Evolution and spillover dynamics of yellow fever at the forest-urban interface in Brazil

Yellow fever virus (YFV) continues to threaten human and wildlife populations in the Americas, yet its transmission at the forest-urban interface remains unclear. Here we integrate ground- and canopy-level mosquito surveillance, systematic monitoring of non-human primate carcasses and viral metagenomics to describe the dynamics of a sylvatic YFV outbreak in a 186-hectare Atlantic Forest fragment embedded within metropolitan São Paulo, Brazil, between 2017 and 2018. Our analyses reveal that transmission was primarily driven by a single genetic cluster introduced during a period of high abundance of the main vector, Haemagogus leucocelaenus mosquitoes. A near-complete hepatitis A virus genome was detected in a YFV-infected howler monkey, suggesting potential co-infections at the human-wildlife interface. Phylogenetic and epidemiological modelling estimated a basic reproduction number, R₀, for sylvatic yellow fever of 8.2 (95% CI 5.1-12.2), substantially higher than previous estimates for urban outbreaks. Our findings demonstrate that multisource surveillance could provide actionable early warnings in regions at risk for zoonotic spillover.