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Coronavirus can be airborne, but that’s not how it typically spreads: experts

Global News logo Global News 2020-07-11 Meghan Collie and Laura Hensley
a crowd of people at a train station: Passengers arrive at Cadorna railway station on May 4, 2020 in Milan, Italy. © Getty Images Passengers arrive at Cadorna railway station on May 4, 2020 in Milan, Italy.

The World Health Organization (WHO) acknowledged emerging evidence of the airborne spread of the novel coronavirus on Tuesday after a group of scientists urged the organization to update its guidance on how the respiratory disease passes between people.

On Monday, 239 scientists in 32 different countries sounded the alarm over the possibility that the novel coronavirus may be more airborne than previously thought.

In an open letter to the WHO, the group warned that the virus can linger in the air in the form of smaller particles known as aerosol droplets. The researchers plan to publish their evidence in a medical journal next week.

This was widely reported as a groundbreaking development in the fight against COVID-19, given that the WHO had previously said the virus spreads "primarily from person to person through small droplets from the nose or mouth," also known as respiratory droplets.

READ MORE: Scientists warn coronavirus could be airborne. What does this mean for Canadians?

According to experts, however, airborne transmission isn't exactly new information ⁠— nor is it a reason to be overly concerned.

"I think the main message is there's nothing new that's come out here," said Dr. Rupert Kaul, director of the division of infectious diseases at the University Health Network, of the open letter.

"There have been very strong opinions from either side of this (debate) for quite a long time within the infectious, respirology (and) epidemiology (communities)."

Kaul says doctors have always known that airborne transmission plays a role in the spread of COVID-19 ⁠— but it's not the main way the virus is spreading on the ground.

"Nobody is saying that they think COVID-19 or SARS-CoV-2 (the virus that causes the disease) is completely aerosol-transmitted," Kaul said. "The only debate is, in real life, how much of it is transmitted through the air and how much through the droplets?"

Dr. Sumontra Chakrabarti, an infectious disease specialist at Trillium Health Partners, agrees.

"There's a disconnect between what ... can happen and what is actually, practically happening," he said.

The current recommendations from public health ⁠— including physical distancing, wearing a mask and good hand hygiene ⁠— are protections against respiratory droplets, Chakrabarti says, and they wouldn't be nearly as effective as they have proven to be if the virus was being transmitted through aerosol particles at a high frequency.

"If SARS-CoV-2 was truly airborne, there would be way more infections and all this physical-distancing stuff wouldn't work," he said.

Aerosol particles versus respiratory droplets

The main difference between aerosol particles and respiratory droplets is size.

Respiratory droplets are larger, heavier and often visible. Because they're heavier, they typically fall to the ground rather quickly, Kaul said. Aerosol particles are so small you typically can't even see them, and their light weight allows them to hang in the air and "dry out."

When you sneeze, cough or sing, you expel both aerosol particles and respiratory droplets.

The larger droplets can travel only about six feet, which is why Canada has a physical-distancing rule in place. However, aerosol particles can hang in the air for a while ⁠— especially if you're somewhere where the air is stagnant.

That's why aerosol particles are "much more of an issue for indoor transmission than they are outdoor transmission," Kaul said.

READ MORE: Canadian experts say not to get hung up on term 'airborne' transmission

In lab settings, researchers have previously reported that infectious droplets can linger in the air for hours and on surfaces up to days. In one study, researchers used a device to dispense an infectious aerosol that duplicated the microscopic droplets created in a cough or a sneeze.

But real-life situations are different than controlled lab experiments, experts said.

Based on how both sizes of particles travel, there is little circumstantial evidence to support the claim that COVID-19 is effective at spreading via aerosol particles.

Chakrabarti points to an outbreak in a call centre in South Korea as an example in which 97 people tested positive for the virus. Of those who fell ill, 94 sat on the same floor and 79 sat in the same section on that floor.

"It's clear that the transmission is occurring in people who are close together in a space for a prolonged period of time," Chakrabarti said.

