By David Stanway
SHANGHAI – Epidemiologists studying the largest Chinese outbreak of COVID-19 in two years are trying to understand why the proportion of asymptomatic cases is so high and what it could mean for China’s future containment strategy.
The number of newly confirmed community-transmitted cases in major financial hub Shanghai hit 4,477 on Tuesday, a record high, but only 2.1% showed symptoms. The proportion of symptomatic cases in the previous seven days was about 1.6%.
Although outbreaks overseas have shown that Omicron was less deadly than its predecessors, the rate of symptomatic infections was relatively high compared to China and the number of hospital admissions was lower.
In the UK, estimates for the proportion of asymptomatic omicron infections range from 25% to 54%, government data show, although testing has not been carried out systematically.
The UK has also been ahead in lifting all restrictions as it and other countries adopt a live-with-COVID policy, while the Chinese government has remained cautious and international travel is still restricted.
The country’s lack of symptomatic infections and very low number of deaths – just two COVID-related this year – have raised hopes that China can achieve a “soft landing” if it eases “dynamic release” restrictions since it is a policy of lockdowns and compulsory testing.
Below are some explanations as to why the rate of asymptomatic cases is so high.
SURVEILLANCE TEST
China is also the only major country conducting untargeted mass surveillance tests, which will inevitably reveal more asymptomatic cases, although more symptomatic cases could also be expected to be revealed.
“Certainly, a high level of testing will reveal more, rather than fewer, asymptomatic cases,” said Adrian Esterman, an expert in biostatistics at the University of South Australia.
Elsewhere, many people who have tested positive with home kits are not reporting it, and official data also shows that a drop in infections outside of China has coincided with a drop in the number of tests being carried out.
The story goes on
On Monday alone, Shanghai conducted more than 8 million tests at over 60,000 stations in its lockdown districts. Other countries, while still imposing mandatory testing programs, are now taking a more targeted approach.
LOWER VIRULENCE, HIGHER VACCINATION
China’s uncompromising response to the new variant was in part a result of uncertainty about the level of immunity and resistance in the population after nearly two years of strict containment.
But Shanghai COVID expert Zhang Wenhong wrote on the Twitter-like Weibo platform last week that while the new Omicron variant is harder to eliminate, it’s significantly less “scary” than its predecessors.
Chinese experts, including Zhang Boli, who advises the government on treating COVID-19, said Omicron’s inherently lower pathogenicity combined with the country’s relatively high vaccination rates could reduce the number of symptomatic infections.
However, vaccination rates 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 could be due to early detection in China, which allowed authorities to spot and isolate cases before they became symptomatic, and it was still possible for large numbers of people to get sick.
Wu Zunyou, chief epidemiologist at the China Center for Disease Control, told a news conference on Saturday that “asymptomatic” is not an established condition. People could get sick in a matter of days and continued attention must be paid to the infection rate, he said.
CO-INFECTIONS
It’s also possible that many of the symptoms seen in overseas cases are caused by “co-infections,” with particularly virulent strains of cold often presenting similar to COVID-19.
Researchers said lockdowns abroad had led to a noticeable drop in other infectious diseases, including influenza. As much of the world is now learning to “coexist” with COVID, there has also been an opportunity for old viruses to make a comeback.
(Reporting by David Stanway; Editing by Barbara Lewis)