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This week, the matter of asymptomatic transmission of COVID-19 has caused much confusion — and sparked a lively debate on Twitter.

It started on Monday when the World Health Organization discussed the current understanding of asymptomatic transmission at a press conference.

"Asymptomatic" refers to people who are infected by the novel coronavirus but never develop any symptoms.

"From the data we have it still seems to be rare that an asymptomatic actually transmits onward to a secondary individual," said Maria Van Kerkhove, technical lead for WHO's health emergencies program. In other words, it seems unlikely that people who infected by the virus but don't develop symptoms are spreading the virus to others.

 and led to concern among researchers that WHO was confusing the public.

 has engendered considerable confusion today (WITHOUT DATA) about people without symptoms not transmitting

 Dr. Eric Topol, a scientist at Scripps Research — a view representative of the research community Monday.

 to clarify their comments. "I was responding to a question at the press conference. I wasn't stating a policy of WHO or anything like that," Van Kerkhove said. "I think that's a misunderstanding to state that asymptomatic transmission globally is very rare."

According to Van Kerkhove, what she meant to convey is that she has not seen evidence indicating that transmission from asymptomatic individuals is widespread. "What we need to better understand is, how many of the people in the population don't have symptoms? And separately, how many of those individuals go on to transmit to others?"

Here's what we currently know and do not know about asymptomatic transmission:

1) How many people who get infected with the virus never develop symptoms?

Nobody is certain. "Estimates suggest that anywhere between 6 percent and 41 percent of the population may be infected but not have symptoms," said Van Kerkhove on Tuesday. Some research, such as a May write-up in the journal 

 on which 81 percent of people who tested positive showed no symptoms for journey's 21-day duration, suggests the prevalence could be even higher. Asymptomatic cases seem to be more common among young people and those without 

 that could make them more vulnerable to the virus, such as diabetes or severe asthma.

The existing data is spotty because the so-called "silent spreaders" are a difficult population to find and study — people usually don't get tested unless they think they may have it.

Asymptomatic cases of COVID-19 were first discovered by testing family members who lived with known patients, Van Kerkhove said.

"Now," she said, "asymptomatic individuals are being found in outbreaks"  in closed communities such as 

 with active infections. At such locations, health authorities have conducted mass testing to understand the extent to which the virus has spread and to help bring it under control.

One issue with pinpointing asymptomatic individuals is that the category relies on self-reporting, says 

, an epidemiologist at Kent State University. "You're relying on people to be in tune enough with their bodies that they would know if it felt any different," she says. Having no symptoms can be in the eye of the beholder. Someone without a fever or cough might still feel tired — and may not consider 'feeling tired' to be a reportable symptom.

Or they might not be aware that food is tasting different or that smells have diminished.

Van Kerkhove said that, on follow up, some reportedly asymptomatic patients describe feeling "a little bit unwell, a little bit under the weather," which would indicate that they actually have mild disease. And in some cases, people who report feeling fine and being asymptomatic show signs of 

Identifying asymptomatic people also requires follow-up, says Smith. People testing positive without symptoms could be "presymptomatic," meaning they will develop symptoms in coming days. Research shows that presymptomatic individuals can test positive for the virus 

2) To what degree do people who never develop symptoms contribute to transmission?

Documented cases of onward transmission from patients who never develop symptoms are "rare," Van Kerkhove said. Still, it 

, and the research community is divided on whether the few published cases of an asymptomatic individual infecting others represent anomalies or serve as signs of a more common occurrence.

 from Scripps Research that was published in A

 suggests that asymptomatic cases may be responsible for 30-45 percent of coronavirus transmission, based on studies in Iceland, Italy and Indiana.

Then there's the question of how an asymptomatic case could spread the virus.

Viral particles are exhaled when an infected person coughs or sneezes — but if you're asymptomatic, you're not showing these symptoms.

 that people who are infected but feel fine can have the same amount of virus in their throats as people who are feeling unwell.

For an asymptomatic person, "the question is how does the virus get from being inside your nose or inside your [throat] to being on somebody else?" said Michael Ryan, director of WHO's health emergencies program.

In the absence of sneezing or coughing, Ryan said, spreading the coronavirus seems to require situations where people are in close proximity and projecting their voice and breath at others, such as 

 — activities that have all been implicated in COVID-19 transmission based on reports.

 on mask use: The degree of spread depends on "the amount of viable virus being shed by a person, whether or not they are coughing and expelling more droplets, the type of contact they have with others, and what [infection prevention and control] measures are in place."

3) What are the implications of asymptomatic transmission?

People who are asymptomatic fit into the larger category of undetected cases that are potentially infectious. 

 also includes people who are presymptomatic and will go on to develop symptoms, and those that are mildly ill but don't realize it.

The fact that many people can spread the new coronavirus without knowing they're infected is a key difference between COVID-19 and other diseases, such as SARS, a disease caused by a coronavirus that swept through Asia 

With SARS, "people didn't tend to be infectious as early in the course of disease because the virus was in their lower respiratory airway," Ryan said, so it took actions like coughing and sneezing to expel the virus. The new coronavirus infects both the lower and upper airways, which makes it much easier to spread through activities like shouting and singing.

"It makes [the new coronavirus] much harder to contain," says 

, a virologist at Hong Kong University, "You cannot wait for people to develop symptoms and isolate them. By the time a person is symptomatic or sick or knows she or he is not well, they [may] have already transmitted to many, many people."

Individual behaviors are key to protecting yourself and others, says 

, an infectious disease modeler at Georgetown University. "We actually have some pretty decent tools that are at our disposal," she says, including hand-washing, mask-wearing and keeping a distance from others. "Those tools do actually work, regardless of whether you're thinking about asymptomatic or symptomatic transmission."  

Health

COVID-19

Pien Huang

Author

That's the word some doctors used for a World Health Organization statement. At issue: 1) How many people are infected with COVID-19 but show no symptoms and 2) Can asymptomatic people infect others?

WHO creates 'confusion' about asymptomatic spread. Here's what we know

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