Incorporating delays into iterative tracing does not generally change the concept of R c , but it makes the calculation of Y(p c ) very complicated, because there is no longer a single time of cluster quarantine. Two complications arise: first, delays may cause contactees in the chain emanating from a symptomatic infected to become symptomatic themselves before the tracing process reaches them. They then initiate a new tracing process of their own within the same cluster. Second, if the infection process is faster than the tracing process, the cluster size grows infinitely large and iterative tracing becomes ineffective. We determined p c * by stochastic simulation of clusters until quarantine of the final infected (see Supporting Information for all details).