Symptom free? You could still be seeding new coronavirus outbreaks

As many as six out of ten cases of coronavirus may be caused by people who suffer mild or no symptoms at all, according to new research.

And those invisible infections may partly explain the rapid spread of the virus. While they don’t show up in the health system or in the official reports, they are key to halting the COVID-19 pandemic.

Scientists have suspected for some time that there is a pool of undiagnosed cases playing a role, especially among people who have not visited any disease hotspots. And this is backed up by some of the first detailed estimates of how the virus may be spreading, according to studies outlined in the journal Nature.

With the pandemic spreading around the globe, the World Health Organization has stepped up it’s rhetoric, calling for a mix of aggressive measures. It has urged nations to “test, test, test” instead of ignoring cases that don’t need urgent attention.

Iceland is doing just that, testing every one of its citizens and while a small proportion have tested positive so far, half of them showed no symptoms.

Even so, many nations are not equipped to conduct mass screenings and with resources stretched testing is often reserved for patients with obvious symptoms. In many countries people with mild symptoms aren’t required to inform the authorities.

The latest studies suggest that people who are infected without showing symptoms may outnumber those officially confirmed as suffering from coronavirus, even in areas where testing has been extensive.

Silently infected

A study using data from Hubei, the province of China that was the epicentre of the original outbreak, estimated that there were 37,400 asymptomatic cases in addition to the 26,000 confirmed cases in early March.

“By our most conservative estimate, at least 59% of the infected individuals were out and about, without being tested and potentially infecting others,” says Wu Tangchun, a public-health expert at Huazhong University of Science and Technology in Wuhan, who led the study.

“This may explain why the virus spread so quickly in Hubei and is now circulating around the world.” Another study of 700 infected schoolchildren in China found that 58% had mild or no symptoms.

Research published in the New England Journal of Medicine traced COVID-19 infections which resulted from a business meeting in Germany attended by someone infected but who showed no symptoms at the time. Four people were ultimately infected from that single contact.

Meanwhile, a study of Japanese evacuees from Wuhan found that a third of those infected with coronavirus did not develop symptoms. Similarly, when passengers from the quarantined cruise ship Diamond Princess were tested, 18% of those infected had no symptoms.

Other research has shown people are most infectious during the early stages of infection. “Some people can be highly contagious when they have mild or no symptoms”, says Michael Osterholm, director of Minnesota University’s Center for Infectious Diseases.

Stay at home

Researchers quoted in Nature say urgent action is needed across the world to slow the rate of infection, calling for the closure of all schools, the cancellation of public gatherings and generally keeping people at home and out of public spaces.

“Implementing strong social-distancing measures is the only way to stop the virus from spreading,” Professor Gerardo Chowell, a mathematical epidemiologist at Georgia State University in Atlanta, told Nature.

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