The flu shot only protects against 3 strains; each year, the CDC makes projections on what strains will be most prevalent (based on what strains have been most prevalent in the southern hemisphere during their winter). There are usually about a dozen strains out there wandering around, and, depending on if the CDC spun the Wheel of Pestilence right, you may be covered, or you may not be. Personally, I've never really noticed any kind of correlation between getting the flu shot or not getting it, and getting the flu or not getting it.