WHO backtracks on claim that asymptomatic spread of COVID-19 is 'very rare'

A top official with the World Health Organization has walked back statements that the spread of COVID-19 from people who do not show symptoms is “very rare,” amid backlash from experts who have questioned the claim. 

Experts say research on extent of asymptomatic spread of COVID-19 still emerging

Maria Van Kerkhove, the COVID-19 technical lead at WHO, said on Tuesday that scientists need to learn much more about asymptomatic transmission, which the WHO defines as a person who doesn't have symptoms and does not go on to develop them. (Denis Balibouse/Reuters)

A top official with the World Health Organization has walked back statements that the spread of COVID-19 from people who do not show symptoms is "very rare," amid backlash from experts who have questioned the claim due to a lack of data. 

Maria van Kerkhove, an infectious disease epidemiologist and the COVID-19 technical lead for the WHO, said Monday that the available data from published research and member countries had shown asymptomatic cases were not a significant driver for the spread of the virus. 

"From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual," she said at a media briefing in Geneva on Monday. "It's very rare."

On Tuesday, Van Kerkhove aimed to clear up "misunderstandings" about those statements in an updated briefing, stressing that she was referring to "very few studies" that tried to follow asymptomatic carriers of the virus over time to see how many additional people were infected. 

"I was responding to a question at the press conference, I wasn't stating a policy of WHO," she said. "I was just trying to articulate what we know." 

Van Kerkhove said she didn't intend to imply that asymptomatic transmission of the virus globally was "very rare," but rather that the available data based on modelling studies and member countries had not been able to provide a clear enough picture on the amount of asymptomatic transmission.

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"That's a big, open question," she said. "But we do know that some people who are asymptomatic, some people who don't have symptoms, can transmit the virus on."

WHO did 'questionable job' of communicating

Dr. Isaac Bogoch, an infectious disease physician at Toronto General Hospital, said there has been confusion over the evolving science on the amount of asymptomatic transmission since the start of the pandemic.

"At a fundamental level it's extremely important to explain the science well and explain what our current knowledge is and also explain what the unknown questions are," he said.

"I don't think the WHO did a very good job of that yesterday and they did a questionable job of that today when they were trying to clarify their comments." 

Bogoch said there are key discrepancies between people who have no symptoms, those who are presymptomatic and those who are subclinical with less severe symptoms, which can make studying the true amount of asymptomatic carriers of COVID-19 extremely challenging. 

"We still don't understand the role of people who have no symptoms the entire time [they're infected] versus people that have very, very mild symptoms that are misclassified as having no symptoms, versus people that have no symptoms for the first few days and then go on to develop them," he said. 

"So, when we heard the WHO say that people without symptoms rarely transmit this infection, an eyebrow went up, because we certainly know that there are different types of people without symptoms and it's a little more complicated than what they had reported."

Some experts say it is not uncommon for infected people to show no symptoms, but data is sparse on how likely such people are to transmit the disease. (Evan Mitsui/CBC)

Timothy Caulfield, a Canada Research Chair in health law and policy at the University of Alberta who studies health misinformation, said it was notable how quickly van Kerkhove's statement was criticized and shows how desperate people are for clear information on asymptomatic carriers of COVID-19. 

"It demonstrates how incredibly important it is to take great care in how you're communicating science that is hyper-relevant to the public right now," Caulfield said, adding we still don't have a clear picture from the research community on how much asymptomatic spread is actually occurring. "The science is very uncertain and it's evolving rapidly."

He said van Kerkhove didn't appear to have the intention of making a definitive statement on behalf of the WHO in her comments on Monday, and instead was speculating about her interpretation of the emerging research on the topic. 

"This is exactly the World Health Organization's job, right? This is why they exist: to lead the world in these moments," he said. "So when it's less than ideal, it's not surprising that people are critical. We need [WHO] to be a trusted voice." 

Data on asymptomatic spread 'flawed'

Some experts say it is not uncommon for infected people to show no symptoms.

A non-peer-reviewed study from Germany in May based on 919 people in the district of Heinsberg — which had among the highest death tolls from COVID-19 in Germany — found that about one in five of those infected were symptomless.

But data is sparse on how likely such people are to transmit the disease.

The co-head of Singapore's coronavirus task force told Reuters on Monday there had been asymptomatic transmission cases there, between people living in close quarters.

China said last week that 300 symptomless COVID-19 carriers in the central city of Wuhan, the pandemic's epicentre, had not been found to be infectious.

Dr. Lynora Saxinger, an infectious disease physician at the University of Alberta who has been reviewing the available literature on the topic, said studying asymptomatic transmission is extremely difficult. 

WATCH | How many asymptomatic people could there be?

COVID-19: How many asymptomatic people could be walking around?

4 years ago
Duration 4:20
Doctors answer your questions about the coronavirus, including if there’s any way of knowing how many asymptomatic people could be walking around when only symptomatic people are being tested. 

Van Kerkhove was referencing modelling data that estimated anywhere between six per cent and 41 per cent of the population may be infected but not have symptoms.

Saxinger said that modelling data is "flawed" because it makes assumptions about how many people are asymptomatic and then runs a simulation on how many people could then transmit it.

"There's a big question mark at the actual data in real-world observations with asymptomatic [carriers]," Saxinger said.

"Asymptomatic spread is a dumpster fire in terms of data."


Adam Miller

Senior Health Writer

Adam Miller is a senior health writer with CBC News. He's covered health and medical science news extensively in Canada for over a decade, in addition to several years reporting on crime, politics and current affairs throughout Asia.

With files from Reuters, Christine Birak and Marcy Cuttler

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