Why isn't there a cold vaccine? Each year, multiple strains of the flu are circulating. If we can vaccinate against the most common strains of the flu, it seems as if we should be able to do the same thing for colds. But it doesn't quite work that way. Public health officials cannot predict the spectrum of rhinovirus types for an upcoming cold season. Plus, even if you could, Thomas Smith of UTMB says, "somehow stuff 100 different strains into one shot," that would take care of only the most common cold-causing virus. Colds are not usually life-threatening, so the FDA would have a very low threshold for the kind of side effects that would be considered worth it. "It really had to be nearly as safe as water for approval for the general public," Smith says. Few drugs are. The challenges did not stop there. "Only humans show symptoms of [rhinovirus] infection," Smith says, making it nearly impossible to do any testing between petri dishes and human trials. Even then, researchers would first have to find a rhinovirus that test subjects had not already been exposed to — a difficult task with so many strains circulating every year.