A new study suggests smallpox vaccine immunity may last far longer than expected. Scientists had believed that the vaccine generally only conferred protection from the deadly virus for about a decade. But a study released this week found evidence that people may be covered for 35 years or more, meaning many Americans could retain some level of immunity. The study looked at blood samples from laboratory workers who had been immunized in the last five years and those who had been vaccinated up to 35 years earlier. All samples reportedly showed the ability to stimulate an immune response that would suppress the smallpox virus, though there was some drop off in activity over time. The finding could change the nation’s strategy for vaccinating against smallpox in the event of a bioterrorist attack, said the study’s lead author Jeffrey Frelinger, chairman of microbiology and immunology in the University of North Carolina School of Medicine.