Explanation – Why are almost all COVID infections in Shanghai showing no symptoms?

Written by David Stanway

SHANGHAI (Reuters) – Epidemiologists examining the largest outbreak of COVID-19 in China in two years are trying to ascertain why the proportion of asymptomatic cases is rising and what that might mean for China’s future containment strategy.

The number of new confirmed community-transmitted cases in Shanghai’s main financial hub reached 4,477 on Tuesday, a record high, but only 2.1% showed symptoms. The proportion of asymptomatic cases during the previous seven days was about 1.6%.

Although outbreaks abroad showed that Omicron was less deadly than its predecessors, with low levels of hospitalization, the symptomatic incidence rate was relatively high compared to China’s figures.

In Britain, estimates for the share of asymptomatic omicron infections ranged between 25% and 54%, government data show, although testing was not systematic.

Britain has also been ahead of the curve in lifting all restrictions, as it and other countries adjust to the policy of living with COVID while the Chinese government has remained cautious and international travel remains restricted.

The country’s lack of asymptomatic infections and the very low number of deaths – only two cases linked to COVID this year – have raised hopes that China can achieve a “soft landing” when it relaxes “dynamic clearance” restrictions as it refers to a policy of mandatory lockdowns and testing.

Here are some explanations for why the rate of asymptomatic cases is high.

Monitoring test

China is also the only major country to conduct untargeted mass surveillance tests, which are bound to detect more asymptomatic cases, although more asymptomatic cases are also expected to be detected.

“Higher levels of testing are sure to capture more asymptomatic cases rather than less,” said Adrian Easterman, a biostatistician at the University of South Australia.

In other countries, many people who have tested positive with household items do not report this, and official data also shows that the decline in infections outside China has coincided with a decrease in the number of tests performed.

The story continues

On Monday alone, Shanghai conducted more than 8 million tests at more than 60,000 stations across its closed areas. Other countries, even if they still have mandatory testing programmes, are now taking a more targeted approach.

Less ventilation, higher immunization

China’s stringent response to the new variable was in part the result of uncertainty about levels of immunity and resistance among the population after nearly two years of heavy containment.

But writing on the Twitter-like Weibo platform last week, Shanghai COVID expert Zhang Wenhong said that while the new Omicron variant was difficult to weed out, it was clearly less “scary” than its predecessors.

Chinese experts, including Zhang Boli, who advises the government on the treatment of COVID-19, said that the inherently low pathogenicity of omicron, combined with the country’s relatively high vaccination rates, could lower the number of asymptomatic infections.

However, vaccination levels in South Korea and Singapore are higher than in China, and they have more symptomatic cases.

Catch it early

Zhang also said in an interview with China’s Science and Technology Daily on Tuesday that the large proportion of asymptomatic infections is not necessarily a feature of the virus itself.

The high rate may be a result of early detection in China, allowing authorities to detect and isolate cases before they develop symptoms, and large numbers of people can still be infected with the disease.

“No symptoms” is not an established case, Wu Zunyu, the chief epidemiologist with the Chinese Center for Disease Control, said at a press conference on Saturday. People can start getting sick within days, he said, and the rate of infection still needs to be taken into account.

common infection

It’s also possible that many of the symptoms that are picked up in external cases are due to “co-infection,” with virulent strains of the common cold often manifesting in ways similar to COVID-19.

The shutdowns abroad have led to a marked decrease in other infectious diseases, including influenza, the researchers said. With much of the world now learning to “live” with COVID, there was also a chance for the old viruses to make a comeback.

(Reporting by David Stanway; Editing by Barbara Lewis)