What will the upcoming flu season in Canada look like?

Last year's flu season in Canada saw extremely few cases, amid stringent public health restrictions across the country. But as fall approaches, experts say we could see another relatively mild flu season.

University of Toronto infectious disease expert Dr. Isaac Bogoch told CTVNews.ca he "wouldn't be surprised" if we saw more flu cases this year compared to last year, given the looser public health restrictions we have now, but he doesn't expect the number of cases to jump to pre-pandemic levels.

"I really think that it won't be a 'regular influenza' in terms of having the same volume of cases," Bogoch said over the phone on Monday, "But I'm completely prepared to be way off on this because it's always very challenging to predict influenza."

Dr. Greta Bauer, who is a professor at Western University's Schulich School of Medicine & Dentistry, believes it's too early to tell how the flu season in Canada is going to shape up, but says that the flu season in the southern hemisphere can offer a few clues. The southern hemisphere experiences winter during our summer months, which means that flu season has already wrapped up in those parts of the world.

"In the southern hemisphere, they've already wrapped up their influenza season for the year. And it was a very, very mild influenza season, so that usually bodes very well for us when we're going into it knowing that there hasn't been a lot of influenza in circulation that year in South and Central America," said Bauer.

During the 2020-2021 flu season, the Public Health Agency of Canada (PHAC) reported just 79 laboratory-confirmed cases of influenza. The previous year, PHAC reported over 54,000 influenza cases.

The dramatic drop in flu cases was largely due to the strong public-health restrictions to combat COVID-19 that were also effective at stopping other respiratory viruses, experts say.

"Public-health measures to combat COVID-19 have a pretty profound effect over influenza, like masking, hand hygiene, and fewer people in indoor congregate settings together," Bogoch said.

"Travel was down dramatically as well. And people bring infectious diseases with them as they move around. We had a tiny fraction of the international travel that we were used to seeing," he added.

Bauer says that many of these measures, such as distancing, wearing masks and sanitizing surfaces, were unintentionally more effective at stopping the flu than COVID-19.

"I wouldn't say they're ineffective against COVID, but they're designed for droplet spread, which is primarily the way that influenza spreads and they were highly effective against influenza last year," Bauer said during a phone interview with CTVNews.ca on Tuesday.

While the flu primarily spreads via droplets, the growing consensus among scientists and doctors is that transmission of COVID-19 is primarily airborne.

Dr. Donald Vinh, an infectious diseases expert at the Research Institute of the McGill University Health Centre, says in addition to the public-health measures, the low flu cases last year could also be partly a result of "diagnostic bias."

"What we've seen, at least in the last 18 months ago or so, is that most of the laboratory resources to diagnose infections were focused on COVID. And as a result, diagnostic tests for other viruses like influenza also scaled down," he told CTVNews.ca over the phone on Monday.

With low influenza cases in the southern hemisphere, Vinh says he's more worried about an uptick in other respiratory viruses, such as respiratory syncytial virus (RSV), which primarily affects infants, young children and older adults. Vinh says that cases of RSV have increased in the U.S. and parts of Canada.

"RSV is a virus for which there is no vaccine, and has the capacity to make you quite sick, especially if you're immunocompromised," said Vinh. "There is a 'twindemic' brewing in North America. And right now, that partner of COVID is RSV."

Regardless of how severe the flu season turns out, Bogoch calls it a "no-brainer" to get the flu shot when it becomes available.

"Sometimes there's this perception that the flu is just a little cough or cold. That's not true. The flu is pretty bad. It kills about a half a million people per year and kills about 3,500 Canadians in a typical year. It's a nasty infection, and it's a vaccine preventable infection," he said.