Three-fifths of new coronavirus cases in China showed no symptoms of the illness when they were diagnosed, according to data that are likely to complicate moves by governments around the world to lift strict lockdown measures.
A Financial Times analysis of the more complete data into Covid-19 that the Chinese government began publishing at the start of April found that 60 per cent of confirmed cases recorded over the past month were non-symptomatic at time of testing.
The prevalence of non-symptomatic cases will be a concern for the authorities not just in China but around the world as they seek to reopen their countries after months of lockdowns. It suggests that large numbers of people are likely to be out in the community spreading the virus without knowing it.
This is a particular worry as many public health experts believe that carriers are at their most infectious in the pre-symptomatic stage of the illness.
A lack of testing capacity globally means it is extremely difficult to know how many virus carriers are non-symptomatic. But what evidence there is suggests it is a substantial proportion.
In the Italian town of Vò, which tested all of its 3,300 inhabitants, about half of those initially found to be positive had no symptoms. In Iceland, health authorities found that 43 per cent of those infected showed no symptoms at the time of testing. Research by South Korea’s Centers for Disease Control and Prevention into two particulate clusters in the country concluded that the ratio of non-symptomatic people was 30 per cent to 36 per cent.
Beijing has made great fanfare of its success in combating the spread of the virus and bringing down the new case-count that peaked at more than 5,000 a day in February. It claims to have effectively ended the local spread of the virus, and has been reporting fewer than 100 new symptomatic cases a day for the past two weeks.
Most of the new symptomatic cases are imported by people bringing it into the country from abroad and so are in two-week minimum quarantines — although Chinese media have criticised some leaky quarantine procedures.
The local cases are of greater concern, since they pose a bigger risk of spreading a second wave of infection as the lockdown measures are lifted. While the outbreak centre of Wuhan has had no new symptomatic cases for almost a month, about 20 new non-symptomatic carriers have been detected every day through mass testing.
China’s data on non-symptomatic cases are a feature of its approach to testing and tracing contacts of confirmed cases, meaning that the system can catch victims who otherwise would be unlikely to visit a hospital for testing. South Korea, too, has turned up non-symptomatic cases through testing infection clusters.
Zhong Nanshan, the Chinese government’s de facto medical spokesperson, has sought to reassure citizens about the non-symptomatic cases while reminding them to be vigilant. “I don’t believe that non-symptomatic cases are a large risk,” he told state media this month. Yet only a few days earlier, the National Health Commission warned that these cases were “contagious and carry the risk of spreading the disease”.
China’s biggest cluster of recent cases, in the north-eastern town of Harbin, includes non-symptomatic carriers in the transmission chain. According to the authorities, a returnee from the US caused the infections of at least 71 people, including non-symptomatic spreaders.
“There is very good evidence now that maximal shedding of [the virus] is from the pre-symptomatic phase of the infection. There is also strong mathematical modelling evidence that 40-80 per cent of transmission is from a- or pre-symptomatic individuals,” said Babak Javid, professor of medicine at Tsinghua University in Beijing. “As such, pre-symptomatic infection very much poses a contagion risk.”
Additional reporting by Donato Paolo Mancini in London and Jung-a Song in Seoul