Rapid antibody response after vaccination with a virosomal hepatitis A vaccine

Background: This study was designed to assess the early antibody kinetics after a priming dose, and the extent of the antibody increase after a booster dose of an inactivated virosomal hepatitis A virus (HAV) vaccine (Epaxal(R)). Patients and Methods: This was an open, uncontrolled study in 30 healthy subjects. The vaccine was injected intramuscularly on day 1 and month 12. Serum antibody titers were measured by ELISA on day 1 (pre dose) and at various time points thereafter until month 12 (pre-booster dose). After the booster dose, antibody titers were measured at various intervals until month 24. Neutralizing antibody titers were measured in 12 subjects a number of times during the 1st month by an antibody neutralization assay. Titers greater than or equal to 10 mIU/ml were considered seroprotective. Results: ELISA antibody titers showed a rapid increase post vaccination. By day 15, 96% of subjects were seroprotected, which increased to 100% by day 22 (n = 27 evaluable subjects, aged 18-43 years; 13 male, 14 female). ALL subjects achieved seroprotective HAV-neutralizing antibody titers by day 11 (n = 12). The booster vaccination at month 12 resulted in a strong response in all subjects, with a sustained anti-HAV antibody titer (1,155 mIU/ml) at month 24. Both the priming and booster doses were well tolerated. Conclusion: Primary vaccination with this virosomal HAV vaccine is well tolerated and induces a rapid HAV-neutralizing antibody response resulting in seroprotection in all subjects within 10 days. In addition, the booster vaccination results in prolonged seroprotective antibody levels.