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A growing chorus of experts is calling for the widespread use of masks to slow the spread of COVID-19 and suggesting it may be a key factor in why some countries seemingly have their outbreaks under control while others are completely overwhelmed.

The World Health Organization does not officially recommend the general public use masks as a way to reduce spreading the virus, only saying that if they do decide to use masks, they should do so "safely and properly."

"We encourage countries that are considering the use of masks for the general population to study their effectiveness, so we can all learn from their experience," a spokesperson for WHO said in a statement to CBC News.

"Currently, there is insufficient evidence for or against the use of any type of mask among healthy people in public settings. As new information and evidence about masks and non-medical masks emerges, WHO will update existing guidance."

WHO does recommend medical masks, respirators and other personal protective equipment (PPE) in health care settings, but says only people who are sick or caring for a sick person at home should use them in the community.

WATCH | An epidemiologist explains why masks are effective for asymptomatic carriers

'The point of the mask,' says Christopher Labos, 'is to protect others from you,' especially if you are an asymptomatic carrier of the virus. 2:58

"Masks alone cannot stop the pandemic. Countries must continue to find, test, isolate and treat every case and trace every contact," WHO Director-General Tedros Adhanom Ghebreyesus said earlier this month.

"Mask or no mask, there are proven things all of us can do to protect ourselves and others: keep your distance, clean your hands, cough or sneeze into your elbow, and avoid touching your face."

WHO's recommendations on masks not 'logical'

But the specific language WHO uses has come under fire from experts who suggest that even non-medical masks could be beneficial in curbing the spread of the virus — or, at the very least, couldn't hurt.

Babak Javid, a professor at the Tsinghua University School of Medicine in Beijing and a consultant in infectious diseases at Cambridge University Hospitals in the U.K., thinks there's a double standard in WHO's messaging.

Yes, he says, "there's no direct evidence that masks can prevent transmission."

"But there's no direct evidence that hand-washing can prevent the transmission of COVID or lockdowns can prevent the transmission of COVID because none of those trials have ever been done in the context of COVID."

One infectious disease expert says WHO's recommendation on masks doesn't take into account the significant number of asymptomatic or pre-symptomatic carriers of the virus in the general population. (Evan Mitsui/CBC)

Javid said there is a "logical gap" in WHO's recommendation, because it doesn't take into account the significant number of asymptomatic or pre-symptomatic carriers of the virus in the general population, also known as "silent spreaders."

Asymptomatic carriers are those who show no symptoms, while pre-symptomatic carriers are people who have not yet displayed symptoms — usually in the first few days of infection.

"What I think is indisputable is that the face covering can reduce the amount of both droplets and virus that we can transmit," he said.

Research on virus's spread suggests masks could help

Emerging research has revealed more about how the virus spreads from person to person, suggesting it doesn't just transmit through coughing but also through simply talking.

A U.S. National Institutes of Health study published as a research letter in the New England Journal of Medicine this week provided visual evidence that "speech-generated droplets" can become suspended in the air and inhaled by other people, potentially exposing them to viruses like the one responsible for COVID-19.

That provides some preliminary evidence that the potentially deadly coronavirus, also known as SARS-CoV-2, could be transmitted between people just by speaking normally.

Emerging research has revealed more about how the virus spreads from person to person, suggesting it doesn't just transmit through coughing but also through simply talking. (Evan Mitsui/CBC)

"There's plenty of research showing that even just when we talk and breathe, we release these very small respiratory droplets, smaller than we can see," said Dr. Linsey Marr, an expert in the transmission of viruses by aerosol at Virginia Tech in Blacksburg, Virginia.

"Masks really could help make a difference because they will — if you are sick without knowing it — block those droplets from flying into the air and then staying in the air or contaminating surfaces."

A paper by researchers in China set to be published in July in the U.S. Centers for Disease Control and Prevention's journal Emerging Infectious Diseases, found an infected individual without symptoms was apparently able to spread the virus to nine others at a restaurant in Guangzhou, China.

The direction the air-conditioning system was blowing may have helped transport the virus particles to other diners, who otherwise had no contact with one another, while those elsewhere in the restaurant who weren't near the airflow didn't get sick.

The study has limitations because the findings were not corroborated with experiments to simulate the airborne transmission. Still, the research is raising concern.

