Hepatitis A vaccine: indirect evidence of immune memory 12 years after the primary course.

Vaccine-induced hepatitis A antibodies persist up to 10 years in adults, with mathematical models estimating further persistence up to 20-25 years. Thirty-one adults received booster inoculations 12 years after their initial vaccination (Havrix 720 El.U at months 0, 1, 6). At the time of booster inoculation, all still had detectable antibodies. All but one subject met pre-defined criteria for anamnestic response 14 days after the booster, and all subjects did so after 30 days. The subjects' geometric mean titre (GMT) increased rapidly from 242 IU/L at baseline to 3,832 IU/L at day 14 and 5,282 IU/L at day 30. This study shows a substantial immune response to re-exposure to hepatitis A antigen after 12 years, which occurs rapidly to ensure protection within the average incubation period of hepatitis A virus.