Perhaps the most important model parameters incompletely considered in this work are the social networks and contact processes that dictate disease transmission patterns. Clearly, there is a trade-off between the inclusion of a large degree of detail and heterogeneity in the social structure in a model and the complexity of the resultant model (Ferguson et al., 2003). We have included the level of social detail that we believed necessary to capture the transmission dynamics of smallpox. Although we explicitly modeled person-to-person contacts in hospitals, households, schools, and workplaces, our representations of these social units were admittedly crude. Although we addressed a range of model parameterizations and model structures, a larger sensitivity analysis may reveal surprising results. In future work, we will continue to examine the sensitivity of our results to specific model parameters.

Another limitation of this work is not the model itself but its proper interpretation and use. We caution that the numbers of cases generated in various scenarios should not be taken as quantitative predictions, but instead as a basis for comparing and evaluating different intervention strategies. We also note that in this exercise we modeled only a single geographically confined attack on a relatively small discrete social unit (6,000- or 50,000-person town). In the event of a real smallpox attack, response strategies would have to consider larger social networks and possible repeated introductions over a wide geographic area.

Conclusions

Our simulation exercise revealed that contact tracing and vaccination of household, workplace, and school contacts, along with prompt reactive vaccination of hospital workers and isolation of diagnosed cases, could contain smallpox at both epidemic scales examined. Individual-based simulations of smallpox epidemics provide a valuable tool in crafting policy regarding outbreak response.

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