Shanghai will no longer have medium-risk areas on Tuesday when authorities downgrade a residential neighborhood in Zhuqiao town, Pudong New Area, to a low-risk area.

The neighborhood has not had new COVID-19 cases emerge in the past 14 days, according to authorities.

Speaking about the epidemiological investigation into the six confirmed cases reported from Nov 20 to 23, Sun Xiaodong, deputy director of the Shanghai center for disease control and prevention, said the first three cases were colleagues at the same cargo station of the Pudong International Airport. The first case had been in the cabin of an overseas cargo flight and was in contact with cabin crew before symptoms emerged.

The fourth case worked in another cargo station of the airport and had contact with the crew of an overseas cargo flight at work.

The fifth and sixth cases were the wives of the first and second cases.

Wu Jinglei, director of the Shanghai Health Commission, said the city will strengthen measures to reduce the risk of COVID-19 entering Shanghai through the imported cold chain food.

"All high-risk imported cold chain foods that enter the city's ports or are stored, produced, and sold here, will go through additional external packaging sterilization and nucleic acid testing at designated regions first, after customs inspection and sterilization," Wu said at a conference held on Monday.

Wu pointed out the city has monitored cold chain food and related environments in 1,854 locations, including large-scale wholesale markets, supermarkets, and e-commerce companies. A total of 147,000 pieces of cold chain food and their environmental samples have been tested for the virus. All samples have returned negative.

"We've also improved technical guidance and training for staff members in the cold chain food industry and produced regular nucleic acid testing for them," he said, before reiterating the importance of personal hygiene and prevention measures.