The Chinese port city of Qingdao plans to test some nine million people in the coming five days after 12 locally-transmitted coronavirus cases were reported sparking concerns of a wider outbreak.

China has been largely coronavirus free since mid-August, with all cases reported by the country imported from elsewhere. But as of October 11, Qingdao has reported a dozen locally transmitted cases, all of which have been linked to a hospital treating imported infections, the city’s Municipal Health Commission said in a statement Monday.

Mass testing has already been carried out at the Qingdao Chest Hospital, which has been placed on lockdown, with 114,862 people, staff and patients, having tested negative as of Monday.

Testing will now be rolled out citywide for the entire population of nine million, repeating the type of mass response previously seen in Beijing and other cities across China where clusters of infections have been detected.

Those responses have been successful in keeping China’s overall infection rate low since the country’s initial outbreak was suppressed in March. That has enabled life to return to relative normality, with more than 600 million tourists travelling this month for Golden Week, a national holiday around the Mid-Autumn Festival.

The cluster in Qingdao, a city on the eastern coast home to numerous beaches and popular with tourists, will raise fears of a potential spike in cases related to the holiday.

Authorities in the port city said the specific source of infection was still under investigation.

As of this week, China has recorded 85,578 confirmed cases and 4,634 deaths from the coronavirus, according to the country’s National Health Commission.