Eighteen samples tested positive after the second round of blanket testing in Ruili City, southwest China's Yunnan Province, as of Friday. 

The city, with a population of around 300,000 reported a cluster infection last week and launched the first round of mass nucleic acid testing on March 31. 

Almost all the recent local transmissions in China occurred in the border city, which leads to the dismissal of the city's party chief on Thursday.