China’s Qingdao city has said it will conduct Covid-19 tests for the entire population of more than 9 million people over five days after new cases appeared linked to a hospital treating imported infections.

The city reported six new Covid-19 cases and six asymptomatic cases as of late October 11. Most of the cases were linked to the Qingdao Chest Hospital, Reuters reported.

The National Health Commission’s tally of 21 confirmed Covid-19 cases in mainland China for Ocober 11 was published after Qingdao’s announcement, but did not include any confirmed infections in the city and it was not immediately clear why.

Daily Covid-19 infections in mainland China have fallen drastically from peaks early this year, but the country remains on high alert in order to prevent painful lockdowns.