Explainer: Why are Shanghai's COVID infections nearly all asymptomatic?

Explainer: Why are Shanghai's COVID infections nearly all asymptomatic? Epidemiologists examining the biggest Chinese outbreak of COVID-19 in two years are trying to ascertain why the proportion of asymptomatic cases is so high, and what it could mean for China's future containment strategy. David Stanway Reuters

SHANGHAI, March 29 (Reuters) - Epidemiologists examining the biggest Chinese outbreak of COVID-19 in two years are trying to ascertain why the proportion of asymptomatic cases is so high, and what it could mean for China's future containment strategy.

The number of new confirmed community transmitted cases in the major financial hub of Shanghai reached 4,477 on Tuesday, a record high, but only 2.1% showed symptoms. The share of symptomatic cases over the previous seven days was around 1.6%.

Although outbreaks overseas have demonstrated that Omicron was less deadly than its predecessors, with lower levels of hospitalisation, the rate of symptomatic infection was relatively high compared to China's numbers. ...

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

SURVEILLANCE TESTING

China is also the only major country to do mass, untargeted surveillance testing, which is bound to uncover more asymptomatic cases, although it could also be expected to reveal more symptomatic cases.

"Surely, high levels of testing will pick up more rather than less asymptomatic cases," said Adrian Esterman, an expert in biostatistics at the University of South Australia. ...

LOWER VIRULENCE, HIGHER VACCINATION

China's uncompromising response to the new variant was partly a result of uncertainty about levels of immunity and resistance among the population after nearly two years of heavy containment. ...

The high rate could be a result of early detection in China, allowing authorities to catch and isolate cases before they became symptomatic, and it was still possible that large numbers of people could get ill.

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