The numerator represents the number of new infections at calendar time t, while the denominator was originally intended to represent "the total number of infectious individuals" who can potentially be primary cases with an equal chance at time t. Nevertheless, in order that the estimator of the actual reproduction number coincides with that of R 0 , the concept of the denominator is better replaced by "the total number of effective contacts (which can potentially lead to secondary transmissions with an equal probability)". Therefore, the corrected R a is better written as: