"It is more likely to have been imported via contaminated seafood from abroad," Wu said.

As of Wednesday, Dalian, a port city, had reported 57 confirmed COVID-19 cases in the latest outbreak, starting on July 22. Thirty-two asymptomatic cases are under medical observation, said Zhao Lian, deputy director of the city's health commission, at a news conference on Thursday.

All the confirmed cases (30) and asymptomatic cases (25), involved employees of Dalian Kaiyang World Seafood Co, she said.

Kaiyang is a well-known enterprise that processes imported seafood and sells frozen seafood gift packages produced both at home and abroad.

The scale of the latest outbreak in Dalian has been basically contained, and no further large-scale spread is expected, said Wu, the epidemiologist.

In the beginning, Dalian reported three confirmed COVID-19 cases and 12 asymptomatic infections. Such a large proportion of asymptomatic infections indicates that the outbreak was tamped down quickly, he said.

The city immediately began a large-scale testing program and has identified more asymptomatic people. The newly confirmed cases basically stem from the previously identified asymptomatic carriers, Wu said.

"Because of the timely quarantine, there is no great risk of transmission," he said.

According to Zhao Lian, deputy director of the city's health commission, Dalian has increased its screening of imported aquatic products, frozen and refrigerated meat and other key products, and also strengthened the daily supervision of storage facilities and markets.

Since Sunday, Dalian has been arranging free coronavirus tests for all residents. As of Wednesday, about 4.1 million samples had been collected, Zhao said.

Zhang Xiaomin in Dalian, CGTN contributed to this story.