The CDC has just issued an advisory stating that the flu vaccine for this season is not a good “match” for the strains that are circulating this flu season. This means that if you received the flu vaccine this year, you will probably not be protected from this season’s flu strain, even though you got vaccinated.
Every year, researchers attempt to predict the flu strain that will be prevalent in the upcoming winter months, by flying to Asia before the winter to test flu strains. If the strains mutate and the vaccine is already prepared and distributed (and people receive the vaccine), then the vaccine can be rendered almost ineffective. This occurs if the mutations were faster than the production of the vaccine.