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 during May-July following the long rains season (March-May). Projections under global change scenarios suggested that disease risk will generally decrease over time. Nevertheless, owing to expected human population growth, we estimate that > 90 million people in the study region will be exposed to interepidemic RVF by 2061-2080, which is nearly double the historical (1970-2000) estimate of approximately 49 million people. Mitigating the future health impacts of RVF will require increased disease surveillance, prevention and control effort in risk hotspots.