Two shots of measles vaccine given during childhood protect a person for life. Four shots of polio vaccine do the same. But flu shots must be taken every year. And even so, they provide less than complete protection.

The reason is that the influenza virus mutates much more rapidly than most other viruses. A person who develops immunity to one strain of the virus is not well protected from a different strain.

That is shaping up to be a major problem as the world prepares for a possible pandemic this fall from the new strain of swine flu. It is impossible to know how many people might die before a vaccine matched to that strain can be manufactured.

But scientists and vaccine manufacturers are hard at work on a so-called universal flu vaccine that would work against all types of flu. The goal is to provide protection for years, if not a whole lifetime, against all seasonal flu strains and pandemic strains, making flu inoculation much more like that for measles and polio.