Why COVID-19 and flu could be in a 'tug of war' in the years ahead

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.

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 avid 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 COVID-19 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.