Although vaccination was responsible for the dramatic drop in smallpox deaths during the first decades of the twentieth century, its success was at least in part due to the use of vaccination in conjunction with public health strategies of surveillance and containment. Even compared to mass vaccination, surveillance and containment are thought to have provided the more effective means of controlling the spread of smallpox disease (Fenner et al., 1988).
Epidemiologic study of the spread of smallpox in Pakistan and Bangladesh in the 1960s demonstrated that the disease was not widely disseminated, but occurred in clusters, transmitted through close personal contact (Fenner et al., 1988). To cope most effectively with this type of disease distribution, smallpox eradication teams emphasized the identification of cases and the containment of outbreaks, a strategy termed surveillance and containment. A critical component of this strategy was the program known as ring vaccination. When a smallpox case was identified, all immediate contacts and their households were identified and vaccinated (ACIP, 2002). Any individuals who then developed a fever were isolated. In this way, an initial case was effectively surrounded with (or ringed by) vaccinated individuals, virtually stopping transmission to others in the population (CIDRAP and IDSA, 2004). Historic evidence also suggests that surveillance and containment worked well not only in populations with a high level of immunity, but also in areas where population immunity was relatively low (e.g., due to lack of vaccination) (IOM, 1999).
A smallpox release in today’s world would present new clinical and epidemiologic challenges. For example, a significant proportion of the population in the United States (most individuals born after 1972) has never been vaccinated against smallpox. This means that there is little or no herd immunity, and previously identified patterns of disease spread may not apply (Gani and Leach, 2001). Furthermore, the current population includes more very elderly people and individuals with immune systems impaired due to chemotherapy, preparation for organ transplantation, or HIV infection.
Vaccination strategies that were successful in the past might be less successful in the contemporary context. Ring vaccination that was an effective means of controlling disease transmission among developing country populations that may have been significantly less mobile may not work for today’s highly mobile populations. There are further concerns about ways in which the deliberate introduction of smallpox virus could differ from