Changes in HBsAg carrier rate in Goto Islands, Nagasaki Prefecture, Japan.

Annual mass examinations in an area where hepatitis B virus (HBV) infection is very prevalent revealed that 12.1% of inhabitants born during 1946-50 were positive for hepatitis B surface antigen (HBsAg), compared with only 0.6% of those born during 1971-75. To find out why the HBV carrier rate has fallen, changes in the modes of HBV infection were examined. The HBsAg positivity rate among mothers who gave birth to HBV carrier children in 1965 and before was 26.8% and that for such mothers whose babies were born in 1966 and after was 66.7%, whereas the HBsAg positivity rate among children born to HBV carrier mothers in 1965 and before and in 1966 and after were 30.8% and 28.3%, respectively. None of the 503 inhabitants who had no HBV markers in 1976 had become carriers by 1981. These findings indicate that the decrease in the prevalence of HBV carriage is caused mainly by the reduction in occurrence of horizontal transmission of HBV in infancy.