What does the flu mean for the 6th COVID-19 wave in Quebec?

This story was updated after it was published to include the opinions of other medical experts about the study in question.

As the sixth pandemic wave has peaked in Quebec, the flu has returned to epidemic levels in the province after a lull of nearly two years. 

A team of Quebec researchers has studied how the influenza virus and SARS-CoV-2 virus interact. 

In the study published in February in the journal Viruses, the team of researchers from Université Laval in Quebec City, along with Swiss researcher Clément Fage, found that being infected with one of the viruses could help prevent infection with the other.

The findings are the results of lab tests and experts warn that results outside of a lab could differ. 

"The biggest caveat of this study, of course, is that it's just done with cells in a lab-based setting," said Dr. Matthew Miller, an associate professor of infectious diseases and immunology at McMaster University in Hamilton. 

"But I think it's an important step in the right direction."

Miller noted that learning more about how other viruses interact with the one that causes COVID-19 has "very important public health implications," as more restrictions are lifted and those other viruses begin circulating again.

Flu infections once again reached epidemic levels in early April, meaning the proportion of positive tests for the virus has surpassed five percent. It has now reached 11 per cent, a level typically seen in January and February in Quebec.

"It's very unusual to have a flu epidemic begin so late in the year," said Dr. Guy Boivin — one of the study's authors, and a microbiologist and infectious diseases specialist based at the Centre Hospitalier Universitaire de Québec, a network of teaching hospitals in Quebec City.

"We are in a situation that is completely out of the ordinary. The pandemic really upends all the theories we have about flu epidemics."

The immunity provided by flu vaccines could be waning, too, as most were administered last fall, Boivin added.

But Boivin believes the rise in influenza cases could have an impact on the number of COVID-19 cases in the coming weeks. 

"If our findings are confirmed at the population level, as the flu epidemic grows, we could see a decrease in the number of COVID-19 cases in the coming weeks," he told Radio-Canada.

Co-infection rare but possible: study

After first infecting nasals cells with one virus, the researchers found it was a lot harder to infect them with the other, and vice versa.

Boivin said the team's hypothesis is that, at a population level, the findings could mean the resurgence of the flu could contribute to a reduction in the number of COVID-19 cases in the coming weeks.

Still, Boivin cautions, the negative effects the viruses had on each other in his study were not absolute.

"Some people can still be co-infected by both viruses and we need to keep a close eye on that," he said, noting there have been cases reported in Europe and even recently in Quebec where people have been infected by the influenza virus and SARS-CoV-2 at the same time. 

A British study found that such "co-infections" can lead to symptoms that are more severe.

The study — published in the International Journal of Epidemiology last summer and which has not yet been peer-reviewed —  found that patients who had tested positive for both viruses were more likely to require hospitalization than those who had tested positive for only one of the viruses.

"We can expect harmful effects from being infected by both viruses simultaneously," Boivin said.

'Healthy skepticism' required

Dr. Matthew Oughton, an infectious diseases specialist at Montreal's Jewish General Hospital, says the study in itself isn't broad enough to be able to predict the outcome of the sixth wave.

"There are some significant limitations before you could extrapolate this data toward a real-world setting," Oughton said, noting the researchers used only one strain of each virus, and that the strain of influenza used to infect the nasal cells in the lab, H1N1, was not the same as the one that has since come into circulation in Canada, H3N2. 

He agreed with Boivin that the immunity in the population against that strain of influenza has probably weakened since the flu vaccination campaign.

But Oughton also noted that the warmer, moister weather may decrease the ease with which respiratory viruses circulate. 

"There's a lot of different factors happening at the same time. All this to say, to me, it means you have to be very careful in making predictions; you have to make predictions with a healthy skepticism," he said.

All three infectious diseases specialists say loosening pandemic restrictions may be a factor in the unusually late flu epidemic.

While COVID-19 has been considered more contagious and more dangerous for much of the pandemic, the flu can kill hundreds of people per year. In 2018, more than 1,000 Quebecers died after being infected with the virus. 

"The mortality rates are starting to look a lot more similar than they were earlier in the pandemic, when it was quite clear that SARS-CoV-2 was causing much higher mortality than we normally observe with seasonal influenza," said Miller. 

"During a pandemic, the mortality rate of a virus is generally intrinsically different from when it becomes endemic."

In a news conference last Thursday, interim public health director Dr. Luc Boileau said people with flu symptoms should take the same precautions as they would for COVID-19 — including isolating for five days after the onset of symptoms, wearing a mask to minimize contaminating others, and avoiding public gatherings and people at increased risk for the following five days.