China’s Data on Symptom-Free Cases Shows Most Never Get Sick

(Bloomberg) — Two-thirds of people who test positive for the novel coronavirus without signs of being sick remain symptom-free throughout infection, according to a breakdown (link in Chinese) published by the Chinese government.

Among 6,764 people without symptoms who tested positive for Covid-19, only one-fifth — 1,297 — later developed symptoms and were reclassified as confirmed cases, China’s National Health Commission spokesman Mi Feng said Wednesday at a briefing in Beijing.

Still being monitored are 1,023 people in medical quarantine. The rest — 4,444 — have been discharged from medical observation after recovering.

The phenomenon of asymptomatic transmission is a puzzling feature of the virus that’s allowed the pandemic to spread faster and more widely than previous outbreaks. While researchers earlier thought that most patients ultimately end up developing symptoms, China’s data showing that a significant group remains symptom-free throughout infection illuminates the challenge of containing the raging pandemic.

Researchers are still struggling to understand asymptomatic cases. There’s a possibility that patients who appear to be symptom-free are actually just manifesting symptoms that doctors don’t yet know to look for. For months, a fever and dry cough were understood to be the disease’s main markers. It’s only recently emerged that a loss of smell and taste is also a sign of infection. China has not disclosed the range of symptoms it looks for.

China continues to detect asymptomatic infections even after new confirmed cases dropped to zero for the first time in March. The virus has officially sickened 82,000 and killed more than 3,000 in the country.

Ten provinces and cities in China are conducting surveys so researchers can learn more about asymptomatic cases and antibodies in people with coronavirus infections, the Hubei Daily reported. The survey in Wuhan, where the disease emerged in December, will cover 11,000 people randomly selected from 100 neighborhoods.

The number of asymptomatic infections is likely to be higher than the 6,764 China has detected. These cases were found through efforts to test the contacts of confirmed patients. Otherwise, those who show no signs of being sick have no reason to seek out testing.

Once found, asymptomatic patients are placed under quarantine for monitoring and discharged only when they no longer test positive for the virus. Those who develop symptoms during the two-week quarantine will be reclassified as confirmed cases under China’s counting method and treated in a hospital.