R 0 estimation for COVID-19 pandemic through exponential fit.

We provide an easy and accurate method for approximating the reproduction number R 0 defined in an SIR epidemic model. At first, we present a formula extracting the exact R 0 in case of constant rates of infection and recovery assumed in an SIR model. Then, we proceed proposing an exponential fitting to various data taken from the real-world epidemics. Certain applications for current COVID outbreak are considered, and figures describing the fluctuation of R 0 in various countries are given.