In 1991, ACIP updated its recommendations regarding the use of smallpox vaccine for occupational exposures to include “health-care workers involved in clinical trials using recombinant vaccinia virus vaccines” and lengthened to 10 years the recommendation for revaccination of relevant groups of laboratory workers (CDC, 2001). A series of domestic and international terrorist attacks occurred over the decade that followed, ranging from sarin gas attacks on the Tokyo subway to anthrax attacks by mail in the United States. Those developments stimulated robust discussion of the need for new public health policy and legislation to confront the possibility of bioterrorist attack. When the 1991 ACIP recommendations were updated in June 2001, they included “recommendations for the use of vaccinia vaccine if smallpox (variola) virus were used as an agent of biological terrorism or if a smallpox outbreak were to occur for another unforeseen reason” (CDC, 2001). ACIP concluded that recommendations regarding pre-exposure vaccination should be on the basis of a calculable risk assessment that considers the risk for disease and the benefits and risks regarding vaccination. Because the current risk for exposure is considered low, benefits of vaccination do not outweigh the risk regarding vaccine complications. If the potential for an intentional release of smallpox virus increases later, pre-exposure vaccination might become indicated for selected groups (e.g., medical and public health personnel or laboratory workers) who would have an identified higher risk for exposure because of work-related contact with smallpox patients or infectious materials.

After the events of 2001, the possibility of future bioterrorism and the specter of deliberate exposure to the smallpox virus, a dangerous category A pathogen,² caused CDC to reconsider smallpox vaccination as a tool for preparedness. CDC requested that ACIP provide an update of recommendations for the use of smallpox vaccine. The Department of Health and Human Services (DHHS) began to assess the status of smallpox vaccine stocks and initiated planning and activities for increasing the vaccine stocks, and CDC, the National Institutes of Health, and the Department of Defense intensified their work in the development of new vaccines (such as safer or less reactogenic smallpox vaccines) to prepare effectively for a potential smallpox virus release (Cohen and Marshall, 2001). The Public Health Security and Bioterrorism Preparedness and Response Act of 2002, signed into law on June 12, 2002, included provisions for supporting smallpox vaccine development.