After more than 100 COVID-19 cases were traced to a 45-year-old salesman who tested positive on Saturday, Jilin province has launched an investigation of health clubs that may be part of superspreader events.

The salesman, surnamed Lin, held four workshops in different health clubs in Gongzhuling and Tonghua, Jilin, between Jan 8 and 11, resulting in a cluster infections involving more than 100 people with an average age of 63.

"We will investigate whether there was any illegal business behavior when the clubs promoted their products through such workshops," said Liu Shunchang, deputy director of the provincial administration for market regulation on Sunday, adding that if any violations are spotted, severe punishment will be forthcoming.

Starting on Monday, Gongzhuling began to carry out mass testing. Residents were ordered to stay at home in self-quarantine, according to a statement by the city's epidemic prevention and control headquarters.

Community staff and volunteers will help residents purchase daily necessities during their home quarantines.

The province reported 30 new locally transmitted confirmed cases of COVID-19, driving the caseload to 40 as of Sunday, according to the provincial health commission.

Seventeen asymptomatic carriers were identified, bringing the total to 90, the commission said on Monday.

The recent local transmission in Jilin province came after the first asymptomatic carriers returned from neighboring Heilongjiang province, which has seen infections climbing.

As of Sunday, Heilongjiang had reported 145 confirmed cases and 243 asymptomatic carriers since Jan 10, when the first asymptomatic carrier of the current outbreak was reported in Suihua's Wangkui county, officials said at a news conference on Monday.

"Wangkui is preparing to launch a third round of mass nucleic acid testing across the county," said Dong Wenqin, deputy mayor of Suihua. "The fourth round of testing will start in Huiqi town, which has been hit worst."

"We must continue to focus on epidemic prevention and control in rural areas," said Liu Yancheng, a commission official, adding that clinics, pharmacies and other medical institutions in rural areas should report immediately if staff members suspect a person is infected, with symptoms such as fever or cough, he said.

Related staff members should strictly observe migrants, including tracing their daily movements, monitoring their health condition and conducting nucleic acid testing as needed, he said.

He also urged rural residents to avoid gatherings, such as weddings, funerals, banquets and games of chess and cards to prevent spreading the virus.