This winter’s flu season is one of the worst in a decade; it started particularly early, and by January, Boston Mayor Thomas M. Menino had declared a public health emergency. Nationwide, there have been large numbers of hospitalizations and deaths, particularly among those over 65. Our only protection, aside from hand-washing and shunning social interaction, is the flu vaccine.

Still, as we have all heard by now, this season’s flu vaccine is only 62 percent effective (an average of the protection against each of the major strains prevalent this year). While that’s far better than crossing your fingers and hoping for the best, the risk of getting sick even when vaccinated is much greater than with vaccines for diseases such as polio. Why is it so hard to develop and maintain a highly protective vaccine for flu? The reason is, in a word, evolution.