The Chinese port city of Qingdao in Shandong province, will test 9 million people over the next five days after 12 new cases of the coronavirus were reported, all linked to a hospital now in quarantine.
The mass testing comes after a dozen cases were found linked back to a hospital treating coronavirus patients arriving from overseas.
The new measures taken by the health authorities of the region were introduced to stem the spread of the virus in the province.
Since about mid-August, China has not experienced any new significant outbreaks in the country. The new cases were generally imported from abroad. As of 11 October Qingdao has reported 12 new cases, all linked to a hospital, a center for infectious diseases.
The first mass tests were carried out at the Qingdao Chest Hospital, which was quarantined, and involved nearly 115,000 people, including staff, patients, and relatives, which tested negative.
The tests will be carried out on a city scale, repeating what happened in Beijing and other Chinese cities, where outbreaks had been reported.
The Chinese response has kept the virus under control in the country. More than 600 million tourists are now traveling for the Golden Week, a Chinese holiday that takes place around the mid-autumn festival.