04/03/2020 21:04 EDT | Updated 04/04/2020 10:24 EDT

Asymptomatic Coronavirus Cases Reinforce Need To Keep Your Distance: Dr. Theresa Tam

The CDC urged Americans Friday to wear cloth masks in public to protect themselves against COVID-19.

OTTAWA – As the United States’ Centers for Disease Control and Prevention moved to recommend Friday that Americans wear cloth coverings in public to protect themselves against asymptomatic carriers, Canada’s top doctor insisted masks aren’t needed but said Canadians should consider anyone they encounter to be infected with the coronavirus. 


For several days, Health Canada has refused to say whether it agrees with the CDC’s assessment that as many as 25 per cent of coronavirus carriers are asymptomatic — that they shed the virus but show no symptoms. 

A spokeswoman told HuffPost Canada on Wednesday that she would provide an answer by Thursday. On Friday, the department suggested it did not feel it needed to provide a response.  

The CDC said it was issuing the recommendation for face coverings after reviewing studies suggesting a “significant portion” of individuals with the virus have no symptoms and that those who may eventually develop symptoms can transmit the virus to others before they start feeling ill.  

“This means that the virus can spread between people interacting in close proximity—for example, speaking, coughing, or sneezing—even if those people are not exhibiting symptoms,” the CDC wrote

“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.” 

The agency said it was not recommending Americans use surgical masks or N-95 respirators, as those are “critical supplies that must continue to be reserved for healthcare workers and other medical first responders.”  

It also stressed that physical distancing remains important in slowing the spread of COVID-19. 

Chief Public Health Officer Theresa Tam gestures as she talks about the pandemic curve during a news conference in Ottawa on Friday.

As she began her daily press conference on Friday, Dr. Theresa Tam, Canada’s chief public health officer, warned reporters that everyone should be vigilant about catching the virus, whether or not their community is reporting any cases.

“We must all consider that anyone could be infected and keep our two-metre distance as the safest approach,” she said.

When pressed about the growing evidence that asymptomatic individuals are helping to spread the coronavirus —  and whether that might change Ottawa’s recommendations on whether people should wear masks, Tam said officials are just starting to gather and analyze some of the recent evidence. What they find may change the advice she provides, she said.

“It wasn’t really until the last few days almost or weeks that a number of studies have come out, so we are actively reviewing those. And, we do understand that they could essentially result in a number of changes, perhaps, in how public health manages the current outbreak.”

Numerous studies now suggest a large percentage of coronavirus carriers are asymptomatic — and that they may be key to controlling the outbreak. 

A nurse holds a swab to test a patient at a drive-through clinic at Ste-Justine Children Hospital in Montreal, on Wednesday.

Provincial governments have not been testing asymptomatic people, and officials at all levels continue to insist healthy individuals don’t need to mask up to go to the grocery store or run other necessary errands.

But that could soon change. 

In a podcast interview with U.S. National Public Radio, released Monday, CDC director Dr. Robert Redfield said what has been “pretty much confirmed now is that a significant number of individuals that are infected actually remain asymptomatic. That may be as many as 25 per cent.”

Asymptomatic people have helped transmit the virus, Redfield said, as have pre-symptomatic individuals — those who will develop COVID-19 but are contagious for about 48 hours before they start to show symptoms. 

“I think this helps explain how rapidly this virus continues to spread across the country,” he said.

Since few people without symptoms of COVID-19 are tested in North America, it’s difficult to estimate how prevalent the virus may be in society. The sample sizes for most of the studies released so far are small and limited.

This week, for example, China’s National Health Commission began releasing data on asymptomatic cases. The first figures released suggest approximately four in five coronavirus infections are asymptomatic, The BMJ said.

In Iceland, screening of 9,000 self-selected individuals in the general population found 1 per cent were positive, but 50 per cent of those people said they were asymptomatic, the lab company’s founder told CNN.

A paramedic stands on the back of an ambulance as a patient is brought into a Toronto hospital, on Wednesday.

An analysis published Friday on CDC’s website of testing done at a long-term-care skilled nursing facility in King County, Washington, in March, found that pre-symptomatic and asymptomatic residents were shedding as much virus as residents with symptoms. 

“This analysis suggests that symptom screening could initially fail to identify approximately one half of [nursing home] residents with SARS-CoV-2 infection,” researchers wrote. “Unrecognized asymptomatic and presymptomatic infections might contribute to transmission in these settings.”

CDC now recommends staff be closely monitored for any symptoms at the beginning and end of a shift, and that once a COVID-19 case is identified, health care personnel wear gowns, gloves, eye protection and N95 respirators when caring for residents.

A March study by Italian researchers who tested all 3,300 inhabitants of the small isolated town of Vò, where the first European died of COVID-19, suggest community infection rates could drop completely with mass testing and the isolation of all asymptomatic carriers. 

The isolation of asymptomatics is essential to be able to control the spread of the virus and the severity of the disease.Sergio Romagnani, a professor of clinical immunology

After reviewing that case data, Sergio Romagnani, a professor of clinical immunology at the University of Florence, wrote an open letter to his regional government stating that the “vast majority of people infected with COVID-19, between 50 and 75 per cent are completely asymptomatic.”  

Young people were especially likely to be asymptomatic, he said.

“The isolation of asymptomatics is essential to be able to control the spread of the virus and the severity of the disease.” 

Romagnani noted this was especially true for doctors and nurses who could be spreading the infection among themselves and their patients without knowing it. 

