The coronavirus is washing over the U.S. These factors will determine how bad it gets in each community

On March 15, the governor of Washington called the Seattle area the “hotbed of this outbreak.”

“Code Red,” blared the tabloid cover of the New York Post eight days later.

And this weekend, the Tampa Bay Times forecast that Florida, where the governor has not imposed statewide stay-at-home measures, would “see tens of thousands of infections in the coming weeks” from the coronavirus pandemic.

Two months after the first case of Covid-19 was confirmed in the United States, the virus has sprung from its initial toeholds and started to shift the country’s infection map — from one of pockmarked hotspots to one that blankets the entire nation. Detroit. New Orleans. Dallas. These are among the cities that could be the next hubs of the U.S. emergency, and where the virus has likely been spreading for weeks.

As the outbreak rolls across the country, residents are asking: Who will be hit next? And how fast and how hard?

“It’s very feasible we’re going to see multiple city- or state-level outbreaks across the country in the next few weeks and months,” said Maia Majumder, a computational epidemiologist at Boston Children’s Hospital. “They will start at different times and they will peak at different times.”

Experts told STAT that a host of factors are at play: demographics like the age and health of the population; proximity and connections to other regions; and how densely people live, whether in crowded apartment buildings or more secluded family homes. There are also factors that can be addressed through action, like local response efforts and physical distancing requirements.

Other elements are impossible to predict. With outbreaks, chance occurrences can push one city’s crisis to the forefront, like Seattle’s was early on.

“Part of the reason we found it in Seattle is also an element of bad luck, in the sense that it seemingly quickly jumped into a nursing home, and there were fatalities, so we caught it,” said Samuel Scarpino, an assistant professor at Northeastern University’s Network Science Institute. “It was probably spreading in other parts of the U.S. We just didn’t see it because those places didn’t have those early fatalities.”

As epidemiologists track the infiltration of the coronavirus in the United States, their efforts have been hampered by flawed information. The bungled rollout of testing has created a gulf in our knowledge about where and how widely the virus has spread — and where a simmer is starting to boil over.

Models showing how other cities could soon face the health care demands that New York hospitals are experiencing now helped convince President Trump on Sunday to extend physical distancing recommendations through April.

“When you look at all of the states together, all of them are moving at exactly the same curves,” Deborah Birx, the physician coordinating the White House coronavirus response, said Monday. “And so, that’s why we really believe this needs to be federal guidance, so that every state understands that it may look like two cases today — that become 20, that become 200, that become 2,000.”

A major factor in determining which places are hit soonest is simply where the virus is likely to land first.

Local outbreaks get “seeded” when cases are imported there, but there’s an inherent randomness at the earliest stage of an outbreak. If three infected travelers from Wuhan — the Chinese city where the outbreak is thought to have originated — had landed on the same January day in three different U.S. cities, one might not have spread the virus to anyone; one might have spread it to a few people but the transmission petered out there; and one might have passed it to a few people, who passed it to a few more people, which created a cluster of cases, and on from there.

As we try to anticipate the path of the outbreak, experts told STAT that the more imported cases a location has, the more likely some are to ignite ballooning chains of transmission. It helps explain why the Seattle area and San Francisco Bay Area — which have strong ties to Asia — were home to some of the first confirmed cases of community spread of the virus. As Europe lit up with infections and Americans abroad fled home, more cities on the East Coast started to be affected.

Experts are quick to caution, however, that the cities that seem to have had an explosion of cases aren’t necessarily the only places with transmission.

The coronavirus (SARS-CoV-2 is its scientific name) starts with a slow burn. People infected might not show symptoms for five or more days, but during that time, the virus can be passed to others. By the time enough people get sick enough to get on health systems’ radars, cases could be spreading unchecked. It takes much longer to go from one case to 1,000 than to go from 1,001 cases to 2,000.

“Large numbers of cases may suddenly appear, but a large epidemic has not occurred suddenly,” Ben Cowling, a University of Hong Kong epidemiologist, told STAT in an email. “It has probably built up over many weeks.”

The picture of an outbreak in any given place on a given day is more like a Polaroid coming into focus from a few weeks ago than a snapshot of the spread of the virus that day — particularly because of the U.S. testing problems.

Take, for example, the Seattle area. Experts say Seattle leapt onto the radar not because it was necessarily ahead of other cities, but because there was more information there, and a more dramatic impact. Crafty researchers, frustrated by the lack of coronavirus testing, uncovered cases by adapting a separate influenza surveillance project, showing the virus was moving through their community. The outbreak also struck a nursing home, sending many people to the hospital with what in some cases became fatal respiratory infections — generally, older people are more likely to suffer severe Covid-19 infections — and signaled the virus’ presence.

A s the viral spread picks up speed, another factor that will influence which places are hit the hardest is when and how aggressively local officials implement physical distancing policies, experts said.

