, both in New Jersey and New York and across the rest of the nation. She said the 1918 flu spread in this manner, popping up in groups of cities rather than all at once.

“This isn’t so much a pandemic, but a series of epidemics,” Silvera said.

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Michael Sol Warren may be reached at mwarren@njadvancemedia.com. Follow him on Twitter @MSolDub. Find NJ.com on Facebook.

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