China, which has been praising itself for its draconian response in shutting down much of the country for most of February and thus halting the spread of the coronavirus pandemic domestically (at least if one ignores the glaring discrepancies from reports on the ground), may have a new problem on its hands.

According to the South China Morning Post, which cited classified data from the Chinese government, as many as a third of the people who test positive for the coronavirus - also known as "silent carriers" - may show delayed symptoms or none at all.

The data show that 43,000 people in China tested positive by the end of February, but had no actual symptoms, the newspaper reported. They were quarantined and monitored, though as we noted last month, in a radical change in how China is underreporting the severity of the disease, asymptomatic cases aren’t included in China’s tally of those infected with the virus.

“The number of novel coronavirus cases worldwide continues to grow, and the gap between reports from China and statistical estimates of incidence based on cases diagnosed outside China indicates that a substantial number of cases are underdiagnosed,” a group of Japanese experts led by Hiroshi Nishiura, an epidemiologist at Hokkaido University, wrote in a letter to the International Journal of Infectious Diseases in February. Based on his research, Nishiura put the proportion of asymptomatic Japanese patients evacuated from Wuhan, ground zero of the outbreak in China, at 30.8 per cent – similar to the classified Chinese government data.

Another useful point of reference is the data collected from the Diamond Princess cruise ship, which was quarantined for weeks in Yokohama, Japan. All of its passengers and crew were tested, with 712 people testing positive – 334 of whom were asymptomatic, according to official Japanese figures.

In Hong Kong, 16 of the 138 confirmed cases as of March 14 were asymptomatic or presymptomatic, according to Ho Pak-leung, a professor with the microbiology department of the University of Hong Kong. All of these numbers point to a significantly higher ratio of asymptomatic cases than indicated by data publicly released by China so far. There were 889 asymptomatic patients among the 44,672 confirmed cases as of February 11, epidemiologists from the Chinese Centre for Disease Control and Prevention wrote in a paper published online in JAMA Network Open on February 24.

A separate study by scientists from the University of Texas at Austin estimated that people who had not yet developed symptoms transmitted around 10 per cent of the 450 cases they studied in 93 Chinese cities. Their findings are awaiting publication in the journal Emerging Infectious Diseases.

Ho from the University of Hong Kong said some asymptomatic patients had a viral load similar to those with symptoms.

“Of course it is hard to say if they may be less infectious if they don’t cough. But there are also droplets when you speak,” he said, referring to how the respiratory virus is transmitted.

As Bloomberg notes, the novel virus has been described as “insidious” because many infected people are well enough to go about their daily business, unwittingly spreading it to others as symptoms begin to appear on average five to six days after infection. While scientists have been unable to agree on what role asymptomatic transmission plays in spreading the disease, they do know that the virus appears to cause a mild illness lasting about two weeks in children, adolescents and younger adults in most cases, and potentially more severe disease lasting three to six weeks in older people.

The bottom line is that the propagation of "silent carriers" and high rate of asymptomatic cases can complicate efforts to stop the spread of the disease which has infected more than 280,000 people and killed nearly 13,000 globally, because many countries aren’t testing people unless they’re seriously ill, the newspaper reported. It also means that with thousands of asymptomatic carriers roaming around the population, confident they are free of the disease, secondary and tertiary break outs are virtually guaranteed even for populations which, like China, claim to have defeated the pandemic.