Scientific American Online. 2003 Mar 3.
Excerpt from Introduction: "Edward Jenner, the English physician who first developed the smallpox vaccine in 1796, believed that vaccination caused a fundamental change in personal constitution and would lead to lifelong immunity to smallpox. Unfortunately, this proved to be incorrect. It is now clear that immunity wanes over time. Exactly how long the vaccine confers protection, however, is difficult to assess.
"Immunity to smallpox is believed to rest on the development of neutralizing antibodies, levels of which decline five to 10 years after vaccination. This has never been satisfactorily determined, though. And because smallpox has been eradicated in the wild, correlating antibody levels with susceptibility is not possible. Revisiting historical data is difficult because of incomplete information in a number of areas. These include how many times the subjects were vaccinated (revaccination produces longer-lasting immunity), whether the vaccinations were carried out successfully and whether or not subjects ever had a subclinical smallpox infection that would boost their immunity (this situation is particularly likely in endemic areas). The last natural smallpox infection occurred in 1977, so recent advances in immunology and medical testing cannot be brought to bear on this question."
Note: Full article available on publisher's website.