Half of people with coronavirus have no symptoms, data shows

About half of people who have the coronavirus show no symptoms, according to data from Iceland, the Italian town of Vo, and the Diamond Princess cruise ship.

The data from those places is of particular interest to researchers because in those places even people without symptoms were tested – which is not the case in the U.S. and most countries, where desperately needed tests are reserved mostly for people who show symptoms.

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Experts say the discovery has critical implications and shows that even more widespread testing may be needed to identify all carriers.

The Centers for Disease Control and Prevention (CDC) is also expected to recommend face coverings in virus hot-spots soon because that may help prevent spread from people who show no symptoms.

Here’s some of the data that experts are looking at.

Cruise ship

All 3,711 passengers on the Diamond Princess cruise ship were tested, 712 tested positive, and of those, 331 (46 percent) have never shown outward symptoms, according to Japan’s health agency, which has also been cited by the CDC.

“Many cases are apparently asymptomatic. This is both good and bad news, because it means the virus lethality may be lower than initially thought, but also that people can unknowingly spread the virus,” Patrick T. Dolan, a virologist at University of California, San Francisco, told Fox News.

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“It is still too early to be certain of the numbers, but it is clear that asymptomatic infection is contributing significantly to the spread of SARS-CoV2. This is exactly why strict adherence to social distancing measures is critical,” he said.

On the cruise ship,  nine speople have died, with 10 still in critical condition. After adjusting for expected further deaths and for age – all passengers who died were over 70, and the average age on the ship was 58 – London researchers used the ship’s data to estimate that the death rate for a general population is around 0.6 percent.

Vo, Italy

In the small northern Italian town of Vò, where Italy’s first coronavirus death occurred, the entire population of 3,000 people was tested.

“We tested all residents of Vò... including those who did not have symptoms,” two Italian researchers wrote in the Guardian. The researchers were surprised to find that “a significant proportion of the population, about 3 percent, had already been infected – yet most of them were completely asymptomatic.”

“This allowed us to quarantine people before they showed signs of infection and stop the further spread of coronavirus [in  Vò],” the researchers noted.

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Health experts reached by Fox News all said that asymptomatic carriers are a major issue. The situation calls for “a better national policy of widespread testing, to isolate cases,” said Ali S. Khan, the dean of the College of Public Health at the University of Nebraska Medical Center.

The U.S. got a late start in testing, but is rapidly becoming a world leader in it after many regulations were waived in mid-March.


In Iceland, more than 6 percent of the entire country has been tested, by far the largest proportion in the world. Nearly half of those tests have been done as part of a “screening program," in which anyone who asks for a test can get one, even if they have mild or no symptoms.

“Of the 5,502 samples taken in the screening program between March 13-19, 50 tested positive for SARS-CoV-2... 66 percent of those individuals reported symptoms, such as coughing and body ache,” Kjartan Hreinn Njálsson, at the Icelandic Directorate of Health, told Fox News, adding that the other third had no symptoms.

Non-symptomatic people can spread the disease, but, “we know that the virus is much more likely to spread from person to person if the infected one is showing symptoms,” Njálsson said.

The good news is that a high rate of non-symptomatic cases means more people will be fine than researchers first thought. But virologists note that even accounting for that, more than a million people could die if no precautions were taken.

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“Respect social distancing and other measures like wearing a face mask of some kind, maybe even homemade, when you go out to buy food or pick up medicines,” Dr. Peter Kolchinsky, the author of The Great American Drug Deal: A New Prescription for Innovative and Affordable Medicines, told Fox News. He trained as a virologist, and is managing partner of RA Capital Management, which invests in new medicines.

“We all have to try to stay uninfected to keep the burden lower on hospitals until we get effective treatments and vaccines. When we get widespread testing, people can come back to work with others they trust to have been tested,” he said.

Maxim Lott is Executive Producer of Stossel TV and creator of ElectionBettingOdds.com. He can be reached on Twitter at @MaximLott.