Wang Xijiang, deputy director of the Kashgar Centre for Disease Control and Prevention, said frequent testing was needed to detect cases early so that the outbreak could be swiftly brought under control.

He said asymptomatic patients could test positive at different stages of the incubation period.

“Nucleic acid testing is the gold standard for confirming infections, and it is also the most important method for detecting and identifying asymptomatic infections,” Wang said. “To get the epidemic under control as soon as possible, we will launch a fourth round of free nucleic acid testing for all residents in Shufu county on November 4.”

00:39

Xinjiang officials complete 4.7 million Covid-19 tests in Chinese city of Kashgar

Xinjiang officials complete 4.7 million Covid-19 tests in Chinese city of Kashgar

Located near the border with Tajikistan and Kyrgyzstan, the county is home to 250,000 people. Mass testing began in Shufu, and in the Kashgar area of 4.7 million that includes Shufu, on October 24 when a 17-year-old garment factory worker in the county tested positive without symptoms in a routine screening.

The Kizilsu Kyrgyz autonomous prefecture, which neighbours Kashgar, also carried out two rounds of mass testing in October that detected 15 cases in Aktau county. Aktau has also begun a fourth round of testing for its 220,000 residents after finishing the third on Tuesday.

01:42

China’s city of Kashgar finds 137 new coronavirus cases after infected teen sets off mass testing

China’s city of Kashgar finds 137 new coronavirus cases after infected teen sets off mass testing

China has largely brought local transmission of Covid-19 under control and has moved fast to respond to recent outbreaks in Dalian, Qingdao, Beijing and Urumqi with partial lockdowns and citywide screening, affecting millions of people.

But some experts do not agree with mass testing.

It is usually done with the aim of achieving eradication and should be done at short intervals of not more than a few days, according to Lee Shui-shan, deputy director of the Stanley Ho Centre for Emerging Infectious Diseases at Chinese University of Hong Kong.

“The idea is to capture all existing infections, and there is a time element attached,” Lee said. “Doing this over a period of weeks will mean that you are identifying some infections and letting others go so there is no end to the chasing game.”

Coronavirus: China ‘not headed for second wave’ despite new surge around the world

He said mass testing needed to be supported by a health care system that could cope with an influx of patients, including enough isolation facilities.

“The scale can be very large, so that’s why we are seeing authorities using this measure only after the epidemic is already under some form of control,” Lee said.

In Europe, where authorities are scrambling to slow a resurgence of infections across the continent, Slovakia has launched mass testing for anyone aged over 10 among its 5.5 million people. So far more than 38,000 people have tested positive. Britain, meanwhile, will begin mass testing all residents of the northwest city of Liverpool on Friday in a pilot scheme that could be expanded to other cities.

According to Lee, if the goal is to reduce the impact of the coronavirus rather than achieving zero infections, mass screening is not necessary.

And he cautioned that it should not be the key strategy to contain the spread of the virus.

“The main measures are still social distancing and universal mask-wearing, depending on the situation and the stage of the epidemic,” he said. “It’s finding the balance between controls and normal social activities that poses the challenge.”