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

The article provides a quantitative assessment of the risk of yellow fever virus (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. To do so, we adapted a stochastic transmission model, previously developed to assess the transmission risk of chikungunya virus (CHIKV) in the same area, to account for relevant epidemiological dynamics of YFV, using existing field data on Aedes albopictus abundance and biting rate on humans. Overall, the present work reveals a low, but nonnegligible risk of YFV transmission in European areas characterized by substantial A. albopictus infestation and medium-to-low human density. Considering the severity of YF, this result highlights the need for public health authorities to ensure early diagnosis (not trivial since YF has not been reported in Italy since the 19th century), prompt notification of infected cases and swift responses targeting mosquito populations through vector control interventions and the human population via reactive vaccination campaigns. The recent unexpected rise of YF has caused a worldwide shortage in vaccine stockpiles, which has led to the adoption of fractional dosing immunization during the recent Brazilian outbreaks. Although this approach seems to have been effective in controlling the epidemics locally, many questions remain open. More generally, the availability of YFV vaccine stockpiles at the global scale may be an important challenge to outbreak control in the future. For these reasons, public health authorities might consider preventive general-purpose risk reduction measures such as larviciding, given their demonstrated costeffectiveness in simultaneously preventing outbreaks of different arboviruses, even in areas with limited risks.