Smallpox Bioterrorism Vaccination Policy Decision Analysis Using Computer Modeling and Simulation

Abstract

ISSUE: With the threat of smallpox bioterrorism, health officials face the decision of pre-emptive or post-release vaccination. One argument is that preemptive vaccination can reduce the severity of outbreaks. However, the possibility of adverse and even fatal side effects plus the limited immunization period of smallpox vaccines (typically 3–5 years) make the evaluation of the post-release policy a necessity. A computer simulation model was developed to compare the viral infection rates for the two policies. PROJECT: A graphical finite-state computer simulation model was developed to simulate the deliberate release and spread of the smallpox virus through random human roaming and “chance encounters” in a typical urban setting. The model took into consideration realistic smallpox biological factors (such as disease infectiousness during the incubation, prodromal, and symptomatic periods) as well as social/spatial structure and demographic characteristics. Simulation runs were conducted to compare preemptive and post-release vaccination scenarios over various mass vaccination coverage levels (vaccinating from 0–100% of the population). In all post-release scenarios, vaccination took place upon the earliest detectable onset of an outbreak. RESULTS: Simulation results indicated that preemptive vaccinations showed no benefit over the post-release vaccination strategy when the mass vaccination coverage levels were below 70%. Above 70%, the preemptive vaccination strategy achieved an exponential growth in effectiveness and significantly improved the control performance (e.g., the simulation study found that preemptive vaccination at 75% coverage level equated to that of 95% for the post-release case). In order for the infection to be confined to a small percentage of the population (i.e., below 5%), it is necessary that over 70% of the population be vaccinated for the preemptive case and over 90% for the post-release strategy (when no other outbreak control measures were taken simultaneously). LESSONS LEARNED: Unless mass vaccination at a coverage level of 70% or above is an achievable goal, the study shows that it is best to invest resources in the early detection and rapid rollout of a post-release vaccination program. Preemptive vaccination could be an effective outbreak-control mechanism, but only when there is a substantial vaccinated population base to effectively curb the roaming bioterrorist contagion. The developed computer model proved valuable for supporting health policy decisions. Its application can be extended to analyze the control policies for other epidemics.