Stamping out fires! Controlling smallpox with targeted mass vaccination

Background. More than 30 years have now passed since the last naturally occurring case of smallpox; however, the variola virus still exists in at least 2 locations. The possibility that any clandestine stocks could be used for bioterrorism is a continuing concern for the public health community. Objective. Mathematical modeling is used to assess the impact of mass vaccination following a smallpox release when either standard public health controls are failing or political/public opinion is urging more comprehensive methods. Two mass vaccination strategies are considered: a blanket nationwide campaign v. an approach targeted only at those geographic areas that experience smallpox cases. The study evaluates which intervention strategy results in the fewest combined disease and vaccine-related deaths. Results. Outbreaks that go unnoticed until up to 50 cases have occurred are optimally controlled with targeted mass vaccination of the affected administrative districts in the majority of scenarios considered. The number of people vaccinated is approximately two thirds fewer than when implementing a nationwide campaign. Similar results arise when contact tracing is either highly unsuccessful or reduced in favor of reallocating limited resources for a policy of mass vaccination. Conclusions. Reactive nationwide mass vaccination remains a suboptimal strategy for controlling an expanding smallpox outbreak in all but the most extreme circumstances. Rather, targeted mass vaccination of affected areas is likely to result in fewer deaths. The vaccines administered are also likely to be much fewer because they would probably be distributed to a much smaller number of districts, thus relieving pressure on potentially stretched public health systems.