A new Canadian study has found that this year’s flu shot is 74 percent effective at preventing influenza that is severe enough for people to seek medical treatment.  This is significantly better than last year, when the same researchers found that the flu vaccine only cut medically attended flu by about 40 percent.

The H1N1 strain of influenza is causing more than 90 percent of flu cases this year in Canada and the United States.  Last year a different virus, H3N2, caused most flu illness.  This year’s H1N1 virus affects mostly young and middle-aged adults, while last year’s strain was typically harder on seniors.

With media reports highlighting stories of previously healthy adults dying from flu, huge demand for the vaccine was experienced in Canada in January, the time of year when influenza vaccine efforts are typically dying down.  Many provinces had to scramble to meet the unexpected demand.

With this season’s influenza epidemic hitting early, epidemiologists were given a unique opportunity to give an early estimate of just how effective this year’s shot really is.  To determine the effectiveness of the flu vaccine, patients with flu-like symptoms are tested for flu and asked if they received a flu vaccine.  Efficacy means the percentage of people who were vaccinated that did not actually have influenza, despite having flu-like symptoms.

Since the 2009 pandemic, the H1N1 viruses have made up less than 20 percent of seasonal influenza virus detection in Canada. This winter’s strong H1N1 resurgence in North America has led to concerns that the virus may have mutated or that the vaccine is not working.  But the data in the new study suggests neither is true.

The U.S. Centers for Disease Control estimates that flu vaccine cuts the risk of infection by between 50 percent and 70 percent.  This year’s vaccine is at the high end of effectiveness one might expect from a flu shot.