A bike courier wears an alternative protective face mask in downtown Vancouver, Monday, April 6, 2020. THE CANADIAN PRESS/Jonathan Hayward

OTTAWA - Wearing a simple cloth mask is a way for someone who might have COVID-19 without realizing it to avoid spreading the illness to others, Canada's top public health doctor said Monday.

Dr. Theresa Tam said the new advice, a shift from previous instructions, flows from increasing evidence that people with the virus can spread it without knowing they're sick.

Masks worn this way protect others rather than the people wearing them, and don't exempt wearers from all the other measures they should take against COVID-19, including physical distancing and regular handwashing, Tam said during a media briefing by federal officials.

"We are still in a very critical stage of Canada's COVID-19 epidemic, and this is no time to relax our preventive measures."

Medical masks still need to be preserved for front-line health workers, so cloth masks and other alternatives are the way to go, Tam said.

A non-medical mask can reduce the chance of respiratory droplets coming into contact with other people or surfaces — for instance, on a bus or at the grocery store, she said.

Scientific knowledge of COVID-19 continues to grow and it is clear that transmission of the virus is happening more often than previously recognized from infected people right before they develop symptoms, something called presymptomatic transmission, Tam said.

There is also evidence that some infected people who never develop symptoms are also able to spread the virus, a phenomenon known as asymptomatic transmission, she added.

Officials still do not know how big a role these two circumstances play in transmitting the virus, "but we know that it is occurring," Tam said.

"Wearing a non-medical mask in the community has not been proven to protect the person wearing it. It is an additional way that you can protect others."

Health Minister Patty Hajdu said the advice was not being characterized as a universal recommendation because it applies in cases where someone may feel they cannot stay two metres away from others and "would like to take additional measures to protect the people around them."

Some of the emerging evidence has only recently been published and, in the international community, various public health experts and countries have been looking "very intensely" at the issue, Tam said.

The federal advice follows a similar message last Friday from the U.S. Centers for Disease Control and Prevention.

The U.S. organization says the virus can spread between people who are speaking, coughing, or sneezing, even if they are not exhibiting symptoms.

"In light of this new evidence, CDC recommends wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain (e.g., grocery stores and pharmacies) especially in areas of significant community-based transmission."

This report by The Canadian Press was first published April 6, 2020.

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News from © The Canadian Press, 2020