California is not in the clear yet, but its experience so far suggests early action is crucial.

As the coronavirus started to spread in the US, California, not New York, might have seemed a likelier place for the pandemic to peak.

California, the nation’s most populous state, was among the first to report cases. The first possible case of community transmission in the US was reported in California on February 26; the state reported its first death on March 4. New York lagged by days, reporting its first community transmission case on March 3 and first death on March 14.

But just over a month after California’s first coronavirus death, as of April 8, the state has seen more than 17,000 cases and about 450 deaths — while New York state has more than 140,000 cases and about 5,500 deaths.

Emergency medical workers transport a sick prisoner to an ambulance outside Elmhurst Hospital Center, in Queens, New York on April 6.

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Experts say it’s too early to definitively say why California is faring so much better than New York. One factor, though, is that California simply acted more quickly than New York once it became clear that coronavirus was starting to spread in the US. If cases in California remain under control while those in New York soar — still a very big if — the experience could carry important lessons for how to deal with Covid-19, the disease caused by the SARS-CoV-2 coronavirus.

California’s experience likely reflects, at least in part, the value of quick, more proactive action — along the lines of what experts say is needed across the US, even in places that might not feel exposed to coronavirus right now. We “need to shift to a proactive mentality rather than reactive,” Krutika Kuppalli, an infectious diseases physician and emerging leader in biosecurity fellow at Johns Hopkins Center for Health Security, told me. The reactive mentality “has been very much the way this outbreak has been from the beginning.”

It’s also important, experts added, that California remains vigilant. With the huge economic harm caused by the coronavirus lockdowns, it can be tempting to ease off social distancing measures early. But to truly avoid a catastrophe like New York’s, experts say, California likely needs to stay at home as much as possible, at least until coronavirus cases appear to drop and proper testing and surveillance are in place to better track and mitigate new outbreak clusters.

Los Angeles County officials said as much, warning about a potential peak in the next two weeks. “If you have enough supplies in your home, this would be the week to skip shopping altogether,” Los Angeles County Public Health Director Barbara Ferrer said on Monday. “Without everyone taking every possible precaution, our numbers can start skyrocketing.”

Hospitals in New York City, which had been especially hard hit by the coronavirus, are facing shortages of beds, ventilators, and protective equipment for medical staff.

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How California has avoided an explosion of coronavirus cases

There are other factors at play in the differences between the two states. One is the density of their largest cities: New York City is the densest city in the US (though San Francisco is second), and a lot of people packed closely together makes it easier for the coronavirus to spread.

New York state has also tested people at more than four times the rate of California, which could partly, though not mostly, explain the difference between both states’ reported cases and deaths.

A big factor — perhaps the biggest — is also chance. “There’s the possibility that there were just more introductions of the virus in the East Coast, in the New York area,” Jeffrey Martin, an epidemiologist at the University of California San Francisco, told me.

But California also acted more quickly than New York once it became clear that coronavirus was starting to spread in the US. The San Francisco Bay Area issued America’s first shelter-in-place order on March 16, and California Gov. Gavin Newsom issued a statewide stay-at-home order three days later.

Officials in seven San Francisco Bay Area counties have announced plans to extend the shelter in place order until May 1.

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Los Angeles County has closed all beaches as a new measure to stem the spread of Covid-19.

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New York, meanwhile, didn’t issue a statewide stay-at-home order until March 22. (New York City didn’t implement its own order beforehand; Gov. Andrew Cuomo said he didn’t believe it would work if only one city did it.)

And there’s evidence that social distancing was taken more seriously in some parts of California even before it was government-mandated. Restaurant data from OpenTable suggests that seated dining on March 1 was down 2 percent in New York City, but it was down 18 percent in San Francisco. (Though it was only down by 3 percent in Los Angeles, so not every place in California acted the same.)

As March began in New York, officials were encouraging people to go about their business. On March 2, New York City Mayor Bill de Blasio tweeted he was “encouraging New Yorkers to go on with your lives” and “get out on the town despite Coronavirus” — offering a movie recommendation for The Traitor. That did come before New York state confirmed a case of community transmission, but it also came after Cuomo, in a press conference with de Blasio, called community transmission “inevitable.”

