Three different patterns of transmission of the virus, with different probabilities of transmission, were assumed according to whether some protection by anti-histo-blood group natural antibodies was present (Figure 4). A strong or moderate group effect denotes a strong or moderate protection, respectively, whereas no group effect corresponds to an absence of protection. The impact of group effect was assessed on the number of infected individuals over time in four different populations with very different frequencies of ABO phenotypes (Figure 5). A strong group effect delayed the initiation of the epidemic as well as it decreased importantly the total number of infected individuals whatever the population considered as compared to no group effect and, to a lesser extent, a moderate group effect. Moreover, a strong group effect evidenced different starting times of the epidemic according to the different populations (Figure 5B). The impact of group effect was assessed on the number of infected individuals over time according to their blood groups in the Chinese (Hong Kong) population (Figure 6). The number of blood group O-infected individuals was always the most important one, closely followed by blood group B-, A-, and AB-infected individuals whatever the transmission pattern considered. A strong group effect also delayed the initiation of the epidemic as well as it markedly decreased the total number of infected individuals. Furthermore, a strong group effect also postponed the starting time of the epidemic in blood group O individuals as compared with the other blood groups. The blood group type of the index case in the Hong Kong outbreak was not provided (Cheng et al. 2005). The modeling was first performed using a blood group O index case, as described above. In order to assess whether the results could be affected by the blood group type of this first case, further simulations were performed with different index cases of either the A, B, or AB blood group. The main results were not modified since a strong group effect always delayed the initiation of the epidemic as compared to no group effect and to a moderate group effect. Likewise, it markedly decreased the total number of infected individuals irrespective of the index case's blood group (data not shown).