Drastic drop in influenza virus likely result of less international travel: experts

·4 min read

Whether it's a slight cough or a scratchy, sore throat, some may be tempted to dismiss mild symptoms as "just the flu" amid a serious global pandemic.

But experts say a drastic drop in the circulation of the influenza virus this season means signs of flu are more likely to be COVID-19 than another respiratory virus.

A FluWatch report from the Public Health Agency of Canada (PHAC) released last week shows laboratory-confirmed incidents of flu are exceptionally rare this season, despite "elevated testing" for it during the pandemic.

Experts say a confluence of factors are playing a role in the abnormally light flu season, including public health measures aimed at curbing the spread of COVID-19 and the reduction of international travel.

Dr. Sumon Chakrabarti, an infectious disease expert in Mississauga, Ont., says the low prevalence of flu underscores the need to get tested for COVID if people develop symptoms.

"You can't tell by looking if somebody has influenza or COVID," he said. "And right now, depending on where they live, if someone has acute viral symptoms, the chances of it being COVID over other things is much higher."

PHAC's report shows there have been 51 influenza detections in Canada to date this flu season — significantly lower than the nearly 15,000 cases averaged by this point in the past six seasons — and there were zero lab-detected cases (from 13,000 tests) over the first week of 2021.

Chakrabarti expects there to be more cases of influenza than what PHAC's data shows, since not everyone with flu-like symptoms is tested for that virus. But in the segment of the population that is getting tested — typically older adults seeking medical care — influenza isn't coming up.

People admitted to hospital with symptoms are given respiratory multiplex tests that can detect multiple viruses at once, Chakrabarti said.

"And we've picked up very little in the way of other viruses. So if you're seeing a reduction in those cases, it suggests that the overall amount of flu in the community has dropped."

While experts assumed public health measures like mask-wearing and physical distancing would also lessen flu prevalence, the level of drop-off has been surprising, says Dr. Zain Chagla, an infectious disease specialist with McMaster University.

He believes travel restrictions have likely played a significant role.

Whereas COVID-19 can continue to spread easily because the virus is already entrenched here, Chagla says influenza is usually brought in each winter from tropical climates. A population confined largely indoors due to cold weather helps it spread.

"Border restrictions, quarantine rules, that probably limits the amount of influenza coming in in the first place," Chagla said. "And the odd case that does come in, it's harder to spread because people aren't congregating."

Raywat Deonandan, an epidemiologist with the University of Ottawa, agrees that a reduction in international travel likely explains the light flu season more than just the implementation of public health measures. He says places in South America are also seeing dips in flu numbers even though mask-wearing hasn't been as widespread there.

A level of immunity to influenza may also be contributing to the stifling of the virus, he added.

"More people got a flu vaccine this year," Deonandan said. "That can't be underestimated."

Chagla says other respiratory viruses also seem to have decreased this season. While there was an uptick in the common cold rhinovirus in the fall — usually correlated with children going back to school — PHAC data shows it's been dropping since.

Hand-washing and sanitizing high-touch areas may be playing a role in controlling viruses that are more transmissible on surfaces, experts say.

Chagla says cold or flu-like symptoms should raise a red flag for anyone right now, and he worries about people mistaking COVID signs for another virus.

"In years past you could say: 'this is just a cold,' doctors would say: 'don't even come in,'" Chagla said. "And now we have to switch the mentality to say: 'actually, no, go get tested.'"

Chakrabarti warns the "just the flu" mentality also diminishes the significance of influenza, which can lead to serious disease in vulnerable people too.

So there's need for caution, even if symptoms are from the flu virus.

"A lot of people say 'it's the flu, who cares? I get it all the time,'" he said. "This is going to sound familiar, but the reason it matters is because you can spread it to somebody else."

This report by The Canadian Press was first published Jan. 19, 2021.

Melissa Couto Zuber, The Canadian Press