Immunogenicity and safety of a new inactivated hepatitis A vaccine in young adults: a comparative study

Objective To evaluate the immunogenicity, safety, and dosage of a new inactivated hepatitis A vaccine administered to young adults. Methods One hundred and four normal adult volunteers, seronegative for hepatitis A virus and hepatitis B surface antigen, were randomly assigned to one of three groups. The high-dose group received a primary dose of 1000 units of the new vaccine, the low-dose group received a primary dose of 500 units of the same vaccine, and the Havrix group received a primary dose of 1440 enzyme-linked immunosorbent assay units of Havrix, a licensed inactivated hepatitis A vaccine. All groups received a booster dose of the same vaccine 6 months after the primary dose. Local and systemic adverse reactions, seroconversion rates, and geometric mean titers of hepatitis A virus antibodies were measured in all three groups. Results Local and systemic reaction types and rates were similar in all three groups after primary and booster doses, although local reactions were more frequent in the Havrix group following the primary dose. No serious adverse reactions occurred. One month after the primary dose, the seroconversion rate was 87.5% in the high-dose group, 70.0% in the low-dose group, and 50.0% in the Havrix group (P=0.001, versus the high-dose group). At month 6 (before administration of the booster dose), seroconversion rates were 96.9% in the high-dose group, 65.0% in the low-dose group (P=0.0029), and 68.8% in the Havrix group (P=0.007). All subjects in all groups seroconverted by one month after receipt of the booster dose. Geometric mean titers were similar in all three groups at month 1, but were higher in the high-dose group (264 mIU/ml) than those in the Havrix group (135 mIU/ml) at month 6 (P=0.0013). One month after the booster dose, geometric mean titers in the high-dose group (2747 mIU/ml) were higher than those in the low-dose group (1657 mIU/ml) (P=0.0223) or in the Havrix group (1316 mIU/ml) (P=0.01). Conclusions This new inactivated hepatitis A vaccine is immunogenic and safe; two doses of either 500 or 1000 units can induce hepatitis A virus antibodies well above the protection level.