New York’s coronavirus outbreak has violently erupted over the past few days, and the state is now driving the national epidemic — while on the West Coast, public health experts are wondering if an early and aggressive response saved California from a similar fate.

California reported some of the earliest coronavirus cases in the United States in late January. And in the first week of March, California and New York were neck and neck on cases of COVID-19, the disease caused by the coronavirus. But over the past week, New York case counts have doubled every few days, and the state now has nearly 10 times the cases California does: 38,000 to 4,200.

Infectious-disease experts say early maneuvers in California, especially in the Bay Area — first discouraging people from gathering in crowds and then ordering them to shelter in place — may have had a dramatic impact, even if they came only a few days ahead of those in New York.

But other factors may also be in play. New York is testing far more people — three times as many as California — and therefore identifying more cases, for example. And it’s possible that what’s happening 3,000 miles away could be California’s future.

“New York may just be three or four days in front of us. We’re going to see an increase in the number of cases here as well,” said Dr. Warner Greene, a senior investigator at the Gladstone Institutes in San Francisco who specializes in HIV but is studying the new coronavirus. “Days matter — they really matter. You think you’re fine, you’re absolutely fine, but this thing is just waiting to explode.

“But we went into shelter in place quicker; we got people apart quicker,” Greene said. “That could be a contributing factor to what we’re seeing in California now. And that’s why I think the whole country should be sheltering in place.”

Back to Gallery New York state has 10 times the COVID-19 cases California... 9 1 of 9 Photo: Misha Friedman / Getty Images 2 of 9 Photo: Santiago Mejia / The Chronicle 3 of 9 Photo: Jim Gensheimer / Special to The Chronicle 4 of 9 Photo: Jim Gensheimer / Special to The Chronicle 5 of 9 Photo: Wong Maye-E / Associated Press 6 of 9 Photo: Victor J. Blue / Getty Images 7 of 9 Photo: Angela Weiss / AFP via Getty Images 8 of 9 Photo: Kevin Hagen / Associated Press 9 of 9 Photo: John Blanchard

















The World Health Organization on Tuesday identified the United States as the next potential epicenter of the pandemic, with China and South Korea both on a path to recovery and Italy starting to see signs of its outbreak slowing down, though gradually.

New York state now makes up roughly half of the United States’ 86,000 cases of COVID-19. On Tuesday, experts on the White House Coronavirus Task Force advised that residents who have fled New York City, where the bulk of cases are located, should place themselves in a two-week quarantine to avoid infecting people in other parts of the country.

New York state is also testing more people than anywhere else in the country — 120,000 as of Thursday, compared with about 77,000 in California, 57,000 of which haven’t been processed yet.

How and why New York’s testing is so far beyond California’s isn’t entirely clear. New York started testing more people sooner than California because the state requested and received emergency-use authorization from the Food and Drug Administration to start using its own test on Feb. 29. At that time, California was using tests supplied by the Centers for Disease Control and Prevention, which was trying to play catch-up after early production errors slowed down distribution to the states.

But testing alone doesn’t explain why New York’s case counts are so much higher than California’s, or why the rate is spiraling up so fast on the East Coast. The death toll in New York was five times higher than California’s — 500 deaths to 85, as of Friday. Deaths tend to be a much more reliable marker of the spread of the disease than cases because determining how someone died is not dependent on the availability of testing kits.

Also, hospitals across New York state, and in New York City in particular, are filling up, but California hasn’t yet seen a similar surge.

New York Gov. Andrew Cuomo issued “stay at home” orders last Sunday, two days after Newsom did the same for California and five days after Bay Area health officers told 6.7 million people to shelter in place.

New York City and the Bay Area had about 450 and 300 cases, respectively, when the Bay Area stay-home orders were issued on March 17. When Cuomo shut down the state on March 22, New York City had 5,500 cases. The Bay Area: 539.

Shelter-in-place orders hadn’t been in effect long enough to entirely account for the dramatic differences, but the Bay Area issued other directives earlier, such as shutting down mass gatherings, advising people to work at home when possible, and asking older adults and people with compromised immune systems to stay home.

“We were more aggressive; we got out there a little earlier,” said Dr. John Swartzberg, an infectious disease expert at UC Berkeley. “We were maybe a week up on New York, and that doesn’t sound like much time, but in terms of the spread of this pandemic, it’s enormous.”

Indeed, lessons from the 1918 influenza pandemic taught public health leaders that speedy, assertive actions could save lives, said Dr. George Rutherford, a UCSF infectious-disease expert who has been consulting with Bay Area officials on the coronavirus response.

“Time is critical. You can’t be late, not even by a week,” Rutherford said. “We moved to lockdown, for lack of a better phrase, relatively quickly. I think we’re reaping the benefits.”

There are other explanations for New York’s particularly brutal outbreak. New York City has the densest population in the country, and for a virus that spreads primarily by close contact, density could make a huge difference. For weeks, if not months, New Yorkers were crowding subways, high-rise elevators and even sidewalks while the coronavirus spread among them.

Even in the Bay Area, where BART trains can be packed at commute hours, there isn’t nearly the same level of close gathering.

“Different places appear to have very different disease dynamic,” said Dr. Robert Siegel, a Stanford infectious-disease expert. “You could have predicted New York was going to have a big problem because of the high density of people, which is ideal for spreading the virus. We’re spread out more in California.”

Cultural issues also could play a role, though infectious-disease experts aren’t sure in what ways New Yorkers might differ from Californians that might explain the differences. New York City has a younger population than San Francisco, and its bars stay open two hours later every night — perhaps that factored into the spread of the disease, experts said.

And some of the explanation may come down to luck. In pandemics, certain individuals are sometimes found to be so-called super-spreaders, capable of infecting dozens or even hundreds of others. In the 2004 SARS epidemic, also caused by a coronavirus, one person in Canada was linked to 128 cases in a hospital.

No super-spreaders have been identified in the current outbreak, but that doesn’t mean a few particularly contagious people aren’t at the center of some clusters in New York, Rutherford said.

Local and state leaders warn that California will surely see its case counts climb in the coming weeks. And California, too, is bracing for a surge on hospitals that could strain resources beyond capacity. New York may be on a more advanced track than California, but that doesn’t mean the West Coast won’t end up on a similar trajectory.

“I can feel confident that things are going to get worse before they get better,” said Dr. Grant Colfax, director of the San Francisco Department of Public Health, at a news conference on Monday, during which he advised residents to take seriously the orders to stay at home.

“Every community where the virus has taken hold has seen a surge in coronavirus patients who need to be hospitalized,” Colfax said. “We expect that to happen in San Francisco soon, in a week or two, or perhaps even less.”

Staff writer Catherine Ho contributed to this report.

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com