A study, published in The American Journal of Medicine, found that in the event of a smallpox bioterrorist attack, the smallpox vaccine should be used first on individuals who have not been vaccinated previously.

Researchers at the National Institute on Aging, the National Institute of Allergy and Infectious Diseases and National Institutes of Health examined 246 participants of the Baltimore Longitudinal Study of Aging.

In the sample, 209 subjects were vaccinated one or more times 13 to 88 years prior to the study; an additional 18 had had childhood smallpox and 29 had no history of vaccination or smallpox.

"We found that vaccinated subjects maintain what appear to be protective levels of neutralizing antibodies indefinitely and do not require booster vaccinations even if they are many decades removed from primary vaccination," Dr. Dan L. Longo of the National Institute on Aging said in a statement.

"These data imply that limited supplies of vaccine can be more usefully applied to individuals who have never been vaccinated -- primarily individuals born after 1972."