Evaluation of the timing of the booster injection after a primary vaccination against hepatitis A

To evaluate the long-term efficacy and the timing of booster injection of a lyophilized, inactivated hepatitis A vaccine (HAV), we studied the acquired anti-hepatitis A virus titres in 45 subjects who received either two or three doses of the vaccine. With three doses greater than or equal to 0.5 mu g administered at 0, 1 and 6 months, antibody titres increased to approximately half those seen in people with a history of natural hepatitis A infection. Although the titres decreased gradually, all subjects remained anti-HAV positive at least 5 years after the third injection. The geometric mean titre exceeded 100 mIU/mL, which approximated the highest titre obtainable by the inoculation of immune serum globulin. A prompt antibody reaction was seen with two doses administered 2 weeks apart, with the titres achieved being 5.6-fold the peak titres obtained by the inoculation of immune serum globulin. All 33 subjects were antibody positive 18 months after the first injection, but some became seronegative by 30 months, suggesting that a third booster dose would be desirable at 18 months.