Significant increased risk of arboviral transmission in Rome in a year of record-breaking hot

In 2023, an unprecedentedly high number of locally acquired cases of dengue virus occurred in Europe - including the first large outbreak in Rome (Italy). Globally, 2023 was the warmest year on record, with each month from June to December being warmer than the corresponding month in previous years. We conducted an extensive collection of entomological data in Rome to investigate how 2023 unprecedented high temperatures have affected the abundance and seasonal dynamics of the local vector Aedes albopictus population and the risk of secondary transmission of both dengue (DENV) and chikungunya (CHIKV) viruses. This was achieved by applying a mathematical dynamic temperature-based model to analyse Ae. albopictus adult female captures by Sticky Traps (STs) in 10 sites within Rome metropolitan area in 2012 and 2023. The simulated Ae. albopictus females’ population was used to compute the basic reproduction number R_0, which represents the number of human secondary cases caused. Finally, we simulated a transmission dynamic associated with DENV and CHIKV using a standard SEI-SEIR approach. Compared to 2012, in 2023 the model estimates suggest a higher mosquito abundance in 9 of the 10 sites, as well as an average increase in female abundance of 64% (95%CI 59%-68%), 54% (95%CI 48%-60%) and 142% (95%CI 133%-151%) in June-July, August-September, and October-November, respectively. The increase of female abundance indicates a prolonged breeding season extending the epidemic period (R0>1) for both pathogens by ~ 1 month. From 2012 to 2023, the proportion of sites with potential arbovirus transmission increased from 33% to 66%. The probability of an outbreak with >10 DENV-cases was heterogeneous among sites, ranging from 0 to 25%, and substantially higher in 2023 than in 2012, extending the risk from July up to August. Furthermore, the probability of an outbreak with more than 10 CHIKV-cases was heterogeneous among sites, ranging from 0 to 40%, and substantially higher in 2023 than in 2012, extending the risk from August up to October. In conclusion, results suggests that the higher temperatures in 2023 drove a rise of Ae. albopictus abundance and led to an increase in the arbovirus epidemic period in temperate areas. In view of the prediction of higher temperature in Mediterranean regions such as Italy in forthcoming years, the model results highlight the need to increase preparedness of the public health system to this increasing risk.