Shanghai will step up epidemiological investigation and disinfection for old neighborhoods in downtown and suburban areas, the major places where new COVID-19 cases have been reported recently.

The city reported 234 confirmed COVID-19 cases, 2,780 asymptomatic cases and six deaths for Monday, health authorities announced on Tuesday.

Sun Xiaodong, deputy director of the Shanghai Center for Disease Control and Prevention, said 90 percent of the new cases were from central downtown areas and suburbs adjacent to the city.

The old neighborhoods, residential quarters and villages in those areas have dense populations and many use communal kitchens and toilets, which lead to high risk of virus spread, he said.

Sun said the quarantine of close contacts of infected cases must be implemented strictly, and explained the definition of close contacts, which refers to people who have been in contact with confirmed cases within four days before the latter showed symptoms. Those who have been in contact with asymptomatic cases within four days before their test is taken are also considered close contacts.

Secondary contacts refer to those living, working or studying with the close contacts.

Sun noted that those living together with a COVID-19 case in an apartment with its own kitchen and toilet will be deemed as close contacts, while those living on the same floor and neighboring floors will be deemed as secondary contacts.

If a confirmed case lives in a building with communal kitchens and toilets, those living on the same floor will be deemed as close contacts, while the rest of the people in the building will be deemed secondary contacts.

"Each case must be handled with on-site investigation and risk evaluation. We should not resort to a simplistic and one-size-fits-all approach," Sun noted.

Jin Chen, deputy director of Shanghai Housing and Urban-Rural Development Commission, said that places where COVID-19 cases have resided must be disinfected.

Community workers must communicate with the infected cases regarding the disinfection of their homes and find out what items need to be protected during the disinfection process, he said.

Jin said that the homes of close contacts living in places with communal kitchens or bathrooms must be disinfected as well.

In the past month, the disinfection team has sent out around 160,000 staff every day and have disinfected more than 13,000 residential communities.

"Next, we will focus on supermarkets, office buildings and street stores as the city resumes its commerce," Jin said.

Zhao Dandan, deputy director of Shanghai Health Commission, said recent epidemiological investigations have found several COVID-19 cases from precautionary zones, which were deemed the lowest risk level in the three-zone epidemic control systems.

"In order to stem the spread of the virus, we will increase the frequency of nucleic acid testing there, at least two rounds of tests for people in precautionary zones this week," Zhao said.

Currently, around 17.92 million people live in some 46,000 precautionary zones, 3.27 million live in nearly 19,000 controlled zones and 2.11 million are confined inside their homes in 9,000 locked-down zones.