Government researchers say tests using diluted smallpox vaccine were "very successful." In a study sponsored by the National Institute of Allergy and Infectious Diseases, 650 volunteers were given varying doses of the 1970's era vaccine. Researchers say participants displayed an imune response even though some of the doses had been diluted as much as 10-fold. Currently there are only 15 million doses of smallpox vaccine on hand in the U.S., and it could take up to a year to get the 288 million new doses the government has ordered from a British company. Michael Osterholm is the Director of the University of Minnesota's Center for Infectious Disease Research and Policy. He's also an advisor to Health and Human Services Secretary Tommy Thompson. Osterholm says even though the U.S. vaccine stockpile is at least 30 years old, it's very potent.