Five out of the six local COVID-19 cases in Shanghai who were employees in the foreign cargo flights work area at Pudong International Airport were likely infected by people or an environment contaminated by imported novel coronavirus.

"The infection might probably be a result of substandard personal protection of these individuals," the city's COVID-19 Pandemic Prevention and Control Office said on Monday.

These cases, two confirmed on Friday and three confirmed on Saturday, were infected with the virus's Delta variant. Gene testing also found that infection had little relation with other cases in other parts of the country.

The local government also announced that by 9 am Monday it had tracked down 143 close contacts and 942 secondary close contacts of the five cases, as well as nearly 80,000 relevant individuals who might be exposed to the virus.

All tested negative twice, the government said.