So here is the deal with flu shots... I believe they generally take the 3 to 5 nastiest and most prolific strains of the current season, and then start pumping out the antibodies which I won't go into. But that process takes almost 6 months I believe. In the meantime the flu has gone down south to Australia, Africa, south america... then the virus migrates back to the northern hemisphere.
During this migration it can mutate, and when there is a sever mutation that's generally when people get super sick because the antibodies that defend against it may not pick it up in your system.
Now there is Flu A and Flu B. Flu B is only found in humans and is generally the more mild. Flu A flip flops around between humans, birds, and pigs. Since Flu A can go between species it tends to get bigger mutations... it can jump into birds, mutate and then infect people (bird flu) or go to pigs and then jump back to people (swine flu). With the big interspecies mutations the flu shot can be much less effective, but depends what variety you get as there are usually 5+ different active viruses in circulation.
So really it's all an odds game on what all has changed and what you are exposed to vs what you were immunized against.
