The results of a new study suggest that the standard smallpox vaccine confers surprisingly long-lasting immunity. Vaccinations are generally thought to last seven to 10 years, but researchers writing today in the New England Journal of Medicine report that people vaccinated decades ago can still have a strong immune response to the smallpox virus's close cousin, vaccinia.

"It's a question that's been largely unexamined," says study co-author Jeffrey A. Frelinger of the University of North Carolina. To address the question, he and Lawal Garba, also at UNC, tested blood samples from 13 laboratory workers who had received the vaccine because they work with vaccinia virus. Four of the subjects had been vaccinated less than five years ago. The others had been vaccinated between five and 35 years ago or more than 35 years ago. Exposing the samples to vaccinia virus, the researchers tallied the number of so-called CD8 T cells that exhibited the proper immune response--namely, making the protective molecule interferon-gamma.

They found that even the cells of those participants who had received the vaccination more than 35 years ago still produced interferon gamma, albeit it at a slightly lower level. Specifically, whereas 6.5 percent of CD8 cells produced interferon gamma in recently vaccinated individuals, 4.8 percent did so in people who had received their shots more than three decades ago.

"Resistance to vaccinia is waning but not rapidly. It is still substantial," Frelinger observes. "We would think that people even 35 years later would still have substantial resistance to smallpox infection." He adds that this could have important implications for developing a smallpox vaccination strategy. "If you had a limited supply of vaccine," he remarks, "I think you'd want to target predominantly previously unvaccinated individuals."