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.