To illustrate the distinction between R c (t ) and R (t ), consider a situation where the transmission rate is abruptly reduced at a time t = t I . The instantaneous reproduction number R (t ), which estimates how many people one case would infect if circumstances were to remain fixed, would abruptly switch from a high to a low value at time t I . The case reproduction number R c (t ), on the other hand, estimates how many people each case actually infects. It will thus account for the fact that someone infected at time t,t I may spend part of their infectious period before and after the reduction in transmission which occurs at time t I and thus R c (t ) will smoothly transition from higher to lower values.