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 percent showed symptoms. The share of symptomatic cases over the previous seven days was around 1.6 percent.

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.

In Britain, estimates for the share of asymptomatic Omicron infections have ranged between 25 percent and 54 percent, government data shows, although testing has not been systematic.

Britain has also been ahead in lifting all restrictions as it and other countries adapt a policy of living with Covid while the Chinese government has remained cautious and international travel is still curtailed.

The lack of symptomatic infections in the country and the very low number of deaths - only two related to Covid this year - has raised hopes that China can achieve a “soft landing” when it eases “dynamic clearance” restrictions as it refers to a policy of lockdowns and mandatory testing.

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.

In other countries, many people who test positive with home kits do not report it and official data also shows falls in infections outside China have coincided with a decline in the number of tests carried out.

On Monday alone, Shanghai conducted more than 8 million tests at over 60,000 stations throughout its locked down districts. Other countries, even if they still impose mandatory testing programmes, now take a more targeted approach.

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.

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

Chinese experts, including Zhang Boli, who advises the government on Covid-19 treatment, have said the inherently lower pathogenicity of Omicron, combining with the country’s relatively high vaccination rates, could be lowering the number of symptomatic infections.

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

Catching it early

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

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.

Wu Zunyou, chief epidemiologist of the China Center for Disease Control, told a press conference on Saturday that “asymptomatic” was not a fixed state. People could start to get ill within days and attention still needed to be paid to the infection rate, he said.

Co-infections

It is also possible that many of the symptoms that are being picked up in overseas cases are caused by “co-infections”, with particularly virulent strains of the common cold often presenting in similar ways to Covid-19.

Researchers said that lockdowns overseas led to a noticeable decline in other infectious diseases, including influenza. With much of world now learning to “coexist” with Covid, there has also been an opportunity for old viruses to make a comeback.