How often do asymptomatic people spread the coronavirus? It’s unclear

People without COVID-19 symptoms rarely transmit the virus, WHO said, but later backtracked

People wearing masks at a restaurant

The World Health Organization recently sparked confusion when it said people without COVID-19 symptoms rarely spread the virus. But there’s a lot that researchers don’t yet understand about asymptomatic spread.

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Don’t put aside your mask. People who aren’t showing symptoms can pass the coronavirus on to others, experts say, despite a comment from a top global health official that it’s rare and not what is driving the pandemic. 

Controversy over whether people who don’t have symptoms are infectious arose during a World Health Organization news conference on June 8.

“It still appears to be rare that an asymptomatic individual actually transmits [the virus] onward,” Maria Van Kerkhove, the WHO’s technical lead for the COVID-19 response, said. Public health officials should concentrate on finding and isolating people who do have symptoms in order to stop the pandemic, she said. 

Her statements about the coronavirus’s contagiousness in the absence of symptoms seemed to run counter to public health messages stressing the need for masks and social distancing to prevent people from unknowingly spreading the virus. 

Van Kerkhove walked back her comment in a news briefing the following day, saying that she had been referring to a small subset of studies that follow people who never show COVID-19 symptoms and their contacts. “We do know that some people who are asymptomatic can transmit the virus on,” she said in the June 9 briefing. “What we need to better understand is how many people in the population don’t have symptoms. And, separately, how many of those individuals go on to transmit [the virus] to others.”

The WHO’s statement that asymptomatic transmission is rare “is not backed up by any data,” says Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases in Bethesda, Md. “We know that there is asymptomatic transmission…. What we do not know is the extent to which that occurs. So when we hear statements that this is very rare, we do not know that as a fact.”

In some studies, about half or more of people tested did not have symptoms at the time they were found to carry the virus. Some went on to get ill, but it’s not clear what proportion never developed symptoms. About 20 percent of a sample of 238 sailors aboard the U.S.S. Theodore Roosevelt who tested positive for the virus remained asymptomatic, researchers report June 9 in Morbidity and Mortality Weekly Report.

U.S.S. Theodore Roosevelt
Among a sample of 238 service members aboard the U.S.S. Theodore Roosevelt who tested positive for the coronavirus, nearly 20 percent were asymptomatic, a new study reports.Alexander Williams/U.S. Navy

Studies have shown that people can be contagious before they develop COVID-19 symptoms such as a fever, loss of smell or taste, or a cough (SN: 3/13/20). But it’s still unclear at what point infected people are most likely to transmit the virus to someone else, or for how long. One study estimated that more than 40 percent of cases were transmitted in the days before symptoms appeared (SN: 4/15/20). 

There’s no debate among scientists that people are contagious for a couple of days before symptoms start, says Marm Kilpatrick, an infectious diseases researcher at the University of California, Santa Cruz. “No one doubts that,” he says.

What’s less clear is how contagious asymptomatic cases are. Because asymptomatic people don’t know they’re infected, they often aren’t identified, making it difficult to determine how often they unwittingly give the virus to others. Determining true asymptomatic infections is also difficult because people may not recall if they had sniffles or a scratchy nose one day. 

Data on how much infectious virus is present in the noses of asymptomatic people are contradictory, Kilpatrick says. Some studies suggest that asymptomatic individuals have as much virus as people who have symptoms, while other studies have found less virus in people who don’t develop symptoms than those who do. 

More data are needed before WHO or anyone else can declare asymptomatic transmission rare. 

 “That’s why this is a disaster public relations–wise,” Kilpatrick says. Media coverage of the statement may have led people to conclude, “I don’t have symptoms, I don’t have to wear a mask…. We’ve been trying to get 7 billion people on the planet to wear masks even though they [feel] fine, so to misinterpret [data] like that is super counterproductive.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

Erin I. Garcia de Jesus is a staff writer at Science News. She holds a Ph.D. in microbiology from the University of Washington and a master’s in science communication from the University of California, Santa Cruz.

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