People with mild or no symptoms could be spreading COVID-19

As many as six in 10 people infected with coronavirus may be unaware they have the disease, according to a number of papers studying the outbreak.

Implications of the research are stark. And scientists are calling for urgent measures – from closing all schools to banning public gatherings – to stop mild and asymptomatic cases fuelling the pandemic.

Researchers suspect there is an undetected pool of people who have mild or no symptoms, according to an article in Nature. That is because an increasing number of infected people have not travelled to epidemic hotspots, or come into contact with people with confirmed cases of COVID-19.

This is different from unreported cases, those that are missed because authorities are not carrying out enough tests or people in the early stages of the virus who are not yet showing symptoms.

A new study suggests there were 37,400 people with the virus in Wuhan who authorities did not know about, and who had mild or no symptoms but could still be contagious.

Wu Tangchun, a public health expert at Huazhong University of Science and Technology in Wuhan, who led the study, told Nature, “By our most conservative estimate, at least 59% of the infected individuals were out and about, without being tested and potentially infecting others. This may explain why the virus spread so quickly in Hubei and is now circulating around the world.”

That research was based on the best dataset currently available on coronavirus: the clinical information from 26,000 laboratory-confirmed cases reported to the health commission of Wuhan, the epicentre of the outbreak.

It throws into question a report by a joint World Health Organization-China mission last month, which said infections by people with no symptoms were “relatively rare” and did not appear to be driving transmission of the disease.

Scott Gottlieb, former head of the US Food and Drug Administration, said on Twitter earlier this month, “It’s becoming clearer that spread of Covid-19 by people who are asymptomatic or mildly symptomatic may be responsible for more transmission than previously thought; making control of the virus more difficult.”

Separately, the South China Morning Post reports that the number of “silent carriers” – people who are infected by the new coronavirus but show delayed or no symptoms – could be as high as one-third of those who test positive. Reporters Josephine Ma, Linda Lew and Lee Jeong-ho write that 43,000 people in China had tested positive for COVID-19 without immediate symptoms by the end of February, according to classified Chinese government data.

In Iceland, which says it has tested a higher proportion of inhabitants than any other country, about half those who tested positive for COVID-19 have no symptoms.

The new research is in line with other studies. The Diamond Princess cruise ship, which had a COVID-19 outbreak in early February, had 3,711 passengers and crew on board, who were repeatedly tested and closely monitored. Around 700 of them were infected and 18% of those never showed symptoms.

One of the authors of the study, Gerardo Chowell, a mathematical epidemiologist at Georgia State University, pointed out that there were lots of elderly people on the ship, who tend to suffer more from the disease. As a result, he said asymptomatic infections in the wider population could be much higher.

A smaller study was conducted on 565 Japanese citizens evacuated from Wuhan in early February. Of those, 13 evacuees were infected; four of them, or 31%, never developed symptoms.

Now researchers are trying to determine whether people infected with COVID-19 but that show no symptoms can infect others. Preliminary findings seem to suggest they can.

Two studies showed people had high levels of the virus in throat swabs early in their illness, when their symptoms were mild, meaning the disease could easily be spread through coughs or sneezes. Another study detailed the case of an infected individual who never developed symptoms but shed a similar amount of virus to those who did.

Researchers say that furthers the case for governments to impose more restrictions on people's movements, in order to halt the progress of the disease. They want schools to be closed, public gatherings cancelled, and people to be kept at home and out of public spaces. Chowell said, “Implementing strong social-distancing measures is the only way to stop the virus from spreading.”