Abstract
Eighty-seven high-risk individuals in Thailand who had received a complete course of recombinant HBV vaccine 18–20 y ago were investigated with regard to their immunological memory. To evaluate humoral immunity, anti-HBs antibody titers were measured. Cellular immunity was determined by ELISPOT to detect HBV-specific IFN-γ–producing cells. Overall 83.9% of participants developed circulating anti-HBs (titer ≥1 mIU/mL) and 58.6% were seroprotected (titer ≥10 mIU/mL). As for cellular immunity, 50.6% were positive on ELISPOT. Moreover, there was no correlation between the level of anti-HBs and positive ELISPOT results. However, the majority of participants (81.8%) who were positive for IFN-γ–producing cells were seropositive, but only 50% of seropositive participants were ELISPOT-positive. Thus, 18–20 y after immunization, it appears that a second booster dose should be considered, especially in high-risk groups.