A pitfall in estimating the effective reproductive numberRtfor COVID-19

The effective reproductive number R t of COVID-19 is determined indirectly from data that are only incompletely known. Approaches based on reconstructing these data by sampling time lags from suitable distributions introduce noise effects that can result in distorted estimates of R t . This, in turn, may lead to misleading interpretations of the efficacy of the various measures taken to limit COVID-19 transmission. We discuss in some detail a study used for real time monitoring of the reproductive number in Switzerland; see https://ncs-tf.ch/en/situation-report . We argue that the method used to derive the above curve is systematically flawed and leads to an underestimation of the efficacy of the lockdown. The method adopted by the Robert Koch Institute suffers from similar deficiencies, their impact is however smaller.