Yellow fever beyond the Amazon: surveillance challenges and the role of Callithrix spp. in sylvatic cycles in Brazil.

Yellow fever virus (YFV) remains a major public health concern in tropical and subtropical regions of the Americas and Africa. Since late 2016, sylvatic yellow fever (YF) outbreaks and sporadic cases affecting both humans and non-human primates (NHP) have been recorded in Southern Brazil. These events raised concerns about viral persistence beyond the Amazon Basin, where YFV is endemic, and the potential reemergence of urban transmission. Neotropical NHPs are central to the ecology of sylvatic YF, but many aspects of their infection dynamics and species-specific roles remain poorly understood. Here, we revisit the role of NHP in YF transmission, focusing on marmosets, commonly found in urban and peri-urban areas in Brazil, as potential cryptic hosts of YFV. There is an urgent need to better understand and incorporate NHP ecology, host immunity, and vector dynamics into public health strategies to strengthen preparedness for arboviral reemergence, as the case of YFV.