A medical worker takes a swab sample from a student for nucleic acid test at a university in Qingpu district of East China's Shanghai, March 14, 2022. Photo: Xinhua

The majority of infected persons amid this round of Omicron outbreak in Shanghai are young and middle-aged people with good health, a Shanghai government official said on Tuesday. There was a large disparity in the proportion of confirmed cases and asymptomatic infections, which is related to Omicron characteristics, human immunity and mass vaccination, the authorities noted.

Shanghai on Monday reported 31 new confirmed locally transmitted COVID-19 cases and 865 local asymptomatic carriers, the authorities said on Tuesday. 

The average age of infected cases amid this round of outbreak in Shanghai is 40.6 years old, and 84.5 percent of them are under 60 years old. "The proportion of asymptomatic patients is correspondingly higher," Fu Chen, director of Shanghai Center for Disease Control and Prevention said at Tuesday's press briefing.

The Omicron variant strain is the main epidemic strain in China, which is characterized by strong hidden features, rapid transmission and short incubation period, Fu said, noting that the proportion of mild and asymptomatic hospitalized infected cases in China exceeds 95 percent.  

The majority of people infected amid this outbreak in the city have been fully vaccinated against COVID-19, and the proportion of asymptomatic cases will increase as people's immunity is enhanced, Fu said. More than 55 million doses of COVID-19 vaccines have been administered in Shanghai, with over 22 million people receiving the full process of vaccination and more than 11 million receiving booster shots.  

The recent mass screening in communities has found more cases positive for COVID-19 including asymptomatic infected persons in the incubation period and early stage, Fu noted. 

Notably, a total of 326 local COVID19 cases, including 186 asymptomatic cases, have been discharged from hospital in Shanghai on Monday, a new record following the country's revised discharge criteria, the authorities said on Tuesday.