Impact of dengue vaccination choice on Zika risk: free riders and the tragedy of the commons

Abstract

Dengue vaccination, long in development, has become controversial as it may cause antibody-dependent enhancement (ADE) in dengue-seronegatives. Under partial vaccine failure, ADE increases case severity and may also affect Zika infections since the two viruses are closely related.
From an individual perspective, the vaccination of others appears beneficial, but becoming vaccinated oneself may increase the risk of ADE and thus serious illness, for both diseases. From a population-level perspective, vaccination is expected to reduce the spread of dengue but increase Zika incidence. Nevertheless, prior mathematical modeling research has shown that in some cases, a small number of dengue vaccinations may reduce the final size of a Zika outbreak despite increasing its ability to spread. This study reconciles these results and then evaluates individual risks to both the vaccinated and the unvaccinated in order to connect to broader themes in complex vaccination decisions, such as free riders and the tragedy of the commons. A substantial new finding is that a dual outbreak may change which vaccination decision minimizes risk, compared to single-outbreak scenarios.