Increasing susceptibility to HAV among members of the young generation in Thailand.

The prevalence of antibodies to hepatitis A virus was studied in 961 children and adolescents, randomly selected from five different provinces in Thailand (Chonburi, Lopburi, Udonthani, Nakhon Si Thammarat and Lopburi). The highest prevalence was found in Nakhon Si Thammarat, with 32.1 percent of those aged 10-14 years and 57.1 percent of those aged 15-18 years showing evidence of protective immunity. However, this high rate could be explained by an outbreak of hepatitis A in 1992. In the remaining four provinces, the pattern was typically age-related in that all individuals showed between zero and 13 percent antibody prevalence until reaching the 15-to-18-year age group where it increased to between 5.6 and 22.7 percent. The overall sero-prevalence among all age groups was 7.9 percent. Thus, the majority of the younger generation is susceptible to hepatitis A virus infection thereby enhancing the impact, should an outbreak occur. Preventive measures that might be taken are education aimed at better hygiene and sanitation, as well as vaccination of susceptible individuals within high-risk populations.

Humoral immune response following hepatitis B vaccine booster dose in children with and without prior immunization.

One hundred and twenty-three children who had received no, incomplete and complete primary hepatitis B vaccination but had negative or very low anti-HBs titer were immunized with a single dose of recombinant hepatitis B vaccine. Blood tests for anti-HBs were obtained at 30 +/- 5 days after the booster immunization. Twelve of 18 (66.7%) children without prior immunization (group 1) seroconverted following the single dose Seroconversion rates in children who had undetectable anti-HBs with incomplete and complete primary immunization (group 2 and 3) were 83.34% and 94.5%, respectively. All children with complete 3- dose vaccination but who had low anti-HBs titer (group 4) also seroconverted. This study confirmed that immunological memory, allowing a protective anamnestic response, lasted at least 8 years in children who had received primary HB immunization with undetectable anti-HBs. Therefore, we conclude that the booster dose after complete vaccination is not necessary in healthy children.