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Since early in the pandemic, China has pursued a policy of “zero COVID”: The country  intends to come as close as it can to eliminating the disease from its borders. But even as other countries dropped their own zero-COVID goals over the past year, and despite a recent unusual statement from the World Health Organization calling the policy unsustainable, China’s government has stuck to its plan.

Setting zero COVID as a goal and enacting policies to meet that goal are two separate things. New Zealand and Australia also pursued zero COVID until last fall, when the Omicron surge overwhelmed their precautions. But China has held its policy, and ramped up its response to meet Omicron in spite of substantial costs. Understanding exactly how China has done so is important for figuring out whether the country can stay the course for Omicron.

Benjamin Cowling, chair of epidemiology at Hong Kong University who helped draft the WHO’s 2019 pandemic influenza guidelines, talked Popular Science through China’s strategy.

He says the policy wasn’t necessarily one that China had prepared before the pandemic. Instead, it grew out of the initial response to the Wuhan outbreak.

“There was no playbook before COVID for this,” he says. “It’s a new idea.” Three years ago, in the influenza playbook, Cowling says that he wrote that “containment is futile.” He’ll have to update that statement, he says: China has shown that, in some circumstances, containment is possible.

Step one: suppression

A national zero COVID strategy has two parts. First, contain existing outbreaks, and let them burn out. Then, stop new outbreaks from entering. Or, as Cowling puts it: “One is get to zero, and the other is stay at zero.”

That first step, suppression, is what has occurred in Shanghai over this spring. While the US’s brief and loose stay-at-home policies in the spring of 2020 were intended to bring transmission down to low levels, China’s lockdown has much tighter controls designed to entirely end transmission.

“They’re very, very good at a total lockdown,” Cowling says, “which then buys time for another thing they’re very good at, which is testing everybody in a city. You can see the strategy in Shanghai: Everybody stays at home, they test everybody repeatedly, and anyone who has a case they isolate outside the home, while any contact identified from contact tracing is quarantined.”

Joan Kaufman, a lecturer in global health and social medicine at Harvard University’s School of Medicine, noted that China has a precedent for wielding its infrastructure apparatus to contain a virus: during the 2003 SARS outbreak. “They built these quarantine hospitals and took a very aggressive approach to fully containing and eliminating the SARS virus,” she says. “I think that was the underpinning of this approach.”

Executing COVID lockdowns involves not just labs and medical workers to conduct millions of PCR tests, but civil servants who can trace contacts, and physical buildings to house thousands of people in government-overseen quarantine.

When cases are identified, Chinese authorities have locked down apartment buildings, neighborhoods, cities, or even entire regions, as in Hubei province in early 2020. That has meant different things depending on the circumstances. Sometimes, people are confined to an apartment block or neighborhood. In others, households are allowed a limited number of trips.

That can have a human cost. In Shanghai, residents have gone hungry, or slipped into desperation after months without work. And international outlets have reported growing unrest in the city at news of callous or even brutal quarantine policies—like the separation of parents and children, or police who broke down a door to bring a woman into quarantine.

While a lockdown is in place, China can mobilize a massive contact tracing apparatus to identify possible contacts of people who test positive—in November, the South China Morning Post reported that the police in the city of Chengdu had traced 82,000 close contacts of COVID cases. “There are people scouring CCTV videos, and looking at mobile phone location data,” says Cowling.

For the most part, people who test positive or who are identified as contacts are required to quarantine outside their home. Cities have built quarantine and isolation facilities that can hold thousands of people, and the country has a bank of mobile quarantine buildings that can be trucked to the site of new outbreaks. “They have them in central depots, and if there’s an outbreak in a particular city,” Cowling says, “those trucks just start driving there with these facilities ready to be dropped and connected to electricity and water.”

The speed of that response has meant that lockdowns often last a few weeks. Cowling contrasts this to Australia, which also pursued zero COVID but had to lock down Melbourne for nearly nine months in the face of a simmering outbreak. China’s lengthier exceptions, he points out, like Shanghai or Wuhan, were situations where an outbreak was already well under way before a lockdown.

[Related: What 1 million COVID deaths looks like from the frontlines]

The downside, Cowland says, is that casting such a massive net inevitably means quarantining people who aren’t sick. And quarantine facilities in hard-hit cities have ended up crowded, or without enough food or showers. But during most of the pandemic, life went on normally across the country—which meant that the country avoided not only deaths, but the emotional toll of school closures and other isolation precautions.

And stay out

Then, there’s staying at zero. The key to that has been travel restrictions.

Although it’s possible to enter and exit China, international flights can only land in a handful of cities. Passengers need to take a series of COVID tests—for anyone coming from the US, that means one PCR test seven days from departure, another two days from departure, and a final antigen test before leaving on a direct flight for China.

Anyone flying in is required to quarantine for 14 days and test as often as every day. According to the US State Department, people who test positive are moved to either a hospital or an isolation facility. “Then there’s another week of soft quarantine,” says Kaufman, “which means they’re allowed to move around that city but they can’t go anywhere else” for a set amount of time.

That’s not dissimilar to the approaches taken by Singapore and Korea for much of the pandemic. And, until fall 2021, New Zealand and Australia allowed virtually no international travel.

But China, unlike those countries, is not an island, and tight controls have had consequences for the southern border with Myanmar, Laos, and Vietnam. While most of the country has been spared the disruption of the pandemic, border cities have experienced waves of lockdowns, prompting local recessions and outmigration.

Those closures seem to have been an acceptable trade-off. “Most Chinese people are not international travelers, so the government may see it as an economic downside, but not a dealbreaker,” Cowling says.

Under pressure

These tactics seemed fairly sustainable—until Omicron. The arrival of that variant , which is so wildly transmissible, has changed that calculus, largely because suppression measures need to go further, and last longer, to be effective. That means the actions have been more disruptive to everyday life.

Although news has focused on outbreaks in Beijing and Shanghai, outbreaks are taking place across the country. On May 5, CNBC reported that an estimated 40 cities were affected by lockdowns this spring.

“I think the fear now in China would be that the disruption seen in Shanghai is going to occur again and again in coming months if Omicron gets into Beijing or Guangzhou,” Cowling says. “It’s inevitable that the virus will get in from time to time.”

[Related: The 5 phases of COVID’s endgame]

A major difference between China’s COVID zero plan and those of other countries, Cowling says, is that Australia and New Zealand had planned off ramps. China’s president, Xi Jinping, has said that the country can maintain zero COVID for the foreseeable future—though the policy has been rebranded “dynamic zero COVID” to acknowledge that COVID cases will still occur.

And that’s led to a contentious debate among both foreign and domestic epidemiologists over the country’s strategy. A letter published in Nature Medicine argued that China now has the medical infrastructure to successfully treat COVID outbreaks without resorting to lockdowns. But another recent paper in the same journal estimated that if the country were to end its current lockdown-and-isolate strategy, and instead rely on a patchwork of school and workplace closures, Omicron would kill between 800,000 and 1.6 million people, depending on the effectiveness of those strategies.

The challenge, Cowling says, is that 14 percent of China’s citizens over 60 and 50 percent of those over 80 aren’t fully vaccinated, in part because the populace hasn’t felt personally threatened by COVID. “If you’re not expecting cases in the community, then that means you most likely don’t need to be vaccinated,” he says. Ending the suppression policies could change that risk perception—but only after people had begun dying. That’s similar to what happened in Hong Kong, where morgues overflowed after Omicron overwhelmed a lockdown. To avoid something similar, China needs to find a way to get more vaccines in arms—or stay its current course.

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