The differences found in the demographic characteristics at different blood centers may have contributed to the differences of estimated residual risks at the blood centers. A higher RR was found at Changzhi (158.35 per 100,000 donations per year), while the lowest was found at Mianyang (20.04 per 100,000 donations per year) (see Table 7). According to the characteristics of the blood donors, most of the blood donors were first-time donors, and only Changzhi had the most repeat blood donors, reaching 67.57%. Theoretically, and according to published data, repeat donors have a lower RR for infectious diseases [12, 14]. However, the results of this study showed that the HBV incidence and residual risks at Changzhi were the highest. With regards to the age of the blood donors, most of them were under the age of 35, but most (70%) donors at Changzhi were over 35 years old. China began nationwide hepatitis B vaccinations in 1992, so the residual risk of donors under 35 years old is lower than that of donors above 35 years old. Perhaps this is thus a major cause of Changzhi having the highest residual risk. This data also proved that the HBV vaccine in China has been successful, and it shows that hepatitis B vaccination can effectively reduce residual risks. Second, the proportion of farmers at the Changzhi Blood Center was 37.39%, much higher than the proportions at the other five blood centers. (These proportions ranged from 3.07% at the Fujian Blood Center to 27.37% at the Kaifeng Blood Center. See Table 3). Due to economic barriers and other multivariate factors, farmers had the lowest vaccination rate, which again could have led to their higher susceptibility to HBV infections. Third, the proportion of donors without a high school education was relatively high at the Changzhi Blood Center. These donors may have had a relatively low level of health knowledge, which also could have accounted for their elevated rates of HBV infection. The findings in this study are very interesting, because the evidence above indicates that the evaluation of residual risks should take into consideration not only the proportions of first time and repeat donors, but also their ages, occupations and education backgrounds.