For more than two years, as Canadians experienced a roller coaster of COVID-19 infections, influenza seemed to take a backseat.
The family of viruses that causes the flu was barely spreading across the country until a recent, belated return of Influenza A caught scientists' attention.
In March — as COVID-19 restrictions lifted and more people began mingling — positive tests for influenza A viruses began rising in Canada. By late April, federal data showed a nearly seven per cent test positivity rate, close to the average level for this time of year and largely driven by a spike in Quebec.
This unusually late flu season may offer clues for how COVID and influenza will impact each other in the years ahead, with one theory suggesting these two rival viruses could ebb and flow — but it's tough to predict what, exactly, that would look like.
"It'll be interesting to see what happens as COVID gets less dominant, if influenza starts to compete with it, and they have a tug of war over the next couple of years," said Dr. Sumon Chakrabarti, an infectious diseases specialist with Trillium Health Partners in Mississauga, Ont.
Positive Influenza A tests by week in Canada since March 1, 2022
Flu typically spikes in winter
In a typical flu year, cases first rise in the fall, spike in the winter and taper off in the spring, with sporadic infections throughout the rest of the year — with influenza A viruses circulating before influenza B.
The viruses' disappearance throughout the pandemic might be tied, in part, to shifts in behaviour throughout much of the last two years as COVID-19 surged unpredictably, variant after variant.
"We were able to successfully suppress influenza circulation with our other public measures related to COVID-19," said Dr. Danuta Skowronski, the epidemiology lead for influenza and emerging respiratory pathogens at the BC Centre for Disease Control.
The lifting of restrictions, coupled with a rise in global and domestic travel, likely played a role in influenza returning in recent months, said Dr. Isaac Bogoch, an infectious diseases specialist at the University Health Network in Toronto.
A mismatched flu vaccine this season may be another factor behind this unusual spike, as U.S. public health authorities reported, or simply waning immunity from shots given back in the fall, according to Skowronski.
But some close COVID-19 watchers say the virus that causes it, SARS-CoV-2, may also have helped keep influenza at bay.
There was once speculation that COVID-19 and flu could create what some dubbed a "twindemic," with both types of infections hitting countries at once, but those fears haven't materialized.
Instead, some medical experts say there could be some level of "viral interference," in which a virus such as SARS-CoV-2 pushes out other pathogens at a population level for a period of time.
"There's some type of interesting viral suppression and competition going on here," said Chakrabarti, who suggested COVID and flu may ebb and flow in circulation.
Throughout the pandemic, he explained, widespread COVID-19 infections may have kept people's immune systems on high alert, giving SARS-CoV-2 a long-term advantage over the flu.
"There were flu outbreaks this year, mostly in pockets of the United States, but still it clearly wasn't a regular flu season and influenza was clearly suppressed," echoed Bogoch.
"Now, people talk about viral interference with influenza and COVID and I wouldn't ignore anything. I think the short answer is you can't get overconfident with how the next year or so is going to play out."
On the flip side, virologist Alyson Kelvin of the University of Saskatchewan's Vaccine and Infectious Disease Organization said the sudden return of influenza may actually be tied to millions of Canadians' immunity being dulled by the fifth and sixth COVID-19 waves — not sharpened by a COVID-19 infection.
"The circulation of SARS-CoV-2 — specifically Omicron — may have left people slightly immunocompromised," she said.
"After the virus comes in and infects people, their immune system is weaker, leaving an opportunity for Influenza A to infect people in the weeks following their initial Omicron infection."
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Different pattern of influenza possible
Several experts who spoke to CBC News did agree there could be unexpected shifts in the influenza season based on how the virus behind COVID-19 operates going forward.
"Will SARS-CoV-2 and COVID-19 cases become more seasonal? If so, we might see a different pattern of influenza cases," Kelvin said.
In the short term — more reassuringly — both viruses are expected to fade away as the weather warms up before returning, at some point, in the fall.
This lull period is the time when health-care systems and governments need to start planning for an uncertain future, according to Dr. Leon Rivlin, chief of emergency medicine at Humber River Hospital.
Right now, as flu cases are ticking up, hospitals are still facing staffing shortages tied to COVID-19, he said. On his team, seven physicians called in sick for suspected SARS-CoV-2 infections just last weekend.
"We're going to have this upsurge again," he warned. "And once again, we're going to be strained for human resources."
The months ahead could be crucial to ensure Canadians continue getting vaccinations that offer protection against severe illnesses, while policy makers ensure there's adequate space and staff to handle patients who do fall seriously ill with COVID-19 or flu.
"There's going to be both short-term and long-term solutions," Chakrabarti said. "Ultimately, we need some type of way to absorb any sudden surge of infections that occurs."