The smallpox virus, while a formidable historic threat to health, was eradicated as a result of characteristics of the smallpox virus, the disease, and the vaccine. These characteristics include: a highly effective and very stable vaccine, a noninfectious incubation stage and a disabling prodromal stage that limited the mobility of infected individuals, a distinctive rash that made smallpox cases readily identifiable and helped to facilitate limiting the spread of the disease, and the fact that humans are the only known reservoir for variola virus (IOM, 1999).
At the time smallpox was endemic in much of the world, smallpox vaccination proved to be highly effective in preventing smallpox infection, and in the rare cases where symptoms of the disease occurred, they were milder, and the disease was far less likely to be fatal. In addition to vaccination’s prophylactic value, there is historic evidence that administering the vaccine within three days of a suspected exposure to smallpox virus can prevent the onset of the disease or significantly lessen its severity (Breman and Henderson, 2002; Lane and Goldstein, 2003). Although the smallpox vaccine is very effective, it was its use in conjunction with surveillance and containment that ultimately brought the disease under control and culminated in the eradication of the disease (Fenner et al., 1988).
Experience documented during the global smallpox eradication campaign has shown that smallpox vaccine is highly effective, but its efficacy has not been measured with precision in controlled studies (CDC, 2003a). The Dryvax® vaccine (used in the vaccination campaign begun in 2003) was used successfully to eradicate smallpox in West and Central Africa and other areas during the global campaign. The scar showing previous vaccination signified that an individual was protected against smallpox, and in household contact studies, there was a 90 percent reduction in smallpox among contacts with a vaccine scar, compared to those without (CDC, 2003a).
The need for vaccination disappeared along with the disease itself. By 2002, some Americans had not been vaccinated against smallpox in over three decades, and the remainder had never been vaccinated. It is unclear what level of vaccine-induced immunity remains in previously vaccinated Americans; past evidence on the efficacy and durability of protection provided by vaccination is limited (Henderson, 1988). According to one estimate, fewer than 20 percent of persons vaccinated before 1972 retain immunologic protection (CIDRAP and IDSA, 2004). Other twentieth-century data show that vaccinated individuals have a high level of protection for up