It appears that the iso-p c * contours in Figure 3 are linear in three of the four special cases (panel 3A,B,D), and approximately linear in the fourth case, with long infectious period and variable incubation time (panel 3C: t inf = '; a = 1). The slopes of all (approximate) lines are always between 0.5 and 1.5. This means that the effect of tracing delays is comparable to the sensitivity to the latent period as observed in Figure 2 , so plots of the critical tracing probability as a function of the delay will resemble the plots in Figure 2 , only mirrored (as in Figure 4 ).