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.