Toronto - A study finds that this year's flu shot gave modest protection against influenza A strains in North America, but no protection against the influenza B strain Canadians were most likely to encounter.
The U.S. Centers for Disease Control says overall, the vaccine was about 44 per cent effective against the three types of flu that circulated and caused illness this flu season.
Experts generally estimate that flu shots are between 70 to 90 per cent effective in protecting healthy adults.
But this year two of the three strains in the vaccine didn't match the strains causing the bulk of disease.
The worst mismatch was in the B strain in the vaccine, which showed no efficacy against the influenza B viruses making the rounds.
Though Canada's flu season started out being mainly caused by the H1N1 subtype of influenza A - which the vaccine protected well against - a late surge in influenza B cases made that the main cause of flu illness this year.
