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.

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.

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.