Canada is seeing an early rise in flu cases. Is a 'tidal wave' of infection coming?

Canada is on track to face its first full flu season in several years — one that's starting earlier than usual, all while the country's healthcare system is already grappling with respiratory infections like COVID-19 and RSV.

It's hard to know how the months ahead will play out, including what level of strain serious influenza infections will put on overrun hospitals, and how this year's slate of viruses will interact now that SARS-CoV-2 is firmly in the mix.

But what's clear is there's already a sharp rise in recent infections, and a "tidal wave" of cases is likely on the way, said Dr. Sameer Elsayed, an infectious diseases physician and medical microbiologist in London, Ont., and a professor with Western University.

"We're going to have a big influenza season, I expect, this year."

At the national level, influenza activity has been "increasing steeply" and crossed the seasonal threshold of five per cent of samples coming back positive by late October. Should those trends hold, the federal government will be declaring the start of an influenza epidemic in Canada in its next update, scheduled for Nov. 14.

Ontario has already soared past that benchmark, with roughly 10 per cent of tests recently coming back positive for this year's dominant strain of influenza A.

In the latest update from Public Health Ontario on Nov. 4, the province said the flu season has started "more than a month earlier than typically observed in pre-pandemic seasons."

That early start comes as the province's pediatric hospitals are already overflowing with children sick with illnesses including respiratory syncytial virus, or RSV, and while positive test results for COVID-19 are back on the rise, most recently hitting 17 per cent.

"In the coming months, Ontario will likely face the triple threat of respiratory illnesses," warned Dr. Rose Zacharias, president of the Ontario Medical Association, a physician advocacy group, during a press conference on Wednesday.

Alberta began experiencing a spike in influenza A cases as well by the end of October, alongside the circulation of other pathogens, and B.C. public health officials are also watching for an ongoing rise in positive samples.

"At the moment we are seeing influenza creeping up, and samples come in through long-term care facilities, children's hospitals, adult hospitals," said medical microbiologist Dr. Linda Hoang, the associate director and program head of the bacteriology and mycology lab at the B.C. Centre for Disease Control.

Ontario influenza A test positivity by week

Clues from Southern Hemisphere

How Canada's flu season will progress from here could mirror, to some degree, what countries in the southern hemisphere experienced earlier this year.

In Chile, where the 2022 flu season has come and gone, influenza A began circulating "months earlier" than during pre-pandemic flu seasons, according to a recent report released by the U.S. Centers for Disease Control. (The U.S. is now experiencing an early start as well, with southern states seeing the largest surges.)

Chilean officials reported more than 1,000 hospitalizations during the season. That's higher than during the COVID pandemic when public health restrictions and other factors kept flu at bay for more than a year, but lower than during recent pre-pandemic flu seasons. The country's flu shots also cut the risk of hospitalization nearly in half.

Data on lab-confirmed influenza cases in Australia this year also show the country experienced both an early start and end to its flu season, and a level of infections far higher than any of the five years prior.

But the 2022 flu season's clinical severity — referring to the overall death toll, and the proportion of patients admitted into intensive care — was deemed "low" by the Australian government.

WATCH | Ontario physicians warn of 'triple threat' respiratory season:

So what do those trends signal for Canada in the months ahead?

Alyson Kelvin, a virologist and researcher with the Vaccine and Infectious Disease Organization at the University of Saskatchewan, said it's possible our flu season may also "peak and decline" earlier as well.

But she stressed the combination of flu and other respiratory viruses, including COVID's first time circulating in the winter months without any public health restrictions, makes the stretch ahead particularly difficult to predict.

It's possible the flu season's early start in the southern hemisphere, echoed now in countries further north, suggests the virus is moving into circulation on the heels of other waves of infections.

"We could see a later rise in COVID-19 cases, maybe in early January," Kelvin said. "But it also could be just the backlash of not seeing influenza for the past two years; I really don't know. And I'll be watching the numbers to see a more clear pattern in the next couple of years."

Time for masks again?

Hospital teams worry that waves of different viral infections would mean months of strain on Canada's health-care system, whether or not influenza cases alone amount to high levels of hospitalizations this season.

"It's only November," said Dr. Fahad Razak, an internist at St. Michael's Hospital in Toronto, during an interview on CBC News Network.

"Typically respiratory virus season, including influenza and RSV, but of course COVID as well, would be expected to peak in the coming months. So we have not seen, likely, how bad this will get."

The months ahead could be "very, very difficult," he added, given the already-lengthy wait times to access care at many over-run and short-staffed hospitals across the country.

Given those concerns, a growing chorus of physicians are now calling for a return to mask-wearing inside indoor settings to mitigate the spread of viral infections.

WATCH | Toronto physician calls for return of mask mandates:

"If it's added to the other layers of protection, including vaccination, then it might actually make a difference in terms of dampening the surge so that the hospitals can cope just a little bit better," Canada's chief public health officer Dr. Theresa Tam said in a news conference Thursday.

A study published this week in the New England Journal of Medicine offered more evidence suggesting masks can help mitigate virus transmission. The peer-reviewed research, which focused on school districts in Boston, found the lifting of masking requirements was associated with roughly 45 more COVID cases per 1,000 students and staff in the months following the end of a statewide policy.

Masks are a blunt, imperfect tool, said Razak, but one that also helped keep influenza at bay in Canada throughout much of the pandemic.

The 2020 flu season ended abruptly after a range of public health restrictions were put in place to combat COVID, and there wasn't evidence of community circulation of influenza the following season, either.

Typically, the flu is thought to kill thousands of Canadians each year, while COVID is currently killing hundreds every week.

Canadians encouraged to get flu, COVID shots

Getting a vaccine to protect against both is paramount this fall, multiple medical experts have stressed in recent interviews with CBC News.

It's even safe to get both a COVID booster shot and an annual flu shot during the same appointment, according to Canada's National Advisory Committee on Immunization.

Kelvin, who has long studied influenza, said Canadians always need to take the threat of flu seriously.

The added fear now, she said, is that it's roaring back while there's another respiratory virus — SARS-CoV-2 — now in the mix.

"That's going to add to increased severe disease cases," she said. "And that's what I want to be watching out for — that we're doing the most that we can to reduce respiratory virus transmission in the community."