Inactivated hepatitis A vaccine booster given >= 24 months after the primary dose

We investigated what happens with the immune response when people come back for their booster dose of inactivated hepatitis A vaccine later than the r ecommended time of 6-12 months after the primary dose. We recruited a group of 124 travellers who received either the primary doses of Havrix 720(TM) (two doses) or of Havrix 1440(TM) (one dose) greater than or equal to 24 mo nths before study entry. They received a booster dose of Havrix 1440(TM) an d blood was drawn 1 month later. As a control group, we recruited a group o f 125 travellers who followed a recommended schedule with a primary dose at month 0 and a booster dose at months 6-12. For both study groups, the GMTs increased dramatically and similarly upon the booster immunisation. Althou gh significantly more late travellers (32%) had lost detectable antibodies than controls (11%) before administration of the booster dose, all these su bjects showed an anamnestic response to the booster dose. Delaying the boos ter dose up to 66 months after primary vaccination did not seem to influenc e the immunogenicity of the booster dose. However, the recommended 6-12-mon th interval remains if detectable antibody titers are to be warranted const antly. (C) 2000 Elsevier Science Ltd. All rights reserved.