Montreal's director of public health is warning that even if there's no recorded case of community transmission on the island, COVID-19 is likely being spread by people who haven't travelled and have no known link to a previously confirmed case.

Dr. Mylène Drouin said people who have travelled remain the priority for testing in Quebec, but that doesn't mean no one else has the virus.

"We probably have cases in the community which we have not yet detected," she said at a news conference Monday.

Drouin pointed to the spike in cases in neighbouring Ontario, where far more people have been tested than in Quebec.

On Sunday, Ontario announced 42 new cases of COVID-19. So far, only five of those have been definitively linked to travel or previously confirmed cases, while 37 are listed as " pending ," as officials investigate further.

Ontario has tested 10,178 people and has confirmed 172 cases.

By comparison, Quebec had completed tests by Monday afternoon on 3,079 people, with 50 cases confirmed and 3,073 still awaiting results.

Testing ramps up

Premier François Legault said the province would add seven new labs by Tuesday, increasing its testing capacity to screen 6,000 people a day. The province has promised to set up a total of 15 new labs by the end of this week.

So far, the province has prioritized testing those with COVID-19 symptoms who recently travelled and those who have been in contact with a traveller with symptoms.

When asked Monday if there are cases of community transmission in Quebec, Dr. Horacio Arruda, the province's director of public health, said: "We will test, test, test."

"We are at the beginning compared to other places. In fact, the transmissions here are at the very beginning. I am not telling you there are no transmissions."

Premier François Legault said Quebec will add seven new labs by Tuesday, increasing its testing capacity to 6,000 people a day. (CBC)

Dr. Nima Machouf, a Montreal epidemiologist, said if the test were simpler, like a pin prick, it would be more widely carried out.

"It's certain that we don't know how many people have it," she said.

For now, she said, focusing on travellers allows health officials to determine, for certain, which individuals have the virus and with whom they may have been in contact.

Early on in an epidemic, testing everyone is "very resource-heavy and not a great return," said McGill University epidemiology Prof. David Buckeridge.

Ultimately, he said, testing may no longer serve a purpose if the virus becomes very widespread.

"If you think about it, we don't routinely test everyone for the common flu," he said.

Self-isolation key

Buckeridge praised Quebec's new restrictions, which he said could help curb the spread from two to three new infections per person with COVID-19 to half that.

"I know some people think they are a little aggressive, but they probably should have been in place a week ago," said Buckeridge.

"The earlier you can get in and drive that number down, you really have a huge impact on the size of the epidemic a few weeks down the road."

For now, Drouin said, getting the word out to travellers remains a priority, which is why Montreal dispatched health professionals to Pierre Trudeau International Airport Monday.

Later in the day, the federal government took the extraordinary step of banning non-residents from crossing into Canada.

"Every time that a traveller does not isolate themselves, it's a breach that allows the virus to propagate in our communities. It also reduces the efficiency of our public health strategies," said Drouin. "You must isolate yourself."

"If you're a friend, a colleague or a loved one of a person who has travelled, insist that they isolate themselves, and support them during that period — whether it's food or medication."