An article at The Blaze.com explains that there could be several reasons although one of them, the particular strain of virus that you came down with was not included in the vaccine, doesn't apply to this year's breakout:
[W]hy does [the flu] seem to be so virulent this year? The most popular strain identified this season is Type A influenza H3N2, which is historically associated with more serious illnesses. It is also among the strains covered by the flu vaccine.The article goes on to give two other problems with the immunization that may apply in cases where a person got the shot this year but still got ill:
But as Cranston said “no vaccine is 100 percent effective.” One reason is the lag time between vaccination and active immunity. A person who received a flu shot but comes in contact with the virus before the vaccine becomes effective — between 10 days and two weeks — they are still susceptible to becoming ill.So, the lesson seems to be get the shot early next year before you're exposed to the virus and hope that you're among the 59%.
[Moreover] recent studies have shown that the flu vaccine as a whole is only about 59 percent effective at preventing the illness... [T]he vaccine [also] appears to be less effective for the elderly, which is a population often highly encouraged to receive the shot in the first place.
