The threat of smallpox resulting from bioterrorism has prompted a reassessment of the current population’s level of immunity. A recent study by researchers in the NIA’s Intramural Research Program  shows that older adults who either had smallpox as children or were vaccinated years ago remain immune to the disease and would be unlikely to benefit from new vaccinations.
Given the limited supply of smallpox vaccine in the United States, it makes sense to immunize people most at risk in the event of smallpox exposure, researchers wrote in the American Journal of Medicine. That population includes individuals born after 1972—about the time when routine smallpox vaccination ended in the United States—but not many older adults, they concluded in a study of 246 participants from the Baltimore Longitudinal Study of Aging .
Older adults who were vaccinated at least once appear to remain immune, according to an analysis of more than 800 serum samples that had been stored for 13 to 88 years after vaccination. The researchers, led by Drs. Dan Longo and Dennis Taub, determined that levels of vaccine-specific antibody titers and neutralizing antibodies remained high enough to indicate continued immunity in 209 individuals who had been vaccinated, based on the comparable levels of their antibodies and those of 8 participants who had had smallpox as children. It is known that survivors of smallpox have lifelong immunity to it. Multiple vaccinations conferred only marginally higher protection than a single one.