"In the call centre cluster, you can see if one person was infected, (they) infected the people they were closest to, but didn't infect the people who were sitting far away."

If COVID-19 was effective at spreading via aerosol droplets, proximity to the person with the virus wouldn't have mattered, Chakrabarti said ⁠— everyone on that floor probably would have caught the virus.

This is consistent with the response from the WHO, released Thursday, which confirmed some reports of airborne transmission but stopped short of saying that the virus spreads through the air.

The WHO acknowledged that some outbreak reports related to crowded indoor spaces have suggested the possibility of aerosol transmission ⁠— such as during choir practice, in restaurants or in fitness classes ⁠— but how frequently the coronavirus can spread by the airborne or aerosol route (as opposed to by larger droplets in coughs and sneezes) is not clear.

The organization said more research is “urgently needed to investigate such instances and assess their significance for transmission of COVID-19.”

If COVID-19 was primarily airborne, infection would be much more widespread in enclosed spaces, Kaul said.

READ MORE: What we know about how the new coronavirus is spread

"If I have an aerosol-borne infection ... then when I'm somewhere where they're recirculating air (like an airplane), we would see widespread transmission," he said.

"It's not something we've really seen with COVID-19. The pattern of spread that we've seen on planes is much more that the person next to (the infected person) gets COVID-19."

Airborne concern not unique to new coronavirus

The debate about how a virus spreads ⁠— whether it's airborne (via aerosol particles) or through respiratory droplets ⁠— is not unique to COVID-19.

"It's a debate that's been going on for way longer ... in terms of influenza and numerous other viruses as well," Kaul said.

Tuberculosis is believed to spread primarily via aerosol transmission, which explains why it's so contagious.

A person can catch tuberculosis by entering a room where a person with the virus had been hours earlier. The particles through which it travels are extremely small and able to pass through all your bodily defences in your nose, mouth and throat with ease, Kaul said.

However, other viruses, like influenza, are believed to be on more of a "spectrum," Kaul said.

"Influenza, we think, is mostly transmitted through (larger respiratory) droplets, but there is a little bit of airborne transmission or aerosol transmission of influenza, as well," he said.

"SARS-CoV-2 is probably a little bit more like influenza."

What this means for everyday life

When it comes to daily life, experts say Canadians shouldn't get tripped up on the word "airborne" and should continue doing what they're doing ⁠— namely, physical distancing, wearing a mask in enclosed spaces and practising good hand hygiene.

READ MORE: WHO says 'evidence emerging' that coronavirus may be airborne

On the public health level, the goal is to make the masses aware of the "most common way that COVID-19 is going to be transmitted," Kaul said.

"Let's not focus on preventing the very occasional transmission event that may happen through aerosol spread ⁠— let's focus on the great majority that's going to be transmitted through droplets."

Chakrabarti agrees and says we do not need to be overly concerned about aerosol transmission at this time.

"We know that even if there is a theoretical possibility of there being aerosol that is in the air that has some COVID-19 particles in it, it's irrelevant because what we've been doing is not airborne precautions," Chakrabarti said.

"And it's working."

Questions about COVID-19? Here are some things you need to know:

Symptoms can include but are not limited to: fever, cough and difficulty breathing — very similar to a cold or flu. Some people can develop a more severe illness. People most at risk of this include older adults and people with severe chronic medical conditions like heart, lung or kidney disease. If you develop symptoms, contact public health authorities.

To prevent the virus from spreading, experts recommend frequent handwashing and coughing into your sleeve. They also recommend minimizing contact with others, staying home as much as possible and maintaining a distance of two metres from other people if you go out.

In situations where you can't keep a safe distance from others, public health officials recommend the use of a non-medical face mask or covering to prevent spreading the respiratory droplets that can carry the virus.

For full COVID-19 coverage from Global News, click here.

⁠— With files from Global News' Rachael D'Amore and Reuters

Meghan.Collie@globalnews.ca

Laura.Hensley@globalnews.ca


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