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

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.

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. 

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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.”