Estimation of the Basic Reproduction Number of COVID-19 from the Incubation Period Distribution

The estimates of the future course of spreading of the SARS-CoV-2 virus are frequently based on Markovian models in which the duration of residence in any compartment is exponentially distributed. Accordingly, the basic reproduction number R0 is also determined from formulae where it is related to the parameters of such models. The observations show that the start of infectivity of an individual appears nearly at the same time as the onset of symptoms, while the distribution of the incubation period is not an exponential. Therefore, we propose a method for estimation of R0 for COVID-19 based on the empirical incubation period distribution and assumed very short infectivity period that lasts only few days around the onset of symptoms. We illustrate this venerable approach to estimate R0 for six European countries in the first wave of the epidemic. The calculations show that even if the infectivity starts two days before the onset of symptoms and stops immediately when they appear (immediate isolation), the value of R0 is larger than that from the classical, Markovian approach. For more realistic cases, when only individuals with mild symptoms spread the virus for few days after onset of symptoms, the respective values are even larger. This implies that calculations of R0 and other characteristics of spreading of COVID-19 based on the classical, Markovian approaches should be taken very cautiously.