IIRC, there are 3(?) strains of influenza active currently. If they mis-guessed on what even one strain would be, that drops the effectiveness level significantly right there. I'll take an extra 60% chance over not having that extra protection. The CDC is currently recommending everyone
over 6 months of age (who does not have serious contraindications) get the flu vaccine.
I get the influenza vaccine yearly. My side effects have been limited to an achy arm. That is worth a reduced chance of contracting the virus, a reduced chance of passing it on to any high-risk groups, and being able to work with our patients without having to wear a mask during flu season.
My husband contracted it a few years back and was utterly laid out by it. He was not vaccinated; I was. I didn't contract it and was able to take care of him, go to work like normal, and so on.
Edit: CDC data on active strains: http://www.cdc.gov/flu/weekly/#51
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So that's about 20% Influenza B, 80% Influenza A, with half of the latter being H3N2, about 1% the H1N1 strain from 2009, and the rest not subtyped. More edit: But there are different varieties of B and H3N2, to the best of my recollection, so you need to hope for good match there. I'd assume they did match the H1N1 correctly since it appears to be the most recent strain.