© Xinhua/Xiong Qi

Around 60% of people who contracted the coronavirus in the central Chinese city of Wuhan were asymptomatic or very mild cases not reported to the authorities, according to a study led by a group of Chinese researchers.

The paper suggests that by February 18 the total number of infections in Wuhan, the first epicenter of the coronavirus, could have exceeded 125,000, more than three times the number of confirmed cases, 38,020, reported at the time.

The estimate adds to a body of research seeking to estimate the true extent of the spread of coronavirus beyond counting patients with clear symptoms of Covid-19, the disease caused by the coronavirus.

It was published on the medRxiv preprint platform early this month and has not been peer-reviewed.

The study called the asymptomatic and very mild patients “unascertained cases” and called for attention to gauge their size and transmission.

These patients “could recover without seeking medical care and thus were not reported to authorities,” the researchers said.

“We found that at least 59% of infected cases were unascertained in Wuhan, potentially including asymptomatic and mild-symptomatic cases.”

The researchers used lab tests as the basis of their assessment rather than Chinese government data on confirmed cases because the authorities used symptomatic manifestations and abnormal lung scans to classify patients.

The study uses the lab test data to create models to estimate the number of infections regardless of the presence of symptoms based on estimated rates of infections for people of different age, sex, occupation and residential districts.

The model also estimates the effect of Wuhan residents’ movement, the travel restrictions and quarantine measures on the transmission of the virus.

The paper said covert cases and their transmission had been underestimated in previous models and more evidence by recent studies, such as data from the Diamond Princess cruise ship, indicated that there was a “non-negligible” proportion of asymptomatic cases.

“Estimation of unascertained cases has important implications on continuing surveillance and interventions,” it said.

According to a report in the scientific journal Nature, Gerardo Chowell, a mathematical epidemiologist at Georgia State University in Atlanta, said the result was in the “right ballpark.”

But Chowell said the model might have overestimated the transmission and number of infections with mild or no symptoms because the model assumed that everyone in the community had the same opportunity to be in contact with anyone else, though in reality they might only have been in contact with a small number of families and friends.

China’s public data of confirmed cases does not reflect asymptomatic cases. These cases were tracked but kept under a separate category, according to state guidelines on control and surveillance published on March 7.

Wu Zunyou, an epidemiologist at the Chinese Center for Disease Control and Prevention, said on Tuesday that asymptomatic cases were identified by testing close contacts of confirmed cases. These people were quarantined in China and would not pose a threat to society, he said.

State news agency Xinhua reported on Sunday that there was a new positive case in Wuhan, but the patient was not counted as a confirmed case because he was asymptomatic and his lung scan was normal.

However, there was another new positive case in the city that had been classified as a confirmed case because the patient, although not showing symptoms, had an abnormal lung scan.

The South China Morning Post reported earlier that, according to classified government data, over one-third of laboratory-confirmed cases in China were silent carriers who were either asymptomatic or presymptomatic.

This story originally appeared on Inkstone, a daily multimedia digest of China-focused news and features.

Copyright (c) 2020. South China Morning Post Publishers Ltd. All rights reserved.

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(Provided by Associated Press)