A port worker surnamed Yu whose nucleic acid test result came out positive Wednesday was confirmed to be an asymptomatic carrier whose case is related with imported cases in Ningbo, East China's Zhejiang province, a local official said.

Li Chengjie, executive deputy director of Ningbo's Beilun district, said Yu, 34, has never gone abroad and visited the domestic medium and high-risk areas two weeks before he tested positive for the novel coronavirus on Wednesday.

"And he has not had contact with any domestic confirmed cases or asymptomatic carriers," Li said at a news conference in Ningbo on Wednesday.

Yu, who is employed by a container company in the port, had a negative result in routine nucleic acid testing on Sunday. But his result came out positive early Wednesday morning when he had the nucleic acid test again on Tuesday.

Yu has now been sent to a designated hospital for further medical observation and isolated treatment, he said.

Yu lives in Baifeng village in Ningbo's Beilun district.

All Yu's close contacts have now been quarantined and the relevant venues where Yu works and visited have been required to cease operation for disinfection.

And 245 out of 331 samples collected from Yu's close contacts have come out negative as of Wednesday evening, Li said.

Zhang Nanfen, director of the Ningbo City Health Commission, said relevant departments have been required to spare no effort and introduce effective new measures to prevent and control the imported cases from further spreading in the city.

All new employees are required to show their negative results of nucleic acid tests taken in the 48 hours before they are allowed to start work, Zhang said.

As of Tuesday, Ningbo had administered 13.52 million doses of vaccines, with more than 7.88 million residents who are aged above 18 years having been vaccinated against coronavirus with at least one dose.

And in Zhoushan city, 15 crews from a foreign shipowner arrived in the city as of Wednesday night and got ready to substitute for the infected sailors on board.

On Monday night, 11 of the 16 sailors from a Panamanian vessel who tested positive for novel coronavirus have been hospitalized for treatment in Zhoushan. They are now in stable condition in wards.

The Zhoushan city government has set up a special task force to handle the matter and has initiated emergency procedures when it received reports from the sailors on board.

Nine sailors on board sought help when they developed a fever on Aug 3 when the vessel entered Zhoushan waters. Sixteen sailors later tested positive for coronavirus.

The vessel, which carries more than 42,000 tons of coal, has a total of 20 Chinese sailors on board.