Diluted Smallpox Vaccine Stretches Supply

Results from a key clinical trial indicate that the 15.4 million doses in the existing U.S. supply of smallpox vaccine could successfully be diluted up to five times and retain its potency, effectively expanding the number of individuals it could protect from the contagious disease.

Expanding the smallpox vaccine supply has been a high-priority of the US bioterrorism preparedness plan. In addition to expanding the existing supply of vaccine through dilution, HHS has contracted with a manufacturer to produce another 209 million doses of smallpox vaccine by the end of 2002. The combination of these efforts would bring the total number of doses in the smallpox vaccine stockpile to at least 286 million.

The trial compared the full-strength vaccine with fivefold as well as tenfold dilutions in 680 young adults with no history of smallpox vaccination. More than 97% of all participants in the trial responded with a vaccine take, a blister-like sore at the injection site that serves as an indirect measure of the vaccine's effectiveness. A report describing these findings is to appear in the April 25, 2002, issue of The New England
Journal of Medicine (NEJM).

The public health implications of this study are enormously important,
said Tommy G. Thompson, secretary of US Health and Human Services (HHS). We now know that in the unlikely event of an intentional release of smallpox, our stockpile of smallpox vaccine can be expanded fivefold as we had planned. The success of this study puts us one step closer to our goal of having enough vaccine for every American if needed to respond to a potential outbreak.

Few people have full immunity to the virus anymore because in the absence of any new cases, the United States discontinued routine vaccinations three decades ago, and immunity is thought to wane over time in vaccinated individuals. The smallpox vaccine used in this trial, known as Dryvax, was made by Wyeth Laboratories (Marietta, PA, USA). The vaccine is freeze-dried, live vaccinia virus, a poxvirus related to smallpox virus. Before use, the vaccine is reconstituted with a sterile-water-based diluent. The current vaccine stockpile remains from 1982, when the company stopped making the vaccine. The study was sponsored by the US National Institute of Allergy and Infectious Diseases (NIAID; www.niaid.nih.gov).