TORONTO -- A new Canadian study suggests this year's flu shot offered good protection to people who received it.
The study says the shot was 74 per cent effective at preventing flu so severe that people needed medical treatment.
That's substantially better than last year, when the same researchers found the vaccine only cut cases of medically attended influenza by about 40 per cent.
To date this year the H1N1 strain of influenza is causing the vast majority of flu infections in Canada. Last year a different virus, H3N2, was responsible for most flu illness.
The two viruses target different age groups, with this year's outbreak hitting young and middle-aged adults particularly hard.
That pattern of illness -- highlighted in media reports of previously healthy adults dying from flu -- triggered huge demand for vaccine in Canada in January, a time when influenza vaccination efforts are typically winding down.
Many provinces ended up scrambling to buy additional doses to meet the unexpected demand.
This study on the vaccine's effectiveness is an interim assessment. The researchers are continuing to gather data and will issue a final analysis after the end of the flu season.
The work was led by Dr. Danuta Skowronski at the British Columbia Centre for Disease Control. It is based on data drawn from Canada's five most populous provinces: Ontario, B.C., Quebec, Alberta and Manitoba.
The findings were published in Eurosurveillance, an online public health journal belonging to the European Centre for Disease Prevention and Control.