Supplementary material from "Interepidemic Rift Valley fever in East Africa: The recent risk landscape and projected impacts of global change"

Rift Valley fever (RVF) is a zoonotic disease that causes sporadic, multi-country epidemics. However, there is limited understanding of RVF virus circulation during interepidemic periods and the potential impacts of global change on interepidemic RVF. To address these knowledge gaps, we built a predictive model using recent interepidemic RVF outbreak data from Kenya, Tanzania, and Uganda. We then projected interepidemic RVF risk for three future time periods (2021-2040, 2041-2060, 2061-2080) under three global change scenarios representing different trajectories for climate and human population distribution (SSP126, SSP245, SSP370). Our model identified interepidemic RVF risk hotspots in east Kenya, east Tanzania, and southwest Uganda. Hydrology was a major driver of disease risk: hotspots emerged in association with lakes and rivers, and risk peaked May-July following the long rains season (March-May). Projections under global change scenarios suggested that disease risk will generally decrease over time. Nevertheless, due to expected human population growth, we estimate > 90 million people in the study region will be exposed to interepidemic RVF by 2061-2080, nearly double the historical (1970-2000) estimate of ~49 million people. Mitigating the future health impacts of RVF will require increased disease surveillance, prevention, and control effort in risk hotspots.