Why the WHO won't say the coronavirus is airborne and driving the pandemic

The World Health Organization has refused to cave to pressure from more than 200 experts calling for it to update its messaging on the threat of the spread of the coronavirus through the air, citing a lack of “definitive” evidence.

Lack of 'definitive' evidence on the threat of the coronavirus through the air divides experts

The WHO's technical lead on the pandemic, Maria Van Kerkhove, said the agency's guidelines on airborne transmission are primarily focused on hospitals. Some experts say that statement doesn't go far enough to inform the public about the potential threat of the virus through the air. (Denis Balibouse/Reuters)

The World Health Organization has refused to cave to pressure from more than 200 experts calling for it to update its messaging on the threat of the spread of the coronavirus through the air, citing a lack of "definitive" evidence. 

In an open letter first covered by The New York Times on Saturday, 239 scientists from 32 countries called on the United Nations agency to acknowledge that airborne transmission of the coronavirus is a potential driver of the pandemic. 

But the WHO stopped short of revising its messaging Tuesday. 

"These are fields of research that are really growing and for which there is some evidence emerging but is not definitive," Dr. Benedetta Allegranzi, WHO's technical lead for infection prevention and control, said during a briefing in Geneva Tuesday. 

"The possibility of airborne transmission in public settings — especially in very specific conditions: crowded, closed, poorly ventilated settings that have been described — cannot be ruled out. However, the evidence needs to be gathered and interpreted." 

How big of a threat is the coronavirus through the air? 

It's widely accepted that COVID-19 spreads from both symptomatic and asymptomatic carriers through respiratory droplets, although the WHO previously backtracked on its messaging around the significance of those without symptoms. 

What the group of international scientists is drawing attention to is the role that smaller, microscopic droplets could play in spreading virus particles when people are talking, singing or breathing. 

Studies of so-called superspreading events or locations, such as a choir practice in Washington state, a call centre in South Korea and a restaurant in China have supported the conclusion that some degree of transmission is occurring through the air, and experts say it should not be discounted.

"The risk of ignoring airborne transmission is that the disease will continue spreading rapidly as we've seen," said Linsey Marr, an expert in the transmission of viruses by aerosol at Virginia Polytechnic Institute and State University, known as Virginia Tech, in Blacksburg, Va., and a signatory of the letter. 

But the exact extent to which it plays a role in the spread of COVID-19 is still unclear. 

"We just don't know," Marr told CBC News. "It seems clear that all of these routes could be happening, and given the scale of the pandemic, I think it's wise for us to do as much as we can to slow down or interrupt all of these different routes." 

She said people need to place more emphasis on the public health measures we're already taking in order to stop the potential spread of airborne transmission.

That includes adhering to physical distancing, wearing a mask when necessary, increasing ventilation indoors and moving activities outdoors whenever possible in order to prevent airborne particles from building up.

Dr. Maria Van Kerkhove, WHO's technical lead on the pandemic, said the agency would be releasing a scientific brief in the coming days that will outline its position on all different modes of transmission — including airborne, droplets, surfaces and fecal-to-oral.

WATCH | WHO experts on airborne transmission:

WHO says evidence of airborne coronavirus transmission not definitive

2 years ago
Duration 3:34
While discussing potential airborne transmission of the coronavirus, the World Health Organization detailed its systematic science- and evidence-based approach to reach its conclusions. 

"We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission of COVID-19," she said during the press conference Tuesday.

The WHO's guidelines on airborne transmission are primarily focused on hospitals, she said.

"But we're also looking at the possible role of airborne transmission in other settings, particularly close settings where you have poor ventilation." 

That statement doesn't go far enough for the experts behind the letter, who went public because, they say, they felt there is enough evidence for the WHO to change its messaging to better inform the public about the potential threat of the virus through the air.

"We were frustrated that they were very dismissive of the evidence," said Jose Jimenez, a professor of chemistry at the University of Colorado specializing in aerosol science who also signed the letter. 

"They don't really have really certain evidence about any of the modes of transmission, whether it goes through contacts, through objects or through droplets — there is no more evidence for those sources of transmission than there is for aerosol." 

'No new data' to make conclusive decision

But that level of uncertainty over how big a role airborne transmission plays has also led some infectious disease experts to question the push to label it a significant threat before all the research is in, backing up the WHO's current position.

"It's creating a false sense of alarm, and it doesn't contribute to our understanding or the management of this infection," said Dr. Isaac Bogoch, an infectious disease physician at Toronto General Hospital.

"Now, could there be some airborne transmission? Maybe a little bit, but I think it's pretty safe to say that the vast majority of transmission falls toward the droplet end of the spectrum." 

Aerosol expert Linsey Marr said people need to place more emphasis on the public health measures we're already taking in order to stop the potential spread of airborne transmission. (Evan Mitsui/CBC)

Bogoch said the letter and subsequent article in the New York Times fractured the scientific community and caused a stir with the public over concerns whether enough was being done to address the threat of airborne transmission — but in reality, it's nothing new. 

"This concept keeps coming up. This issue has arisen in January, and it sort of rears its head from time to time," he said.

No new research has arisen that should lead to a definitive answer one way or the other, he said.

"There's no new data. There's no new information. There's just a letter and some angry headlines." 

B.C.'s provincial health officer, Dr. Bonnie Henry, said the controversy has been overblown.

"I actually think it's a little bit of a tempest in a teapot in that we all agree on the extremes and we're fussing a little bit about how much we need to focus on the bits in the middle," she said in her COVID-19 briefing Monday.

"It is important to continue to look at the data, to look at where we're seeing transmission events and adapt if we need to and put in additional measures." 

Epidemiologist Ashleigh Tuite, an assistant professor in the University of Toronto's Dalla Lana School of Public Health, said if the coronavirus spread significantly through the air, we'd know it. 

"If this was primarily aerosol based, we would have had a much harder time controlling this," she said. 

"Given the success that we've had with controlling it, it really does seem like we don't need to be overly worried about the role of aerosols in terms of spread." 

WATCH | Respirologist on risk of airborne transmission of COVID-19

Respirologist on risk of airborne transmission of COVID-19

2 years ago
Duration 5:41
Dr. Samir Gupta says there is not yet enough evidence to show significant spread of the coronavirus through airborne transmission.

Jimenez acknowledged that the threat of airborne transmission isn't on par with a disease like measles, which is highly contagious through the air but said the WHO should go beyond their current messaging. 

"They're in a very difficult position, right? I mean, they are a very important organization, and they are being asked to do a huge job with limited resources," he said. 

"The important thing is, we're trying to nudge them to change. If we didn't think they were very important and their opinion matters and their guidance was valuable, we wouldn't be bothering with trying to convince them."


Adam Miller

Senior Writer

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

With files from Lauren Pelley and Earvin Solitario

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