Eq. (8), however, is the threshold parameter of a simplified model M (with constant N v ). Therefore, to compute R 0 we assume a constant population of vectors, equal to the average value as predicted by the model in the initial phase of the epidemic, i.e. from June 21 to July 26 2007. By employing the next-generation matrix method [28] [29] [30] , one obtains the number of secondary cases generated either in hosts or vectors [31] , that is the square root of Eq. (8) .