Colds mutate even while they're infecting you, and new strains pop up so often that by the time drug-makers create a vaccine against one variation, the serum is already out of date.

即使在感染你的时候，感冒病毒也会发生变异，而且新的病毒毒株经常出现，以至于当制药商研制出针对其中一种变异的疫苗时，血清已经过时了。