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 recommended 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 (two doses) or of Havrix 1440 (one dose) >/=24 months before study entry. They received a booster dose of Havrix 1440 and blood was drawn 1 month later. As a control group, we recruited a group of 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. Although significantly more late travellers (32%) had lost detectable antibodies than controls (11%) before administration of the booster dose, all these subjects showed an anamnestic response to the booster dose. Delaying the booster dose up to 66 months after primary vaccination did not seem to influence the immunogenicity of the booster dose. However, the recommended 6-12-month interval remains if detectable antibody titers are to be warranted constantly.