Until a few years ago, smallpox was pretty low on the list of things to worry about. (Behind "return of acid-washed jeans" but still slightly ahead of "alien invasion.") But then came the anthrax attacks, and the U.S. government started trying to figure out how to vaccinate people for smallpox—years after the disease was supposedly eradicated—with only 85 million doses of vaccine. One possibility was diluting the available vaccine so it would stretch further.

What the researchers wanted to know: Does smallpox vaccine work if it's diluted?

What they did: Three hundred forty adults ages 18 to 32 who'd never been vaccinated for smallpox were randomly assigned to get full-strength vaccine, 20 percent vaccine (diluted with water and glycerine), or 10 percent vaccine. They came back every few days to get their bandage changed; if a blister or pustule appeared under the bandage six to 11 days after vaccination, the vaccine had worked.

What they found: Almost all volunteers at all three dilutions developed a mark indicating the vaccine worked; there was no significant difference between the dilutions. Blood tests on 140 of the volunteers whose vaccinations "took" showed that all but one had antibodies to smallpox in their bloodstream.

What the study means to you: The smallpox stores do seem to be plenty for vaccinating the entire U.S. population in case there's an outbreak.

Caveats: Because the smallpox vaccine is a live virus, it can make you sick. In this study, a few volunteers had heart episodes that may or may not have been related to the vaccine, and one needed hospitalization a week after vaccination.

Find out more: An overview of the smallpox vaccine from the Centers for Disease Control and Prevention

Read the article: Talbot, T.R., et al. "Vaccination Success Rate and Reaction Profile With Diluted and Undiluted Smallpox Vaccine." Journal of the American Medical Association. Sept. 8, 2004, Vol. 292, No. 10, pp. 1205-121.

Abstract online: http://jama.ama-assn.org