The relationship between the idealised generation time distribution w (t) and the distribution of observed generation times can be rather complex for a number of reasons. First, infections are rarely observed, and thus must be either backcalculated or the generation times must be based on a surrogate such as the appearance of symptoms [1, 12] . Second, right censoring can cause the observed generation times to be shorter or longer than expected for a growing or declining epidemic, respectively [26] . Third, as apparent here, if the reproduction number R (t) changes due to depletion of susceptibles, changes in contact rates or public health measures, then this will also change the observed generation times for infectious individuals during that period of change. Thus the distribution w (t) is really intended as a measure of infectiousness which will correspond to generation times for an index case in an ideal large closed setting where contact rates are constant. It can be inferred from data on the timing of cases, as in [10, 13] .