Widespread community transmission of COVID-19 is likely underway in Canada, experts say, warning that Canadians need to heed advice from public health officials and do their part to prevent it from getting out of control.

Community transmission is the spread of an illness with no known link to travel or previously confirmed cases, which can signal a growing number of cases going unreported across the country.

Dr. Allison McGeer, an infectious diseases specialist at Toronto's Mount Sinai Hospital who worked on the front lines of the SARS epidemic in 2003, believes there is no doubt it is already happening in Canada.

"I don't think there's any question," she said.

Ontario announced 42 new cases of COVID-19 as of 5:30 p.m. ET Sunday. So far, only five of those have been definitively linked to travel or previously confirmed cases, while 37 are marked as "pending" as officials investigate further.

Ottawa's medical officer of health confirmed community transmission is likely happening in the city , with up to 1,000 cases going unreported, and instructed all residents not to leave their homes for "non-essential reasons."

Ottawa Medical officer of Health says could be 200-1000 cases in Ottawa- calling on all people to stay home, social distancing. —@JulieIreton

And Alberta's medical officer said the province is "likely seeing community transmission" and announced 17 new cases Sunday, two of which had no known link to travel or other cases. Calgary has also declared a state of local emergency due to a spike in cases and concerns of growing community transmission.

Canada's Chief Medical Officer of Health Dr. Theresa Tam said Sunday that 25,000 COVID-19 tests have been done across the country to date and there has been a "rapidly increasing" number of cases, particularly in Ontario, B.C. and Alberta.

"Our window to flatten the curve of the epidemic is narrow," Tam said. "We all need to act now. COVID-19 is a serious public health threat."

Tam urged Canadians to embrace social distancing in an effort to slow the spread of the outbreak, but did not say whether she believed there is widespread community transmission of COVID-19 in Canada.

Social distancing is an approach to limiting the spread of an illness in the population by creating physical space between people and reducing the size of large gatherings, or avoiding them altogether.

'I think it's clearly here'

Dr. David Fisman, an epidemiologist with the University of Toronto's Dalla Lana School of Public Health, says the exponential growth of COVID-19 cases in Canada indicates community transmission is already underway.

"Either it's already here or it's coming, because we're just importing so many damn cases that we can't keep up with it," he said. "I think it's clearly here."

Dr. Isaac Bogoch, an infectious disease physician at Toronto General Hospital, said Canadian health officials have been bracing for community transmission of COVID-19 for months.

"It's expected that this would happen and the whole purpose was to get prepared for this scenario," he said.

"The time is now to act so that we can mitigate the spread of this infection in Canada. We're not going to eliminate the spread in Canada, but we can certainly dampen the effects."

Bogoch said this is the reason why public health officials are asking Canadians to avoid all non-essential travel outside of the country, return to Canada if they are travelling abroad and embrace social distancing on a massive scale in an effort to flatten the epidemic curve of the outbreak.

An empty street is seen in Manhattan Sunday amid the outbreak of coronavirus. Experts say community transmission could be well underway here in Canada. (Jeenah Moon/Reuters)

Matthew Miller, an associate professor of biochemistry and biomedical sciences at McMaster University, said the virus can have a relatively long incubation time — anywhere from three to 14 days.

"So there's going to be a lag time between when the community-based transmission actually starts to occur and when it's detected," he said.

"Once it's here, it's almost already too late."

Growing community transmission in the U.S.

Jason Kindrachuk, an assistant professor and Canada Research Chair in emerging viruses at the University of Manitoba, said Canada is moving into a new phase of the outbreak that would see increased community transmission likely.

"We can't be overly surprised that that is going to happen if it hasn't already, based on the fact of what we've seen in the United States in particular," he said. "We are seeing obvious community transmission in multiple regions of the U.S. now."

A health-care worker tends to a woman who has tested positive for coronavirus at the Life Care Center of Kirkland, Washington, the epicentre of one of the biggest coronavirus outbreaks in the U.S. (David Ryder/Reuters)

Bogoch said that widespread community transmission in areas like Washington state could be fuelling the increasing number of confirmed cases in Canada.

"We're likely importing cases from a growing number of countries, not just European, but also an increasing number from the United States," he said.

"As time goes on, without travel restrictions in place, we will continue to import cases."

Social distancing may be key to curbing spread

McGeer said that when many Canadians first heard messaging from public health officials encouraging social distancing, she suspects they may not have taken them seriously.

"Maybe the first time you hear the messages, it doesn't feel like it's that important. It doesn't feel like what you can do as an individual makes that much difference. So I'm not sure people have been listening to them," she said.

"As we see more cases people will perhaps listen better, but one of the important messages about social distancing is that the earlier you start it, the more effective it is."

Worshippers attend a Sunday mass limited to 250 people at the Cathedral of Our Lady of the Angels in Los Angeles, Calif., on Sunday. (Mario Tama/Getty Images)

McGeer said the two most important aspects of social distancing to stop the spread of COVID-19 are staying home when you're sick and limiting contact with as many people as possible.

"People have a tendency to think that what I as an individual do is not important," she said. "But when you sum what each of 35 million individuals do, it's critically important."

Not all experts are as optimistic that social distancing will be effective in this outbreak, however, now that community transmission has likely begun in Canada.

Fisman said Canada "squandered" opportunities to learn from countries such as China, South Korea, Singapore and Taiwan in proactively curbing the outbreak as opposed to countries like Italy that have seen an "explosion" in new cases and deaths.

"We've chosen to go the route of Italy and I think unfortunately at this point the die is cast," he said.

"Hopefully I'm wrong about that."