Long-term follow-up of neonates who have been immunized with hepatitis B vaccine

Although the short-term effectiveness of hepatitis B vaccine has been well established, few long-term studies of the effectiveness of this vaccine in neonates who are born to HBsAg positive mothers have been undertaken. It is also not known if supplementary doses of vaccine are required to protect these children who are continuously exposed to the hepatitis B virus, and if so, how long after primary immunization the additional dose should be given. To determine the hepatitis B vaccine effectiveness up to eight years, a cohort of 770 (66%) of 1166 children who had been immunized with HBIG and hepatitis B vaccine at birth, between 1984 and 1989 inclusive, were tested for the serological markers: anti-HBc, anti-HBs and HBsAg. (Anti-HBc indicates that viral replication has occurred at some time (infection); anti-HBs is the antibody to HBsAg and may appear after natural infection or following immunization; and HBsAg indicates presence of hepatitis B virus (HBV) infection.) The children in the cohort were immunized as part of the British Columbia Ministry of Health Program to prevent HBV infection in infants of carrier mothers. Blood samples were obtained by finger-prick and tested at the Canadian Red Cross Society, Vancouver Centre, Blood Transfusion Service. The IMx assay kits were produced by Abbott Laboratories. Parents were interviewed for information on relevant variables. Associations were determined using chi-square tests, Mantel-Haenszel trend tests, linear and logistic regression procedures. Best prediction models for the outcomes: anti-HBc, anti-HBs and HBsAg were determined in a stepwise fashion. Of the participants, 31% of the mothers were HBeAg positive as well as HBsAg positive. The overall attack rate for ages 2 to 8years was 5.1%; the carrier rate was 2.3% and the seropositivity rate (>=10 mIU/ml) was 87.9%. The geometric mean titres of anti-HBs varied from a high of 272 mIU/ml at age two to a low of 23 mIU/ml at age seven. Vaccine efficacy for infection was 89%. The best predictors for infection were the mother's HBeAg status (P<0.0001), the age of the child when the first dose of vaccine was given (P=0.0001), the number of years the mother spent in her country of birth (P=0.001) and inversely the mother's age (P=0.002). Anti-HBs titres were best predicted by the inverse age of the child (P=0.0001), the number of doses of vaccine (P=0.0007), the age of the child when the first dose of vaccine was given (P=0.01) and the number of months the child spent abroad (P=0.05). Since the infection rate was relatively stable between age groups and the association between the child's age and anti-HBc was not significant (P=0.15), waning immunity is not suggested by the findings of this study. This was in spite of the decline in anti-HBs titres with increasing age. The conclusions of this study are: 1) That a booster dose of hepatitis B vaccine after the six month dose is not necessary at least up to age eight; 2) Because a delay in the first dose of vaccine resulted in increased infections, giving the first dose early is critical; 3) It could not be concluded from the data that HBIG was not important, therefore, continuation of HBIG at birth is recommended; 4) Since the timing of dose two later than two months after dose one was not found to be associated with increased infections, this dose could be incorporated with the regular immunization schedule at two months of age if administratively more feasible; 5) Susceptibility appears to arise earlier rather than later and may be due to non-response to the vaccine as a 14% non-response rate was found at 6 to 18 months of age.

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