PORTLAND, Ore. Final results of a smallpox vaccine study conducted by researchers at Oregon Health & Science University show America's preparedness for a smallpox outbreak may be greater than initially thought. The research shows 90 percent of those vaccinated 25 to 75 years ago maintain a substantial level of immunity. In addition, researchers concluded that in the long term, repeated vaccinations do not result in a higher level of disease protection. The research project is the largest of its kind ever conducted. The study is printed in the September edition of Nature Medicine.

"Previously, it had been widely accepted that smallpox virus effectiveness lasts only 3 to 5 years," said study principal investigator Mark Slifka, Ph.D., a scientist at the OHSU Vaccine and Gene Therapy Institute. "This research shows that significant immunity levels last for many decades, perhaps throughout a person's entire life. It also shows that repeated vaccinations provide a short-term boost in immunity but, over time, do not create a sustained higher level of protection compared to those persons vaccinated only once."

To conduct the research, OHSU enlisted the help of 332 study participants. Of this larger group, 306 participants had received at least one vaccination within their lifetime, some had undergone as many as 14 inoculations. The timing of vaccinations also varied among study volunteers. Some participants had been vaccinated as recently as one month prior to testing and as long ago as 75 years. The remaining 26 participants in the study had never received a smallpox vaccination in their lifetime and served as control subjects. The study group was very diverse; volunteers included those vaccinated in 43 states and 34 foreign countries.

"Some of our study participants had received repeated vaccinations. In one case, a person who at one time worked in a smallpox hospital had been vaccinated 14 times over their lifetime ," explained Slifka. "While man
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Contact: Jim Newman
newmanj@ohsu.edu
503-494-8231
Oregon Health & Science University
17-Aug-2003

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