Because of rapid genetic mutation, new strains of the influenza virus are constantly evolving and differ enough from each other that vaccines that protect against one strain don't offer much protection against others. It takes a while to produce and test a new vaccine, however, so upcoming formulas must be based on a prediction of which strains will be in wide circulation a year in advance. These predictions, made by studying epidemiological patterns (which are different for the Northern and Southern hemispheres), have been, historically, amazingly accurate. But this year, the predictions were off for two of the three strains incorporated into the current vaccine, resulting in a real decrease in protection.