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Smallpox: Study Shows Vaccine Offers Longtime Residual Protection As many as 150 million Americans could still be partially protected by smallpox immunizations they received decades ago, according to a preliminary study released yesterday at a meeting of the American Society for Microbiology in Washington (see GSN, May 12). As the federal government attempts to inoculate health care workers across the country to prepare for the possibility of smallpox bioterrorism, up to 150 million U.S. residents may already have some degree of immunity, USA Today reported. More than 90 percent of people aged 36 to 96 have been vaccinated at least once. Antibodies were present in 90 percent of the 306 people tested in the study, according to researcher Mark Slifka of the Oregon Health and Science University. The level of antibodies was fairly consistent in the subjects, whose vaccinations span as far back as 1928. However, the study showed that the level of white blood cells, also known as T-cells, declined over time. Both antibodies and T-cells are needed for full protection. Eight to 15 years after immunizations the level of white blood cells dropped by half, Slifka said, but “if you begin with very high T-cell levels that could still be a large number.” Slifka said people who have received the vaccination twice show greater immunity, but additional innoculations appear to provide no further protection (Anita Manning, USA Today, May 21).
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