Immunogenicity of two versus three injections of inactivated hepatitis A vaccine in adults

Aim: To investigate the immunogenicity of two versus three injections of inactivated strain CR326F-derived hepatitis A vaccine in healthy adults. Methods: Healthy adult volunteers (n=105) at Utrecht University Hospital, The Netherlands, were randomly assigned to receive intramuscular injections (deltoid muscle) of 25 Units (U) at 0 and 6 months (group A, n=53), or at 0, 2 and 6 months (group B, n=52). Blood was drawn before and at various time points after vaccination for determination of serum antibody to hepatitis A (anti-HAV). Results: One month after the first injection, the seroconversion rates (greater than or equal to 10 mIU/ml, international units) were 88% for group A and 90% for group B. Only 2/ 103 (one in each group) showed IgM anti-HAV. One month after the second injection, seroconversion rates were 100% in both groups. At months 3, 6 and 7, anti-HAV geometric mean titers were significantly different because of the different vaccination schedules, but they were similar at months 1, 2 and 12. The anti-HAV geometric mean titer increase after the second injection was higher when the interval between the two doses was of longer duration. Anti-HAV titers of females were significantly higher than those of males and vaccinees less than or equal to 30 years had higher titers than those >30 years. Conclusions: Two 25 U doses of the vaccine investigated given at 0 and 6 months, induce adequate anti-HAV titers in all adult healthy vaccinees and are as immunogenic as three doses given at 0, 2 and 6 months.