Hepatitis B vaccination of adolescents: significance of non-protective antibodies.

Despite hepatitis B virus (HBV) immunization, a percentage of healthy individuals display an antibody titre below the threshold for clinical protection (10 IU/L). In order to predict the existence of this inducible immunological response, the precise anti-HBs titre required to achieve protection in immunized patients with waned HBs antibodies must first be determined. A total of 4486 vaccinated students attending the University of Padova Medical, Science, and Veterinary School were recruited for study between 2004 and early 2012. The baseline concentration of anti-HBs was measured at enrolment. Participants displaying anti-HBs titre <10 IU/L at the follow-up examination (mean 10.8 years) were given a booster vaccination and retested 20-30 days later. At enrolment, 87.6% of the 4486 vaccinated subjects showed persistence of anti-HBs higher than 10 IU/L. Of the 279 booster-vaccinated subjects, 94.6% achieved the cut-off titre. Booster-induced immunological response was correlated to the pre-booster titre level, with ≥2 IU/L ensuring a robust positive response and less than 2 IU/L being associated with the probability of developing insufficient levels of antibodies. Pre-booster antibody titre higher than 2 IU/L in adults might be predictive of an anamnestic response to booster vaccination, whereas titres below this value may indicate likelihood of non-response.