“It is being decided not to swab doctors and nurses again unless they develop symptoms. But, in light of the results of Vò study, this decision can be extremely dangerous. Hospitals risk becoming areas of high prevalence of infection in which no infected is isolated,” he said, according to La Repubblica.

International studies on the potential impact of asymptomatic carriers are mounting.

In a letter to the editor of the New England Journal of Medicine published last week,  German researchers who tested evacuees from Hubei province to Frankfurt, Germany, in February found two out of 114 individuals without symptoms tested positive for the virus. (One person who later developed a rash and a slight sore throat may have been pre-symptomatic.)

A health care worker wearing protective gear walks between a COVID-19 assessment centre at The Scarborough Hospital in Scarborough, Ont., on Friday.

The  researchers wrote “that shedding of potentially infectious virus may occur in persons who have no fever and no signs or only minor signs of infection.”

A British study published last week by London’s Centre for Mathematical Modelling of Infectious Diseases suggests children may be asymptomatic vectors of the virus. 

Studies counter official recommendations

All this new research seems to fly in the face of recommendations by Canadian public health officials and the World Health Organization. 

In mid-February, the WHO said asymptomatic infection has been reported in China but it was “relatively rare” and most who were not symptomatic on the day of testing subsequently developed symptoms.

“The proportion of truly asymptomatic infections is unclear,” the agency wrote, “but appears to be relatively rare and does not appear to be a major driver of transmission.” 

On Jan. 29, when Tam spoke to the health committee, Canada’s chief public health officer told MPs there was “no evidence” that asymptomatic people should be quarantined.

When Liberal MP Marcus Powlowski questioned whether Canada should be asking travellers coming back to Canada from China to self-isolate for 14 days as a precautionary measure, Tam told him the global community’s effort to contain the virus required the absolute commitment of communities affected. 

“Otherwise, they’ll be stigmatized,” Tam said. “They will be asked to take measures beyond what is currently the public health evidence.”

Powlowski, the MP for Thunder Bay–Rainy River, responded that China itself was saying the virus could be transmitted through the incubation period, when people don’t show symptoms.  

“It [a quarantine] would seem to me to be something that maybe we ought to consider,” he said.  

She said officials needed to be “reasonable” and “balance out the risks.”

“Based on what we know about those coronaviruses,” she said, pointing to the flu, SARS and MERS, “is it possible that an asymptomatic person could transmit the virus? Even if it’s possible, it is, we believe, a rare event. It is not that type of transmission that drives the force of an epidemic.”

When pressed about this again, this time by Vancouver NDP MP Jenny Kwan, Tam said even if asymptomatic or pre-symptomatic people were infectious, they would not be able to “readily transmit” the virus unless they were coughing it up or sneezing droplets.

“We do know that asymptomatic people are not the key driver of epidemics. That is very important to understand.” 

No clear data in Ontario yet

Earlier this week, Dr. Barbara Yaffe, the associate chief medical officer of health for Ontario, told HuffPost that she’s aware of CDC’s assessment of the number of asymptomatic carriers but  that right now her department’s recommendations are based on advice from Public Health Ontario and the Public Health Agency of Canada. 

“In terms of asymptomatic, we do not have clear data from Ontario ...,” she said on Thursday.

Dr. Barbara Yaffe attends a news conference in Toronto on Jan. 27.

Still, Yaffe said that, when a COVID-19 case is confirmed and contact tracing begins, officials assume the person may have been infectious 48 hours before symptoms appeared.

Right now, she said, the province doesn’t have the capacity to test the province’s 14 million residents. There are still people with symptoms, and those who have been in contact with people who have developed symptoms, who have yet to be tested, she said. Those cases remain the priority, she said.

“When you think of it, social distancing, or physical distancing, is really assuming that anybody could be infectious out there, so that is one of the big rationales for why we want people to stay home, and if they are going out that they stay at least two metres away from other people,” Yaffe added.

“But that is something that we keep looking at with the science, with Public Health Ontario.”  

Doctor Peter Donnelly, Public Health Ontario’s president and chief executive officer, told reporters Friday that evidence from around the world suggests some people, especially young people who don’t have symptoms and can “innocently” spread the virus to others.

There clearly are many cases in the province that we don’t know about,” he said.  

Tam said officials are continuing to learn and incorporate their new knowledge in their directives. 

But right now, she said, the main message is that medical masks must be kept for health care workers. 

And, if people want to wear non-medical devices, such as homemade masks to help protect those around them from catching the virus, she said, “that layer of protection is a way that you can protect others.”

There is no scientific evidence that masks can protect you from others, she added, but whether you should be wearing them to protect other people is something officials are currently studying. 

On Friday, New York City Mayor Bill de Blasio told a radio station that the city initially recommended residents not wear masks for fear New Yorkers would buy up supply needed for medical workers.

In an interview with WNYC’s “The Brian Lehrer Show,” de Blasio said that while staying home and practicing social distancing are more effective ways of containing the virus, he now recommends residents wear homemade masks or scarves when they go out to prevent asymptomatic carriers or people with mild symptoms from spreading the disease.

If people choose to wear a homemade mask, Tam stressed that it is very important that they wash their hands before and after putting it on, that they make sure it is not gaping — that it fits snugly against their face — and that it is never shared with other people.

“I think we’re all learning through [this],” she said. “I think particularly western societies that are not used to wearing masks in public, are sort of learning this as we are going along … in real time.”

With a file from Samantha Beattie