A seroepidemiologic study of hepatitis A virus infections: statistical analysis of two independent cross-sectional surveys in Okinawa, Japan.

To investigate the endemic situation of hepatitis A virus infection in the past in Okinawa, Japan, the authors analyzed two sets of cross-sectional data on age-specific prevalence of antibody to hepatitis A virus (anti-HAV) obtained in 1968-1973 and 1980-1981 by fitting a catalytic model. For these two sets of data, the asymptotic level of infectious force of hepatitis A virus, namely lambda infinity, was estimated as 0.121 and 0.149, the maximum slope of the time-dependent force of hepatitis A infection, namely alpha, was 0.566 and 0.529, and the year when the force of hepatitis A infection had decreased to the half of lambda infinity, namely beta, was 1966 and 1964, respectively. In the test for the equality of parameters for the two applications, the difference was not significant. Furthermore, the fitness of the catalytic model to the data on anti-HAV prevalence was good. The results of the analysis by fitting the catalytic model show that hepatitis A infection had been highly endemic, that is, 136 infections per 1,000 persons per year in the area studied before 1955, and it decreased rapidly during the 1960s. Since 1975, hepatitis A has been a rare disease (infection is almost zero per 1,000 persons per year) in Okinawa, Japan.