The incidence rate of HBV was determined by using the HBsAg yield approach as previously described . In the process of the HBV infection, HBsAg becomes detectable earlier than anti-HBc. HBsAg may be transient, while anti-HBc can persist for a long time. Therefore, donations confirmed as HBsAg-positive but non-reactive for anti-HBc can be regarded as newly infected cases that are considered to be HBsAg yield cases. The number of HBsAg yield cases divided by the total number of donations is equal to the HBsAg yield rate. The quotient divided by the length of time a patient has been HBsAg-positive prior to anti-HBc seroconversion (termed the HBsAg yield window, calculated as 44 days) gives an incidence estimate for HBV infections among blood donors .