The seasonal flu vaccine is likely a poor match for the strain of the virus circulating in B.C., offering just 30 to 50 per cent protection — though officials still urge people to get the shot.

So far, it appears H3N2 is the predominant influenza strain making people sick in the province — with three outbreaks at long-term-care homes this fall.

The seasonal flu vaccine is designed to protect against the H3N2 strain — along with H1N1 and influenza B — but it appears it will offer only 30 to 50 per cent protection, said Dr. Perry Kendall, B.C.'s provincial health officer.

In a good year, the protection offered by the seasonal flu vaccine can be 60 per cent or higher, he said.

"It's less than we like," said Kendall, but it is "significantly better protection" than last year, when the flu shot offered nearly zero protection.

That hasn't changed the official advice that anyone over six months old, as well as those at high risk of serious illness, including children, pregnant women and the elderly, should get the shot.

"If I could reduce your chances of having a really nasty illness by 50 per cent, would you take it?" said Kendall.

'Notorious' strain for evading immune system

The mismatch isn't for lack of trying by scientists around the world. But influenza is a difficult virus.

Each year, the World Health Organization makes forecasts which strains are likely to be predominant in the Northern Hemisphere during flu season — in time for drug companies to manufacture the vaccine.

By the time flu season arrives, the fast-mutating, gene-swapping virus may have outfoxed those efforts.

"It's important to emphasize that influenza viruses, particularly H3N2, are really notorious for changing their antigenic makeup ... to try to evade the immune system," said Kendall.

"Until we find a new way of making a vaccine which is consistently effective against all the range of mutations that the H3N2 can come up with, then we're stuck with the vaccine we have."

So far, the strains of flu tested by the National Microbiology Laboratory  in Winnipeg between Sept.1 2014 and Aug. 27, 2015 indicate a better match on the H1N1 and influenza B strains, but they were not appearing as frequently as H3N2 over that period.


To hear the full interview listen to the audio labelled H3N2 flu strain may have outfoxed vaccine makers again with the CBC's Rick Cluff on The Early Edition.

With files from Kiran Dhillon