About four in 10 COVID cases are asymptomatic, making them potential sources of increased transmission of the infectious disease, according to a study.

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The study led by researchers at Peking University in China evaluated the percentage of asymptomatic infections among individuals undergoing testing (tested population) and those with confirmed COVID-19 (confirmed population). They conducted a systematic review and meta-analysis of 95 unique studies with 29,776,306 individuals undergoing testing. The findings, published in the JAMA Network Open, showed that the asymptomatic infections were 0.25 per cent among the tested population and 40.50 per cent among the population with confirmed COVID-19.

"The high percentage of asymptomatic infections from this study highlights the potential transmission risk of asymptomatic infections in communities," said Qiuyue Ma, Department of Epidemiology and Biostatistics, from the University. The study found that the percentage of asymptomatic infections among the confirmed population was 54.11 per cent in pregnant women and 52.91 per cent in air or cruise travellers. The percentage was 47.53 per cent in nursing home residents or staff.

This finding of a high percentage of asymptomatic infections among air or cruise travellers suggests that screening and quarantine on airport arrival is important for reducing community transmissions, especially in countries without local transmission, Ma said.

In addition, the study also showed that the percentage of asymptomatic infections was higher among groups younger than 39 years than in other age groups, possibly because the young adults were more likely to show only mild or moderate clinical symptoms. This indicated that young adults who often presented mild or no symptoms were a potential source of transmission in the community.

"Screening for asymptomatic infection is required, especially for countries and regions that have successfully controlled SARS-CoV-2. Asymptomatic infections should be under management similar to that for confirmed infections, including isolating and contact tracing," Ma said.