  A simplified method to compute $R_t$, the Effective Reproduction Number, is
presented. The method relates the value of $R_t$ to the estimation of the
doubling time performed with a local exponential fit. The condition $R_t = 1$
corresponds to a growth rate equal to zero or equivalently an infinite doubling
time. Different assumptions on the probability distribution of the generation
time are considered. A simple analytical solution is presented in case the
generation time follows a gamma distribution.
