25.1 Case Study: Modeling Fractional-Dose Emergency Vaccination Campaigns for Yellow Fever

Abstract

AbstractYellow fever (YF) is a mosquito-borne disease with no specific treatment. The burden of YF was much reduced around the world in the 1950s, but many mosquito control programs were allowed to lapse thereafter, and the World Health Organization (WHO) has been warning for decades that explosive outbreaks of urban YF were likely. YF resurged and spread widely in urban Angola in late 2015, then to Kenya, China, and the Democratic Republic of the Congo (DRC). The existing WHO YF vaccine stockpile was insufficient, and WHO proposed using a fractional dose (one-fifth of that previously used) against the spreading epidemic. We used mathematical modeling to assess the impact of potentially reduced vaccine efficacy with fractional dosing on the infection attack rate. Our rapid risk assessment model showed that the proposed WHO dose-sparing strategy for the YF vaccination campaign in Kinshasa would be robust and effective and would prevent many more infections than using the available vaccine at standard dosage, even with a large margin for error in case fivefold fractional-dose vaccine efficacy turned out to be lower than expected. WHO implemented the strategy in August 2016, and subsequent studies found it to be a viable control strategy, with possible implications for other situations of vaccine shortage.