Inactivated hepatitis A vaccine-induced antibodies: follow-up and estimates of long-term persistence.

To estimate the long-term persistence of anti-HAV antibodies, 120 (schedule 0-6) and 194 (schedule 0-12) adults were vaccinated and followed-up annually for 6 years. Shortly after the last dose, anti-HAV levels fell sharply (annual decline rate delta > 65%). Thereafter, delta diminished to 10-15%. GMTs 5.5 years after the last dose were 522 mIU/ml (0-6 group) and 749 mIU/ml (0-12 group); all subjects except one maintained detectable antibodies. The average delta over the whole follow-up period was 15-20%, resulting in an estimated persistence of anti-HAV levels > or =20 mIU/ml for 20-25 years. These estimates were similar for both applied calculation methods (GMT or individual based) and both vaccination schedules. Because the individual antibody levels tended to stabilise between the last two measurements, the hypothesis of a slow, log-linear decrease and its matching calculation methods might be subject to reconsideration. With the current methodology, however, detectable antibodies are estimated to persist for 20-25 years.