The second conclusion is that a variable detection time improves tracing effectiveness, possibly even resulting in p c *,121/R 0 pre . This does not mean, of course, that one should aim at late detection of some infecteds (which would increase R 0 pre ), but that apparent variability is an argument to use tracing. The reason is that the few infecteds that are detected late (or not at all, which is essentially the same) will be discovered by backwards tracing which is an additional effect to forwards tracing that in itself may already result in p c * = 121/R 0 pre . It was earlier found that p c * can be smaller than 121/R 0 pre [9] , but by a different mechanism, namely the presence of shared contacts in a network.