Since I’m encouraging New Yorkers to go on with your lives + get out on the town despite Coronavirus, I thought I would offer some suggestions. Here’s the first: thru Thurs 3/5 go see “The Traitor” @FilmLinc. If “The Wire” was a true story + set in Italy, it would be this film. — Bill de Blasio (@BilldeBlasio) March 3, 2020

The same day, San Francisco Mayor London Breed, who had already declared a local state of emergency on February 25, warned the public to “prepare for possible disruption from an outbreak,” from dealing with school closures to caring for sick family members. California had confirmed a case of community transmission, in nearby Solano County, by then.

New York officials seemed to take the threat more seriously in the coming days and weeks, particularly after community transmission and deaths were confirmed.

The difference of a few weeks or days on public action and orders telling people to stay home may not seem like a huge deal. But it really is significant with the coronavirus, because the number of cases and deaths, especially early on in an outbreak, can double every few days if protective measures aren’t in place.

“With this virus, days, and even hours, matter,” Jen Kates, director of global health and HIV policy at the Kaiser Family Foundation, told me.

By March 23, three weeks after Breed and de Blasio’s tweets, New York state reported around 5,000 new coronavirus cases a day. California reported fewer than 500.

It seemed like California might have overreacted. It didn’t.

One of the big lessons from California: “Anytime you are dealing with an outbreak, if it appears like you overreacted, then you probably did the right thing,” Kuppalli said.

That may be especially true for the coronavirus, because it can be a stealthy spreader. People with coronavirus can infect others before they develop significant symptoms or without ever developing symptoms (although we don’t yet know how common this is). Especially during the early stages of a Covid-19 outbreak, that means a lot of people could be walking around with the coronavirus and infecting each other without knowing it.

The silent nature of the coronavirus’s spread was exacerbated by America’s lack of testing. Insufficient testing made it harder for officials to confirm people had the coronavirus, isolate them, then track down and quarantine their contacts. That made it much harder to detect any outbreak in the US and eliminated any chance of stopping it in its tracks.

Member at the Stanford Radiology department take blood samples during a coronavirus antibody study in Mountain View, California, on April 3.

Ray Chavez/MediaNews Group/The Mercury News via Getty Images

From the start, then, America was missing a lot of Covid-19 cases. So once a community confirmed a coronavirus case, and especially after it saw a death, there was a good chance it already had a much more widespread outbreak — since most cases are mild (though still potentially very unpleasant) and even the worst cases can take days or weeks to show major symptoms.

“Your chance of the first case being the one that comes to your attention is very, very, very, very small,” George Rutherford, an epidemiologist at the UCSF, told me. “By the time you have the first death, you have to figure that there’s been three full weeks of transmission, and there are at least several hundred cases in the population.”

So once a city, state, or country is reporting a few Covid-19 cases and especially deaths, it’s typically safe to assume there is a much bigger outbreak going on — just one that’s not fully visible, at least yet, to the public. Given that coronavirus cases and deaths can double every few days, it’s especially important for the general public and officials to act quickly at that point to stop exponential growth.

It’s in this context that a six- or three-day lag in issuing a stay-at-home order could really matter. It didn’t seem at the time that either California or New York had major coronavirus outbreaks just yet. But they couldn’t have known at the time — and the early action the states did take very likely prevented cases from taking off as badly as they would have otherwise.

“I’m loath to criticize, and hindsight is 20/20,” Rutherford said. But “you’ve got to start early. You’ve got to do it before deaths start to accumulate. … And you’ve got to keep your foot on the brake throughout the entire period.”

Some evidence on this point comes from the 1918 flu pandemic, which was linked to as many as 100 million deaths globally and about 675,000 deaths in the US. A 2007 study in PNAS found that the places that took quicker action on social distancing — closing schools and banning big public gatherings — saw better outcomes:

[C]ities in which multiple interventions were implemented at an early phase of the epidemic had peak death rates ≈50% lower than those that did not and had less-steep epidemic curves. Cities in which multiple interventions were implemented at an early phase of the epidemic also showed a trend toward lower cumulative excess mortality, but the difference was smaller (≈20%) and less statistically significant than that for peak death rates.

