GSTM1 and infection: Weaknesses of the study include the use of different laboratory methods for the 1992 survey than those used for the 2006 and 2014 surveys, different nonresponse rates of the 2006 and 2014 surveys, underestimation of immunity indicated by anti-HB levels (because when antibody levels wane, memory B cell mediated anamnestic response to HBV exposure can maintain protection from infection), undersampling of the migrant population (because only those residing >6 months in a given survey area were included), and the fact that HepB coverage levels among teens and adults are not measured in China.