Diluting smallpox vaccine that is more than 50 years old is almost 100 percent effective, which means the current U.S. stockpile can be expanded if needed, a new study found.Researchers at Vanderbilt University School of Medicine in Nashville, Tenn., injected 340 healthy volunteers with three different strengths of the Aventis Pasteur smallpox vaccine (APSV) -- undiluted, one-to-five dilution, or one-to-10 dilution.The vaccine was originally manufactured in the 1950s and stored as a frozen preparation.
