Why flu shots are a lottery

As a rash of measles outbreaks spawns controversy in America, a badly performing flu vaccine is also causing concern. This flu season, the influenza vaccine proved only 23% effective. Last year, it was about 60% effective.

Why did the flu shot perform so poorly this season?

Flu vaccines show the immune system three or four antigens, proteins that mimic those on the surface of viruses. The immune system is then primed to act when it encounters the real thing. There is only one antigenic type of measles virus, which means formulating a vaccination is relatively straightforward. But the flu virus is a different beast.

Every winter hundreds of different strains emerge. Since last October alone, The Centre for Disease Control and Prevention has identified more than 800 different influenza viruses. To figure out which specific strains to target, more than 100 countries study influenza trends and collaborate with the World Health Organization. Then, based on forecasts and a bit of luck, each country decides which strains to include in the next vaccination. They only select three or four out of hundreds.

Production of the new vaccine begins, but it takes time. Companies require at least six months to manufacture enough vaccines for the population. During that period, the flu virus may change. Genetic mutations can alter the virus’ surface proteins. If a strain becomes different enough from the original virus, the vaccine is no longer a match.

This season’s most common flu is a particular strain of the H3N2 virus. At a Congressional hearing into why this year’s flu vaccine formula was so far off the mark, officials explained that by the time they realized that the current vaccine didn’t include protection against the new and increasingly predominant version of the H3N2 virus, it was too late.

But even a low probability of working is better than none. Meanwhile, scientists are working on a way to make a more effective vaccine. They are hoping to find antigens that correspond to the parts of the virus that stay the same, even when the rest of the virus doesn't. Such a universal vaccine could protect against any variant of the flu virus and would become a one shot deal.