Assessing the risk of autochthonous yellow fever transmission in Lazio, central Italy

Yellow fever virus (YFV) causes a highly lethal mosquito-borne disease that has recently reemerged after 30 years of low incidence due to vaccination campaigns. Large epidemics occurred in Angola and Democratic Republic of the Congo (DRC) in 2016 through 2017 (overall about 1,000 confirmed cases and 140 deaths) and in Brazil in 2017 through 2018 (2,037 confirmed cases and 674 deaths) [1, 2]. The high international connectivity of Brazil raises concern about the potential spread of disease to other countries by infected travelers [3, 4]; this possibility was confirmed during spring 2018, with the notification of six infected travelers from five European countries, two of which had a fatal outcome [5]. Recent laboratory experiments suggest that European populations of Aedes albopictus may be competent for transmission of YFV [6], and therefore large areas highly infested by this species in Mediterranean countries are potentially exposed to the risk of outbreaks [7]. Here, we provide a quantitative assessment of the risk of YFV transmission in Lazio, the central Italian region where the metropolitan city of Rome is located and where the largest arboviral outbreak in continental Europe occurred in summer 2017 [8]. To do so, we adapted a stochastic transmission model, previously developed to assess the transmission risk of chikungunya virus (CHIKV) [9] in the same area, to account for relevant epidemiological dynamics of YFV, using existing field data on A. albopictus abundance [10] and biting rate on humans [11].