Each year, the arrival of cold weather brings with it two constants: holiday cheer and the flu vaccine. Chances are you’ve seen one advertisement or another around Toronto urging residents to get their annual flu shot.
The growing number of flu cases around the country this season are bringing the contents of the flu shot — and their effectiveness — into question.
Cases of influenza A and B, the two main strains of the virus, have been rising across Canada. Influenza B cases appeared earlier this season and in greater numbers than seen in previous years. The most common strains of influenza A are H1N1 and H3N2. The majority of influenza cases are being caused by the H3N2 virus strain, which is included in the current flu vaccine.
According to Dr. Allison McGeer, a professor at the Dalla Lana School of Public Health and Director of Infection Control at Mount Sinai Hospital, flu vaccines work better against the H1N1 strain than H3N2 because the H3N2 virus evolves more quickly.
Flu vaccines contain small amounts of egg, which is used to grow the virus. With the H3N2 virus, the egg base used to produce the vaccine may have caused minor mutations that reduced the vaccine’s overall effectiveness.
Similar cases have been reported around the world. In fact, a study in The New England Journal of Medicine found that the flu vaccine in Australia was only 10 per cent effective against the H3N2 strain.
McGeer advised that data used to determine the Australian flu vaccine’s efficacy needed to be interpreted carefully. “Every estimate of vaccine efficacy comes with what are called confidence limits. Their confidence limits go as high as 45 per cent, so really what they’re saying is that in their estimate, the efficacy ranged somewhere between zero and 45 per cent.”
McGreer also noted that more time is needed to make any definite conclusions about the flu vaccine’s effectiveness. In the future, McGeer said that researchers will start to grow viral strains in cell cultures instead of in egg bases, because egg bases tend to induce small mutations and take more time to grow.
Despite the inadequacies of this year’s flu vaccine, McGeer highly recommended getting it.
McGeer recognized that some people may decide against getting vaccinated because they question its effectiveness. “So, let me ask you a question, if I offered you an intervention… that reduced your chances of being in a car accident by 50 per cent, would you take it?”
