MONTREAL — There has been a veneer of normalcy in Montreal.
Couples hunched over steak-frites and poutine at popular neighborhood bistros. Young people thronging comedy shows and CrossFit classes in parks. Shopping sprees, even if armed with masks and sanitizers.
But this week the sense that regular life was gradually returning was upended when Quebec, faced with a resurgence of coronavirus cases, became Canada’s first province to reintroduce tough shutdown measures — including closing restaurants, cinemas, and theaters, and forbidding household visits to friends or family, with some exceptions.
“The situation has become critical,” Quebec’s premier, François Legault, said on Monday evening, lamenting that people had relaxed their vigilance and prioritized fun. “If we don’t want our hospitals to be submerged, if we want to limit the number of deaths, we must take strong action.”
Alarm has also shaken Ontario where some cities are again posting record case numbers, and some new, modest restrictions have been imposed, including shutting down strip bars and banning alcohol sales after 11 p.m.
Epidemiologists attributed the growing numbers of case to inadequate contact tracing; an unwillingness to shut down schools and businesses; and people letting their guards down. For example, the karaoke bar, Bar Kirouac, in Quebec City, was linked to 72 cases of Covid-19, according to Quebec health officials.
The relapse has underlined how even Canada, a country with universal health care and a generally disciplined, rule-bound population, remains vulnerable to the coronavirus.
On Tuesday, Ottawa, the national capital, reported 105 cases — its highest number of new cases in one day since the beginning of the pandemic. It now has the highest number of active cases since late April.
Quebec, Canada’s epicenter throughout the pandemic, reported 750 new cases on Monday, helping to bring the total number in the province to 73,450. In total, 5,833 people have died in Quebec; the entire country has seen 9,289 deaths.
Canadians have until recently taken some satisfaction in the country’s approach to containing the virus. Most provinces moved swiftly in March to shut down schools and businesses. The border with the United States was closed and some provinces limited travelers from other parts of Canada — a restriction that still is in effect.
By spring and through the summer, many of those measures were significantly relaxed in most parts of the country.
In contrast to the hard-hit United States, where President Trump has repeatedly clashed with governors over how to manage the pandemic, partisan and regional grievances in Canada have been largely set aside.
Premier Jason Kenney of Alberta, a conservative who has often sparred with premiers in more liberal provinces, offered surplus masks, gloves and ventilators to Quebec, Ontario and British Columbia.
Without exception, political leaders have also deferred to physicians, scientists and public health experts to inform the country’s approach.
While Prime Minister Justin Trudeau has recently been buffeted by scandal, he has been generally credited for his authoritative handling of the pandemic. Early on, he set an example when he became the first leader of a major industrialized country to go into self-isolation, when his wife, Sophie Grégoire Trudeau, tested positive for the virus.
His government has since offered billions of dollars in financial support for businesses and workers hit by the virus to limit economic harm.
But David Buckeridge, an assistant professor and public health surveillance researcher at McGill University in Montreal, said that in Quebec, the provincial government did not act fast enough this time around.
“There’s so much pressure to maintain the economy that the government was a little slow to act decisively and close businesses,” Professor Buckeridge said. He said certain types of businesses like bars should never have been reopened.
He also said that although Mr. Legault blamed the rise in cases on private social gatherings, the province’s poor record at contact tracing made it impossible to confirm that. Mr. Legault’s analysis, he suggested, may be motivated by economic rather than medical considerations.
Health officials and epidemiologists said Quebec’s party culture has helped make it the pandemic troublemaker, compared with other provinces like British Columbia and those on Canada’s Atlantic coast, where the numbers of cases per capita have been significantly lower.
Speaking on Sunday night on a popular television show, “Tout le monde en parle,” Quebec’s health minister, Christian Dubé, said, “We like to party.”
In Montreal, parks in recent weeks have hosted dense clusters of young people attending performance nights or sunning themselves. As winter looms, and with the city’s usual diet of summer festivals canceled because of the pandemic, there has been a particular urge to have fun. The authorities say private underground house parties have also helped fan the outbreak.
In Quebec, revelers enjoy mimicking Celine Dion, a native daughter, at karaoke nights. But earlier this month, after the outbreak at Bar Kirouac in Quebec City, health authorities suspended karaoke evenings throughout the province.
They noted that the projection of respiratory droplets when people sing and the sharing of microphones had made singing nights potentially deadly.
Under Quebec’s new rules, which will go into effect on Thursday, people will be forbidden to invite houseguests with the exception of individuals who require a caregiver, child care or maintenance services. Museums, bars and libraries will be closed. Churches, synagogues and mosques will be limited to 25 people.
People gathering outside will also be expected to remain two meters, or about 6.6 feet, apart. However, schools, which have been buffeted by dozens of cases of coronavirus, will remain open, as will gyms, hotels and hair salons.
The new restrictions will be imposed for 28 days and apply to three regions in Quebec, including Montreal.
In Ontario last weekend, a group of mostly young motorists poured into a resort community and commandeered its streets for illegal racing. Most of the crowd members were not wearing masks or socially distancing. Overwhelmed, police sealed off the town.
On Tuesday Doug Ford, the Ontario premier, said he will introduce orders barring visitors from long-term care homes in three of the province’s most populous regions.
The resurgence of cases has disproportionately affected young people, a shift from the early days of the pandemic. Dr. David N. Fisman, a professor of epidemiology at the University of Toronto Dalla Lana School of Public Health, said that may not just be the result of irresponsible behavior but of other factors, including the opening of university dorms and the fact that young people work in shops and restaurants.
“Not many 60-year-old baristas and waiters here,” he said, saying that blaming renewed outbreaks on social gatherings and irresponsible youth had “become an escape hatch for the government in terms of any restrictions on businesses.”
Dr. Buckeridge said that Quebec’s new measures will take weeks to have any effect on infection rates, and that the province will likely have to go further.
“There are going to be a few 28-day periods unless we change what we’re doing in a meaningful way,” he said.