A county in central China has been put under lockdown again after a flare-up in coronavirus cases, pointing to the difficulty of sustaining outbreak containment in the face of carriers who show no signs of sickness.

Jia county, whose population numbers around 640,000, issued a directive on Wednesday asking all residential compounds to be sealed off and those visiting and leaving homes to produce identity cards, wear masks and submit to temperature checks. Car traffic will also be limited.The move came after residents were unnerved by an incident in which a woman was infected after she visited a doctor who carried the virus but showed no signs of being sick. Researchers are finding that these stealthy carriers can play an outsized role in spreading the pathogen to others, since there’s no easy way to tell they’re infected.

Many countries, especially those with limited testing capacity, do not give people tests unless they show symptoms like fever and cough.

China has officially declared its epidemic contained after reported new cases fell to zero on March 19, but there’s growing concern over a second wave both domestically and in travelers arriving at its border. The U.S. intelligence community has also concluded that China under-reported both total cases and deaths it’s suffered from the disease in a classified report given to the U.S. government.

Asyptomatic Doctors

Testing conducted by a Jia county hospital on March 25 of medical workers who treat COVID-19 patients revealed infections in three doctors who showed no symptoms. The three doctors dined together in a restaurant on March 13 and one of them had previously traveled to Wuhan and self-quarantined for two weeks upon his return.

All three were placed under quarantine after testing positive. But their infections were not made public as China did not start to disclose its number of asymptomatic infections until this week, after domestic and international criticism of its lack of transparency.

Countries like South Korea and Japan have always included asymptomatic patients who test positive in their confirmed case counts, while China’s practice is to move such cases to the confirmed category only after they start showing symptoms. This means that Chinese people who do not develop symptoms but carry the virus are never identified and tallied as confirmed cases.

In Jia county, the three doctors’ infections came to light only when a 59-year-old woman who developed a fever and a headache in a nearby city was found to have contracted the virus on March 28, after visiting one of the doctors. Her visit occurred before the doctors’ quarantine began on March 25.

China’s National Health Commission said there are 1,367 asymptomatic cases under quarantine nationwide as of March 31.