China has launched mass nucleic acid tests in the eastern coastal city of Qingdao after 12 Covid-19 infections including six asymptomatic cases, mostly related to a chest hospital, were reported in the past 24 hours.

More than 9 million residents of the city, located on the Yellow Sea coast, will be tested for the coronavirus over five days, city officials said on Monday.

Mass testing was launched on Sunday evening after reports of the new cases were confirmed.

By Sunday night, photos were circulating online showing residents lining up for tests.

Six confirmed locally transmitted cases and six more asymptomatic ones were reported on Sunday, triggering worries of a new outbreak.

Local media reports said that as of Sunday night, all the cases were connected to Qingdao Chest Hospital, which was treating “imported” cases of the deadly disease.

According to local health authorities, the hospital has been sealed and all doctors and medical staff were being tested.

Buildings, where the families of the infected live, have been locked down; family members and relatives are being tested.

The five districts of Shinan, Shibei, Licang, Laoshan and Chengyang, home to about six million people, will be fully covered in the next three days, health authorities said.

The rest of the city’s population will be covered by the end of the week.

A coastal tourism hub with a German concession area, millions of domestic tourists from across the country had come to the city in the first week of October during China’s National Day holidays before returning to their hometowns.

“Data from the Qingdao municipal bureau of culture and tourism showed that Qingdao received 4.47 million passenger trips during the National Day and Mid-Autumn Festival holidays,” the tabloid Global Times reported on Monday.

The number of daily Covid-19 cases in mainland China has fallen drastically from peaks seen in February this year. But the country and its health officials remain on high alert in order to prevent lockdowns that had led to a contraction of the world’s second largest economy for the first time in decades.

Since the peak number of cases earlier this year, smaller outbreaks have been reported in Beijing in June, in the northeastern city of Dalian and in Urumqi, the capital of the remote northwestern Xinjiang Uyghur Autonomous Region.

The total number of confirmed Covid-19 cases in mainland China stood at 85,578, while the death toll remained unchanged at 4,634.

More than 3,000 imported cases of Covid-19 have been reported.