\( \lambda (t)={c}_1 exp\left[-\frac{{\left(t-{c}_2\right)}^2}{c_3}\right]F(t), \) (12) where c _{1} is a scale parameter that determines the maximum incidence, c _{2} is the time at which the maximum incidence is reached, c _{3} represents the width of the time-dependent incidence function and F(t) is an ad hoc function introduced to both improve the model fit to data and to set the initial time of infection c _{5}. This function has the “logistic” form: \( F(t)=\frac{1}{1+ \exp \left(-{c}_4\left(t-{c}_5\right)\right)}, \) (13) where c _{4} determines the rate at which the incidence increases. |