Our results suggest that the transmission potential of CHIKV in Italy was similar to the one observed in tropical regions where Chikungunya fever is widespread (e.g., Reunion Island, where the best estimate for the initial R 0 was 3.7 [3] ). Specifically, we estimated R 0 to be in the range of 1.8-6. However, being the reproduction number strongly dependent on the density of mosquitoes, which in turn varies a lot over time as a consequence of seasonal meteorological effects, different (even larger) values of R 0 could be observed in future outbreaks, depending on the time of epidemic seeding. In [3] , by adapting a method originally introduced in [50] for human-to-human infections, R 0 was estimated from the generation interval probability distribution function and the number of gonotrophic cycles of the mosquito. This method can not be applied in our study, as the undertaken control measures have contributed to alter the gonotrophic cycles of the mosquito in a indeterminable manner. We found that the probability of observing a major outbreak after the introduction of an index case depends on the ratio of mosquitoes to humans and was estimated to be in the range of 32%-76%. These results confirm the high risk to Europe of tropical vector-borne diseases as a consequence of globalization, which has been modifying the mobility of humans and vectors. Climate changes could have been playing a role, as the geographical limits of mosquito-borne diseases can be influenced by climate [51, 52] , but this is still debated [53] [54] [55] .