Canada could face ‘worst kind’ of flu season as experts warn evolving strain may be mismatch for vaccine

With flu cases now rising in Canada, medical experts are bracing for a difficult influenza season linked to the global spread of an evolving H3N2 strain that could be a mismatch for this year’s vaccine.

New federal data out Friday shows roughly two per cent of country-wide tests came back positive for influenza in the previous week. That’s still shy of the five per cent bar for Canada to declare a seasonal flu epidemic, but it’s a noticeable uptick from a few weeks before.

The ongoing flu season abroad has been marked by record case counts in the southern hemisphere, and an early start to the season across parts of Asia and the U.K. As Canada heads into the winter, it could be a bellwether of what’s to come.

“It’s the second year in a row where they’ve had above-average influenza detections in the southern hemisphere,” said Dr. Jesse Papenburg, a pediatric infectious disease specialist at the Montreal Children’s Hospital and the McGill University Health Centre.

“In terms of the number of cases, last year was probably the worst influenza season Canada had had in a decade, and it seems like that could occur again this year in terms of severity of disease.”

In Australia, physicians recently sounded the alarm after more than 410,000 lab-confirmed cases across the country, marking the second record high flu year in a row.

In late October, U.K. health officials announced its flu season had arrived more than a month earlier than usual, with cases three times higher than last year, amid expectations of a “long and drawn-out flu season.”

The flu has arrived early in parts of Asia, too, including Japan, which has already declared a flu epidemic and closed down many of the country’s schools, U.K. officials said.

There’s speculation that a mutating type of H3N2 is behind that early surge. It’s a strain of influenza A that’s typically known for more severe infections, especially among older people. But what’s particularly troubling some experts this year is that those latest mutations are widening the gap between this virus and our available flu shot.

Dr. Danuta Skowronski, epidemiology lead for influenza and emerging respiratory pathogens at the B.C. Centre for Disease Control, is among the Canadian researchers watching these mutations closely. 

While this H3N2 subtype hasn’t changed much in several years, she says it recently started showing more dramatic structural changes as it spilled into northern countries, which could mean it’s likely “mismatched” to our latest vaccine.

This season’s flu shot targets two influenza A strains — other subtypes of H1N1 and H3N2 — plus an influenza B strain. 

“The difference between the circulating H3N2 virus and what’s in the vaccine is quite a bit different, actually, and that’s no one’s fault. It’s just the nature of influenza. It’s a very changeable virus,” Skowronski said.

Meanwhile Canadian public health officials aren’t making any predictions just yet.

In a statement to CBC News, the Public Health Agency of Canada said it monitors respiratory virus activity globally to detect emerging trends that could impact Canada. 

However, the agency stressed that published research suggests data from the southern hemisphere is not a “direct predictor” of what could happen here, due to different seasonal dynamics. 

“At this time, it is too early to determine what kind of flu season Canada will experience; that will depend on such things as dominant influenza subtype, co-circulation of other respiratory virus, vaccine effectiveness, and vaccine coverage.”

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Though there’s still only early data on what forms of influenza are circulating in Canada right now, the latest federal figures show a 50/50 split between H1N1 and H3N2 among the influenza A subtypes detected across the country in the last week — but all the outside experts interviewed by CBC News expect H3N2 will continue to rise.

“We’re more likely to see a dominant epidemic due to that mutated virus, and of course, that’s what they are seeing elsewhere in the U.K., in Japan,” said Skowronski. “We may have a different profile, but I think it’s better to prepare for the fact that this is likely coming — and we have already picked it up.”

School-aged children are being struck first in many countries, but eventually the virus will wind its way to adults who are more at risk for severe outcomes, she added. (So far, the most flu cases in Canada are already among older adults aged 65 and up, the latest figures show.)

Dr. Allison McGeer, a researcher and infectious disease specialist with Sinai Health System in Toronto, warned H3N2 seasons are typically the “worst kind of flu season.” 

Adults in their 60s and older are usually hit hardest, echoed Papenburg. “So if it’s an H3N2 season, we might expect to see more hospitalizations and, unfortunately, more deaths.”

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While this year’s shot may not be perfectly matched to the newer form of H3N2 that could dominate in the months ahead, all three medical experts stressed it’s always worth getting, particularly for Canadian seniors.

Seasonal immunization clinics are already open across multiple provinces. 

Health officials are calling on Canadians to consider getting a flu shot alongside other seasonal vaccines, including COVID-19 vaccines for those eligible — though some patients now have to pay out of pocket in certain parts of the country, such as Quebec and Alberta— and respiratory syncytial virus shots, which can prevent serious illness in high-risk groups such as elderly adults and newborns.