Some 90 percent of people vaccinated for smallpox 25 to 75
years ago are still protected against the disease. Although
it was formerly thought that smallpox vaccinations were only
effective for three to five years, researchers found that
significant immunity levels last for many decades and -- possibly
for an entire lifetime.
Further, repeated vaccinations provide a boost in immunity
in the short term, however they do not result in higher levels
of protection over the long term. Repeated vaccinations are
no more effective than a single vaccination in the long term.
Nature
Medicine September, 2003
