The likelihood of estimating R 0 with (15) is proposed as follows. First, the inverse of both sides of (15) is taken: The numerator of the right-hand side indicates the total number of effective contacts made by potential primary cases in year t that have an equal probability of resulting in a secondary transmission, and the denominator is the number of secondary cases in year t. Given that all the potential contacts made by primary cases (black circles) are known using the incidence data and the generation time distribution, the probability that each potential contact resulted in a secondary transmission is given by 1/R 0 . 1 The intrinsic growth rate during the exponential growth phase; 2 the basic reproduction number estimated using equation (5); 3 the basic reproduction number estimated using equation (17); the 95% confidence intervals are shown in parentheses.