One example cited in the study is the difference between Philadelphia, which was slow to act, and St. Louis, which was faster. As this chart shows, St. Louis did a much better job of flattening the curve and averting excess deaths:

The goal with disease outbreaks is to look less like Philadelphia and more like St. Louis. So far, New York state looks more like Philly, while California has hewed closer to St. Louis.

To avoid New York’s situation, states can’t let up on social distancing early

That California has so far avoided an outbreak as bad as New York’s does not mean that the state is in the clear now. To the contrary, experts cautioned, with the coronavirus still spreading quickly across the US, it’s entirely possible at this point that an outbreak could begin in any state where social distancing measures aren’t taken seriously.

To that end, California and other states will likely need to maintain such restrictions for the next few weeks, if not months. Even once states see the number of coronavirus cases and deaths decline, they will need to wait some time from now for the threat to really be over.

Again, the 1918 flu pandemic offers relevant evidence. St. Louis, although it’s now heralded for its early action, still appeared to, like many cities at the time, pull back social distancing measures too early. Based on a 2007 study in JAMA, that led to a spike in deaths.

Here’s how that looks in chart form, with the line chart representing excess flu deaths and the black and gray bars below showing when social distancing measures were in place. The highest peak comes after social distancing measures were lifted, with the death rate falling only after they were reinstated.

This did not just happen in St. Louis. Analyzing data from 43 cities, the JAMA study found this pattern repeatedly across the country. Howard Markel, an author of the study and the director of the University of Michigan’s Center for the History of Medicine, described the results as a bunch of “double-humped epi curves” — officials instituted social distancing measures, saw flu cases fall, then pulled back the measures and saw flu cases rise again.

Notably, the second rise in deaths only appeared when cities removed social distancing measures, the JAMA study found: “Among the 43 cities, we found no example of a city that had a second peak of influenza while the first set of nonpharmaceutical interventions were still in effect.”

For California and other states, the goal now is to not only get those curves flattened and bending downward in the next few weeks, but also make sure there isn’t another bump up.

To some degree, that’ll require vigilance until a vaccine for Covid-19 is developed, which could take another year or more.

A group of friends home from college park their cars in a circle to socially distance while spending time together in Berkeley, California, on March 28.

Jessica Christian/The San Francisco Chronicle via Getty Images

But vigilance may not require a full lockdown until a vaccine. If the US scales up testing and surveillance capabilities, experts say, it could detect early warning signs of the disease and act accordingly: isolate people confirmed to have the virus, quarantine everyone they came into contact with, and, if necessary, take broader community-wide measures to make sure the virus doesn’t spread further.

“If there’s enough testing around and people are willing to be tested, the brushfires can be identified and put out before the wildfire,” Martin, of UCSF, said. He emphasized, “The only way that a society can function is if the brushfires are identified and put out.”

That doesn’t necessarily mean testing everyone, even those without symptoms. That’s largely impractical; for one, people who test negative would have to be retested regularly to make sure they remain negative. But it does mean testing everyone with symptoms and the people they’ve come into contact with on top of getting them to isolate and quarantine. That will allow the US, Martin explained, to reopen society and the economy more broadly.

This is essentially what South Korea has done to contain coronavirus. As Max Fisher and Choe Sang-Hun reported at the New York Times, South Korean officials quickly rolled out thousands of tests — still testing, to this day, at nearly double the rate as the US — to track infections and contain them. The country has earned wide acclaim for its response, with its new coronavirus cases now on the decline after it suffered one of the biggest crises in Asia outside China. But even South Korea has braced for a potential second wave, showing the need for constant vigilance.

To truly avoid more outbreaks like New York’s, the US needs to get closer to South Korea. But testing is still a problem across the country, including New York and California, experts said. The US has to fully address that problem to get back to normal.

Until then, cities and states need to take and maintain the kind of action that San Francisco and California took, both formally and informally, early on.