Part of the problem was a mismatch between the influenza strains circulating and the vaccine available. Here's how annual flu shots are formulated.
There are many flu strains, and those strains can also change and mutate.
New genetic technologies are letting us look at flu evolution right where it starts: within individual people, while they're sick.
The flu vaccine – which prevents one from getting influenza – changes every year, because it is based on the strains of the virus that presented in the previous year.