Could cause panic.” “Will not help end the crisis.” “Could backfire.”

When the Chinese government blocked most travel into and out of the city at the center of the Covid-19 outbreak in late January, many public health experts took to social media and op-ed pages to decry the measure as not only draconian and a violation of individual rights but also as ineffective: This largest quarantine in history — the city, Wuhan, has a population of 11 million, and the lockdown has been expanded — would have little effect on the course of the epidemic, they argued.

As the U.S. and other countries imposed travel restrictions, even the World Health Organization questioned whether they were a good idea. But early evidence is causing some disease fighters to reconsider.

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The last few days have seen a perceptible flattening in growth of Covid-19 cases in China, raising hopes that the epidemic has peaked. (Though there are doubts about the accuracy of China’s count.) That supports the emerging consensus on the Wuhan quarantine in particular: that, at minimum, it bought China and the world time to prepare. Crucially, the time lag allowed public health agencies to devise and distribute a diagnostic test that hospitals can use to identify patients ill with the novel coronavirus.

“Measures on movement restriction have delayed the dissemination of the outbreak two or three days within China and a few weeks outside China,” Sylvie Briand, director of Infectious Hazard Management at the WHO, told reporters this week. “Those measures, if well implemented, could have an impact on the propagation of the outbreak.”

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Scientists are still gathering data on the effect of travel restrictions and the historic quarantine — and, in particular, whether they reduced total cases and deaths or just postponed many of them without lowering the eventual cumulative toll. The answer will have broad consequences for future outbreaks, perhaps putting large-scale quarantines back on the list of health officials’ epidemic countermeasures, including in countries that value individual liberties more than China does.

“We should do what we can to understand their effectiveness, because [quarantines and travel restrictions] may be considered again in this epidemic and in future epidemics,” said preparedness expert Tom Inglesby, director of the Center for Health Security of the Johns Hopkins Bloomberg School of Public Health. “It’s important to know not only if they worked in some way but also to gauge if they did harm.”

There have, in fact, been numerous, and in some cases fatal, unintended knock-on effects of the quarantine, as critics warned. People were unable to reach sick, elderly parents in Wuhan, let alone take them out of the city for treatment of heart disease, cancer, diabetes, and other illnesses. This week, UNAIDS announced that one-third of people in China who are living with HIV reported that because of lockdowns and travel restrictions they were at risk of running out of their HIV medications within days. And China’s economy has slowed to a crawl.

The impact of the movement restrictions extends beyond Wuhan, to other areas in China as well as other countries getting a trickle of cases early this month rather than the flood they likely would have if Wuhan residents and visitors had been able to leave the city. As a result, they have been able to deploy countermeasures with deliberation and not in a rushed panic.

The quarantine “let countries field the fly balls coming at them one by one by one and not drop them,” said David Fisman of the University of Toronto, a leading epidemic modeler. “The fact that Wuhan got sealed off made the numbers something other countries could handle. They’ve been able to figure out things like how to identify cases and what to do with them. I think that made a huge difference.”

If thousands of infected people had traveled from Wuhan starting in late January, Fisman said, “every place new cases landed would have had the potential to be another Wuhan,” whose Hubei province is approaching 2,000 deaths.

Instead, health care systems in countries that have had cases (1,073 and counting outside China, according to WHO) had time to prepare, said Alexandra Phelan of Georgetown University’s Center for Global Health Science and Security: “Delays can be a really useful tool, and maybe one of the only tools you have, to implement screening and training for health care workers, and essentially get your house in order.”

Hospitals could have used the time to give their staffs refresher courses in infection control and practice using respiratory gear, said Inglesby (though it’s not clear how many in which countries did so). Agencies such as the U.S. Centers for Disease Control and Prevention had time to update and disseminate technical information, including the use of diagnostic kits.

“The ability to test for the virus has been distributed pretty widely,” including by the CDC to U.S. states and cities, Inglesby said. Testing capacity grew outside the U.S., too, he said: “Certainly more countries can test for the virus now than three weeks ago,” when without the Wuhan quarantine they might have faced a flood of cases, both suspected and real.

With the breathing room the quarantine provided, China has been able to plan or begin about 120 clinical trials of potential Covid-19 treatments, Inglesby added: “There has been time to acquire data on what medications might work,” including those currently in use for other diseases and therefore relatively available.

