Purpose: Long-term immunogenicity and safety of the human papillomavirus (HPV)-16/18 AS04-adjuvanted vaccine when administered to adolescent girls was evaluated. Methods: This open-label, follow-up study (NCT00316706) was conducted in 31 centers in Taiwan, Germany, Honduras, Panama, and Colombia. In the initial study (NCT00196924), 1,035 girls aged 1014 years received the HPV-16/18 AS04-adjuvanted vaccine at 0, 1, and 6 months. Here, geometric mean titers (GMTs) of antibodies against HPV-16, HPV-18, and monophosphoryl lipid A (MPL), a component of the AS04 Adjuvant System, were reported up to month 48. Results: In the according-to-protocol immunogenicity cohort (N = 563), GMTs at month 48 in initially seronegative participants were 2,374.9 (95% confidence interval: 2,205.72,557.0) EL.U/mL for anti-HPV-16 and 864.8 (796.9938.4) EL.U/mL for anti-HPV-18, that is, six- and threefold higher than the plateau level in a reference study demonstrating vaccine efficacy in young women (age, 1525 years). All participants remained seropositive for anti-HPV-16 and anti-HPV-18 at month 48. Most participants (81.8%) were seropositive for anti-MPL antibodies before vaccination. Anti-MPL antibody titers in initially seropositive participants increased initially, and then declined. Most initially seronegative participants for anti-MPL seroconverted; 69.6% remained seropositive at month 48, with anti-MPL antibody titers similar to the natural background level. The vaccine was generally well tolerated. No serious adverse events were considered related to vaccination. Conclusions: In adolescent girls, the HPV-16/18 AS04-adjuvanted vaccine produces anti-HPV-16 and anti-HPV-18 antibody titers that are maintained for up to 4 years at higher levels than those in young women in whom vaccine efficacy against cervical lesions was demonstrated.
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