Estimation of Rift Valley fever virus spillover to humans during the Mayotte 2018-2019 epidemic

Background. Rift Valley fever (RVF) is an emerging, zoonotic, arboviral haemorrhagic fever threatening livestock and humans mainly in Africa. In the absence of a human vaccine, estimating the transmission potential of RVF virus from livestock to humans is key to assessing the impact of livestock disease control measures on preventing human disease. Methods. We combined a unique RVF dataset, with livestock and human surveillance data of the 2018-2019 RVF epidemic in Mayotte, and used them in a mathematical model. Using Bayesian inference, we quantified the transmission amongst livestock and spillover to humans, and we assessed the impact of livestock vaccination on reducing human disease risk. Findings. Vaccination scenarios indicate that early livestock vaccination (December 2018) would have reduced the human epidemic size by a third, whilst vaccinating one month later required using 50 % more vaccine doses for a similar impact. In addition, the likelihood of virus re-emergence in the next rainy season (2019-2020) was estimated very low, with 55.8 % (90 Credible Interval [27.1-59.5]) of the livestock population being immune in August 2019. Interpretation. Human and livestock health surveillance, early detection, and timely vaccination in livestock are crucial to reducing disease risk in humans. We present the first study quantifying RVF virus spillover using livestock and human data, and use this quantitative information to inform on the impact of potential control programmes. This demonstrates the value of a One Health approach to surveillance and control of this emerging infectious disease. Funding. ARS Ocean Indien, EAFRD, RITA Mayotte, VEEPED, Wellcome.