China was able to prevent the kind of crisis that happened in Wuhan — where the health care system was overwhelmed — from taking place throughout the country in part because of its aggressive lockdowns. Keeping people away from each other means the virus can’t spread among them. It’s why experts are urging leaders in regions that do not think they have broad spread of the virus yet to implement physical distancing measures now — to save lives that could otherwise be lost a month from now.

“The effects will be seen weeks down the road,” said Caroline Buckee, an epidemiologist at Harvard’s T.H. Chan School of Public Health. For people who think that distancing measures are not needed in their communities yet, she said, “that is extremely naïve. Everywhere should be taking social distancing very seriously, because nowhere is immune to this virus.”

More than 30 states have told residents to stay at home as much as possible, though governors in Pennsylvania, Texas, and Florida are among those who have limited their orders to certain counties or left it up to local authorities to issue such orders. Because of resistance from some governors, some experts have called on the Trump administration to strengthen its recommendations to more enforceable actions.

There are tentative signs that existing distancing measures — particularly when implemented early — might be having an impact. In the Boston area, a “superspreader” event at a conference in late February made it clear a problem was looming. Epidemiologists started tracking cases, and local and state leaders moved fairly quickly to convince people to stay home. Cases have continued to increase, but the moves may have pushed the peak of the Massachusetts epidemic out a few weeks, giving hospitals more time to prepare.

“The hope is that it minimizes it,” Scarpino said. “We still have another week [until we] know how the measures are working.”

In the Bay Area, where local leaders imposed the country’s first “shelter in place” orders in mid-March, experts are also watching closely to see if the region’s infection curve will flatten.

If the region’s hospitals avoid the fate that New York’s are now facing, one potential explanation is that New York’s infections were seeded a little earlier and its distancing measures started a little later. When outbreaks grow exponentially, those differences matter greatly.

“Did the Bay Area, for instance, import cases in smaller numbers and later than New York?” said Joseph Lewnard, an epidemiologist at University of California, Berkeley.

From the initial hotspots, the virus is likely to travel next to cities with close connections. Officials in Houston, for example, have watched worryingly as cases in nearby New Orleans have grown, given the number of people who travel back and forth between the two cities. The spread of the virus in Atlanta has raised concerns that it could become more widespread throughout the South.

This is why some governors are asking people arriving in their states to quarantine themselves for two weeks. The federal government on Saturday recommended that most residents of New York, New Jersey, and Connecticut refrain from traveling.

“The probability of nobody transmitting the virus becomes very, very, very low if you have a lot of people infected,” Lewnard said.

Experts describe outbreaks migrating in “traveling waves,” typically following patterns of transit and connection among communities. As those waves widen and pulse out from more cities, they can quickly start converging over the whole country.

“Right now it’s no longer a traveling wave,” Buckee said. “Right now it’s everywhere in the United States. Maybe not everywhere, but it will be soon.”

Once the virus does take hold in a community, a host of factors can color what the outbreak looks like.

The age of the population is a clear factor. One hypothesis for Italy’s high death rate, for example, is that it is the second oldest country in the world. South Korea’s data of those infected skew young, but that might be because its outbreak was seeded in a religious community that was generally young and was then quickly brought under control. Demographics are not fate, however. Japan’s population is the only one older than Italy’s, but its death rate has remained low.

The underlying health of a community will also shape how much harm Covid-19 inflicts. Heart and lung diseases, diabetes, and other chronic conditions are, like age, risk factors for more severe disease. Puerto Rico, Maine, and Florida have the highest percentage of residents 65 and older, while Oklahoma, Alabama, and Mississippi have the highest death rates from heart disease.

Access to care and hospital resources also vary widely across the United States. A Washington Post analysis showed a tenfold difference in ICU capacity among different regions.

Whether a region’s climate matters remains an open question. Other viruses that cause respiratory infections do not transmit as efficiently in hot, humid settings. But some experts are skeptical that the weather in the South, for example, will protect the region, given that this is a new virus that no one has immunity to. It also spread easily in the warm climate of Singapore.

Then there’s density. It’s thought that the crowdedness of New York — where people squeeze into subways and even the parks are packed — contributed to the spread of the virus there. Experts also pointed to another type of density: how many people share a single residence, whether that’s families or roommates. Data from China showed the virus slipped easily among these closest of contacts, and as distancing measures go into effect, the contagion could move indoors as well.

“In some sense transmission has been taken off the streets and pushed back into family units,” Mike Ryan, the head of the World Health Organization’s emergencies program, said Monday. He added that ideally, people who were sick would be isolated outside their homes to avoid spreading it to their families.

As the impact of the virus starts to be felt more dramatically in more U.S. cities, attention will turn away from the earlier epicenters. But that doesn’t mean that places we once thought of as the eye of the storm are in the clear.

Jay Inslee, the governor of Washington, told CNN on Sunday his state was seeing some modest success in bending its curve. But, he added, “we have a long, long way to go.”