"That was pretty strong evidence for the potential that transmission is happening through these microscopic respiratory droplets that can stay floating in the air for a long enough time to kind of go back and forth over the tables," Marr said.

But the lack of comprehensive research on the effectiveness of non-medical masks in limiting the spread of virus particles, either with this coronavirus or other viruses, makes assessing their usefulness in the current pandemic challenging.

The one randomized clinical trial of cloth masks in a health-care setting for protection against respiratory illness — done on influenza, not COVID-19 — found that they let through 97 per cent of virus particles and actually increased the rate of infection among health-care workers. That same study found surgical masks failed to block 44 per cent of virus particles.

The authors of that 2015 study recently wrote an addendum to their research because they have received so many questions about it since the coronavirus pandemic began.

Should masks be mandatory in public?

Last week, Transport Canada made it mandatory for all air passengers to wear non-medical masks or face coverings over their mouth and nose during travel.

Passengers on boats are "encouraged to wear non-medical masks or face coverings whenever possible," while train or bus riders are also "strongly encouraged" to do the same.

The Public Health Agency of Canada says non-medical masks are 'not a substitute for physical distancing and hand-washing,' but can protect those nearby 'even if you have no symptoms.' (Evan Mitsui/CBC)

But the Public Health Agency of Canada only recently changed course on its recommendations to Canadians on the effectiveness of non-medical masks, previously saying masks should only be worn by health care workers.

"Wearing a non-medical mask is an additional measure that you can take to protect others around you," Canada's Chief Public Health Officer Dr. Theresa Tam said on April 6 — while warning that a non-medical mask doesn't necessarily protect the person wearing it.

"A non-medical mask can reduce the chance of your respiratory droplets coming into contact with others or landing on surfaces," Tam said. "The science is not certain, but we need to do everything that we can and it seems a sensible thing to do."

PHAC also says non-medical masks are "not a substitute for physical distancing and hand-washing," but can protect those nearby "even if you have no symptoms."

Last week, Transport Canada made it mandatory for all air passengers to wear non-medical masks or face coverings during travel and 'strongly encouraged' passengers on boats, trains and buses to do the same. (Ben Nelms/CBC)

But some experts say that given that significant role asymptomatic and pre-symptomatic carriers of the virus play in the spread of the virus, Canada should be mandating the use of non-medical masks in public, the way other countries have.

"Public health officials are saying there's no evidence that the mask is going to keep you from getting the disease, but I also think it's important to recognize that absence of evidence is not evidence of absence," said Dr. Joe Vipond, a Calgary emergency department physician.

"Even a small decrease in the risk of me getting the disease is still really important from a population level."

Evidence from other countries

Asian regions that have successfully dropped their curve of new COVID-19 cases, like South Korea, Taiwan and China, all have widespread mask usage, while the Czech Republic recently implemented mandatory mask measures and saw case fatality rates drop, Vipond wrote in a recent op-ed for Maclean's magazine .

There are numerous factors as to why a given country can become overwhelmed by the virus, including the onset of outbreaks, the accuracy of reporting, backlogs in testing, the age of the population and the effectiveness of containment measures.

But Javid said that while this is still just correlative evidence, it could be one reason why countries like Taiwan, which doesn't have total lockdown measures in place but does have widespread mask usage, has a much lower incidence of COVID-19 than other countries like Sweden, for example, where masks are less commonly used.

Jeremy Howard, a University of San Francisco researcher and the co-founder of Masks 4 All , said Japan is another example of a densely populated region with widespread public mask wearing and low levels of COVID-19 despite not prioritizing physical distancing or testing compared with a place like New York.

"There's all this evidence, which strongly suggests, as part of the suite of tools we have available, wearing a mask appears to be astonishingly powerful, and the cost is nearly zero," he said.

"And when we compare the level of evidence we have to something like washing hands, it's vastly, vastly, vastly, vastly higher."

While masks are not a "magic bullet" in curbing the spread of the virus and more research still needs to be done on their effectiveness, Javid said they can be used in conjunction with other measures like physical distancing and widespread testing to slow the spread.

"To get over the COVID problem we're going to need a multi-pronged approach. To me, what's attractive about masks is that they're cheap, they're probably effective and they allow economic activity," he said.

"We can't stay in lockdown forever, so to me, the downsides are so minimal compared to the potential upsides."