U.S. travel restrictions (barring most foreign nationals who have been in China recently from entering, requiring Americans returning from China to be quarantined or self-isolate) were, like the Wuhan quarantine, “really meant to slow down the spread” of the coronavirus, Health and Human Services Secretary Alex Azar, told reporters this month. “It’s not meant to hermetically seal the United States from the virus, but rather to allow us to focus our resources,” including by setting up the quarantine centers. “It’s about slowing spread.”

“I think most health officials agree that at best [quarantine or travel restriction] delays and … kind of pauses things,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told reporters. “What we needed was a delay to essentially prepare better.”

Experts are divided, however, on whether the Wuhan quarantine not only resulted in what the WHO’s Briand called “postponing the peak” of the epidemic — the time when the number of new cases tops out — but also in reducing the total number of cases and deaths.

“I haven’t seen anything that says the overall number of people infected in the world will be less” than without the Wuhan quarantine, Inglesby said.

The reason for pessimism is that although the quarantine reduced the rate of transmission and therefore the number of cases in any given time period, normal commercial activity and travel in China “has to resume at some point, so it’s hard to argue that the disease wouldn’t resume its trajectory,” Inglesby said. “No one has put forth the case that what China has done will eliminate this virus from Earth.” Sustained transmission, however, might look more like bad colds (as other coronaviruses cause) than like severe seasonal flu.

Experts who take a more sanguine view believe that slowing the spread and buying time for countermeasures did have a permanent effect on the epidemic, not only postponing what would have been today’s cases into next week but preventing some of those cases forever. If so, then the eventual toll would be less than without the Wuhan quarantine.

That and other control measures changed how many cases each infected person causes “big time,” Fisman said. The lower this “reproductive number,” the lower the total case count. The Wuhan quarantine, he said, “reduced the likelihood of people in Wuhan coming into contact with people not in Wuhan. The rate of transmission is now pretty flat. This is coming under control in other cities [outside Hubei] in China.”

Fisman’s and other mathematical models of the epidemic’s trajectory, however, are undermined by the fact that key numbers in the equations describing disease transmission (a “model” is essentially a group of equations solved by computers) are uncertain.

For instance, the time between when one infected person becomes ill and when someone he infects does was thought to be five to seven days. But an analysis led by Hiroshi Nishiura of Japan’s Hokkaido University suggests this “serial interval” is only four days.

That could be bad news because half the people who get sick do so less than four days after exposure to the virus that causes Covid-19. “A substantial proportion” of transmission may therefore occur before people show symptoms, Nishiura and his colleagues warned. “Pre-symptomatic transmission … may even occur more frequently than symptomatic transmission,” confounding control measures: If people can spread disease before they’re sick, then isolating only sick people won’t stop transmission.

In fact, physicians in China reported this week in the New England Journal of Medicine that a symptom-free individual had just as much virus (according to nose and throat swabs) as patients with symptoms — all set to be sneezed or coughed onto other people.

Although diseases that can be spread by people who aren’t sick are harder to control, there are hints that the worst of the Covid-19 outbreak in China is nevertheless past. Like several other models, three by researchers at Georgia State University School of Public Health projecting Covid-19 cases suggest that the outbreak is losing steam. Total cases in China outside Hubei should stay below 15,000 through the end of February, with little increase after that, the models say. (Projections for Hubei got scrambled when China changed the case definition for Covid-19, producing a sharp one-time uptick in case totals last week.)

As of Thursday, there were 12,644 Covid-19 cases in China outside Hubei.

“For all other provinces, the models are on track,” said mathematical epidemiologist Gerardo Chowell of Georgia State. “The containment strategies implemented in China are successfully reducing transmission,” he and his colleagues wrote in a paper published last week in Infectious Disease Modeling. “The epidemic growth has slowed.”

Chowell inclines toward optimism, he said: “I think that we’ll control this,” especially if the Covid-19 virus (like influenza and other viruses) doesn’t survive or spread as well in warm, humid conditions.

Barring secondary outbreaks (as happened with SARS, a coronavirus that spread around the globe in 2003), Chowell said, the Covid-19 epidemic in China might be over in three weeks. This pathogen might join the other four coronaviruses that give millions of people colds and, in some cases, pneumonia every year.

If it falls well short of an uncontrollable pandemic, quarantines and other measures widely viewed as ineffective and even counterproductive could well join the bag of tools used in future viral outbreaks.

Andrew Joseph contributed reporting.