Modeling Ring-Vaccination Strategies to Control Ebola Virus Disease Epidemics

Abstract

The 2013-15 Ebola epidemic that primarily affected Guinea, Sierra Leone and Liberia has become the most devastating Ebola epidemic in history [1]. This unprecedented epidemic appears to have stemmed from a single spillover event in South Guinea in December 2013 and rapidly spread to neighboring Sierra Leone and Guinea in a matter of weeks. Here we employ a network-based transmission model to evaluate the potential impact of reactive ring-vaccination strategies in the context of the Ebola epidemic in West Africa. We model ring-based vaccination strategies that incorporate the radius of contacts that are vaccinated for each infectious individual, the time elapsed from individual infectiousness to vaccinating susceptible and exposed contacts, and the number of available vaccine doses. Our baseline spatial transmission model in which the ring vaccination strategy is investigated has been previously shown to capture Ebola-like epidemics characterized by an initial phase of sub-exponential epidemic growth. Here we also extend this baseline model to account for heterogeneous community transmission rates that may be defined as a scalable function of the distance between an infectious individual and each member of that individual’s community. Overall, our findings indicate that reactive ring-vaccination strategies can effectively mitigate established Ebola epidemics. Importantly, we studied scenarios with varying number of weeks elapsed between the onset of symptoms and the day contacts are vaccinated and found that it is still beneficial to vaccinate contacts after the infectious period has elapsed. Our results indicate that while it is beneficial to vaccinate members of the community, the probability of extinction is not very sensitive to which contacts in the community are vaccinated unless transmission varies very steeply on the network distance between individuals. Both of these observations underscore the fact that vaccination can be effective by reducing transmission at the community level.