Canadian experts say an acknowledgment by the World Health Organization this week that airborne transmission of the coronavirus could be possible in poorly ventilated areas does not mean it’s a predominant route of transmission.
“I think the concern is that this form of the virus could theoretically be more prevalent than we initially anticipated, but that’s a theory only,” said Dr. Abdu Sharkawy, infectious diseases expert at University Health Network. “When you look at real-world experience and real-world evidence, it’s not (matched) by what we’ve seen.”
Up until now, the WHO has been steadfast in its assertion that the virus was predominantly spread when people were close enough that air droplets from one person coughing or sneezing could reach another person’s eyes, nose or mouth. Or when a person touched a surface contaminated with the virus.
The organization has said airborne transmission of SARS-CoV-2, the virus that causes COVID-19, can only occur during medical procedures that generate aerosols, such as intubation.
But in a scientific briefing released Thursday, the WHO said for the first time that under some circumstances airborne transmission may occur outside of medical environments, such as in crowded and poorly ventilated settings.
The acknowledgment by the WHO came after an open letter from Donald Milton, a professor of internal medicine and environmental health at the University of Maryland, and Lidia Morawska, a professor in the School of Earth and Atmospheric Sciences at Queensland University of Technology in Australia.
The letter, signed by more than 200 others, appealed to the medical community at large, as well as national and international bodies, to recognize the potential transmission route of the virus through inhalation of microscopic respiratory droplets, and to recommend precautions.
Its authors pointed to studies that have “demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond one to two metres from an infected individual.”
But Dr. Bonnie Henry, B.C.’s provincial health officer, called the letter “controversial” in a news conference this week, according to media reports, and said that SARS-CoV-2 spreads mainly through larger droplets transmitted when close contact with others occurs.
Henry said the virus was different from airborne viruses such as measles and smallpox, which can be spread through microdroplets that can be suspended in air for several hours and pass through ventilation systems.
Sharkawy said his concern is that the letter is arousing more alarm than is helpful.
“There’s absolutely no evidence that anybody working outside of a medical setting or high-risk setting of some kind requires the use of N95 masks for their own safety,” he said.
Although the WHO acknowledged that airborne transmission may have contributed to some outbreaks of COVID-19, it also said other factors such as droplets or contaminated surfaces may have been at fault.
“To date, transmission of SARS-CoV-2 (the virus that causes COVID-19) by this type of aerosol route has not been demonstrated,” according to the briefing, and that “much more research is needed given the possible implications of such route of transmission.”
This interview with Dr. Abdu Sharkawy, infectious diseases expert at University Health Network, has been condensed.
How are viruses that can be airborne typically transmitted? Talking? Coughing?
“That’s an excellent way to contrast what we’re seeing with COVID-19 compared to say measles, or shingles or tuberculosis. Those are aerosolizing pathogens, which means that simply talking, simply actually breathing in the same airspace as somebody else, let’s say in an enclosed room you could potentially be at risk. Those particles will be suspended in the air because they’re smaller, and they will remain suspended sometimes for hours.
“And because they’re suspended over time and can travel over greater distances, all you have to do is walk into a room two hours potentially after an infected person was in the room and you may be at risk.
“We have absolutely no evidence that COVID-19 poses the same threat to people.”
What is the difference between microdroplets and droplets?
“Microdroplets are basically somewhere between aerosols and droplets. Then there’s micro-aerosols as well. Microdroplets are small droplets, aerosols are much smaller than the micro-droplets and the micro-aerosols are even smaller than that.
“The truth is microdroplets are probably still not that much more contagious or transmissible than droplets are.”
Sharkawy said because microdroplets are smaller than droplets, they are capable of travelling further, but not by much. And they are not suspended in the air for any length of time.
What do we know about airborne transmission of COVID-19 and microdroplets?
“We don’t think that microdroplets are a major form of transmission of this virus. As simple as that … I would highlight the fact that outside of high-risk medical, dental and other health-care related settings, the risk of aerosolizing from this virus is exceedingly low. It’s just not reflected in real-world evidence.”
Sharkawy said if you’re near a person breathing heavily during a workout, or coughing, that person may release a small amount of aerosolized particulate infectious matter, but the majority of what’s being released from an infected source is still the heavier droplet variety.
For aerosols, will a cloth mask help?
“No. If we’re going to consider what theoretically will work against aerosols or not, then that’s why N95 masks are made. N95 masks are made to filter small particulate matter and to prevent them from being ingested or breathed in.”
But Sharkawy said people should be more concerned with social distancing when they are in a store or on public transit, as well as other measures such as mask wearing. They are not going to “encounter a situation where somebody is so heavily infected with COVID-19 that they’re going to be expelling a high degree of aerosolizing matter into the air around you.
“The situations where we are concerned with that is in a hospital where someone is being put on a ventilator, for example, or deep suctioning is being pursued on them or some other high-risk medical procedure. Occasionally some dental procedure or ophthalmologic procedures can predispose somebody to that same level of risk, but those are the situations where you do need an N95 mask.”
What is your thought or reaction to this open letter to the WHO?
“I think it’s reasonable to remain cautious,” said Sharkawy, but that means “you should be doing things that are readily accessible to manage your own safety and those around you.”
Does there have to be a certain concentration of the virus in an aerosol or microdroplet for it to be contagious?
“Absolutely. We do believe that the aerosols that are being released actually contain less viral RNA, they actually have a lower viral load in them compared to the heavier droplets, which is deemed to be the more conventional route for transmission.”
Are we concerned about aerosols or microdroplets outdoors?
“There’s very little evidence that there’s really any measurable amount of outdoor transmission. Is it reasonable to wear a mask if you’re in a crowded setting (indoors or outdoors)? Absolutely. Is it a must if you go outside? Absolutely not. I think that there’s no evidence for that.” If you want to enjoy the fresh air in a park or anywhere else with your family and you’re engaging in a safe practice otherwise with respect to distancing, there’s absolutely no need for a mask.”
Would you want to open a window if you have people over?
“No question. Everybody has a different level of comfort. I don’t have anybody inside my house, whether they’re part of a circle or a bubble or whatnot. I’m still comfortable meeting with my close friends only outdoors,” where they are still social distancing.
“If other people feel more comfortable in an indoor setting, absolutely I think it’s a good idea, number one, to be in as spacious a setting as possible. If you have the ability to have windows opened, that’s great. If that’s not an option, having the air conditioning set to a very high level will help but it’s not quite as effective.”