Four key measures are typically used to describe the transmissibility of an infectious disease. First, the serial interval (SI) describes the mean time between illness onset of two successive cases in the chain of transmission. Second, the secondary attack rate (SAR) describes the proportion of susceptible contacts that acquire infection from an infectious person. Third, the basic reproductive number (R 0 ) is defined as the average number of secondary cases per primary case in an idealised entirely susceptible population in the absence of control measures. Finally, the effective reproductive number (R t ) at any given time point represents the actual average number of secondary cases per primary case observed in a population. R t reflects the impact of control measures and the depletion of susceptible persons over time. The initial R t may approximate R 0 in pandemic situations. [2] [3] [4] [5] .