For less than 20% of the cases attributable to human transmission, R is entirely consistent with zero, even when accounting for the uncertainty in the duration of the infectious period (c 21 ). Reported information does not allow at present a precise determination of c 21 for H5N1 influenza in humans, so that different scenarios are possible, which we explore in detail in Supplementary Material S1 ( Figure S4 , Table S1 ). Data permitting, a hierarchical Bayesian estimation method for the distribution of c can also be envisaged [32] . Figure 4 shows the evolution of R and of its corresponding 95% credible interval. The computation of successive probability distributions for R gives a basis for assessing the evolution of transmissibility over time, including the approach to the epidemic threshold RR1. At present we conclude that, even in the unrealistic worst case scenario, where cases are aggregated at the national level and all cases are attributed to human transmission, R remains below unity.