"Recent studies have shown that the flu vaccine as a whole is only about 59 percent effective at preventing the illness. NRP pointed out that the vaccine appears to be less effective for the elderly, which is a population often highly encouraged to receive the shot in the first place."
"Another reason someone might contract the flu even after receiving vaccination is if the strain they were infected with wasn’t included in the vaccine in the first place. "
http://www.theblaze.com/stories/2013...flu-heres-why/