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Abstract
To monitor immune status during clinical trials and after vaccine registration, several assays have been developed to measure type-specific human papillomavirus (HPV) serum antibody levels. These include neutralization assays, single epitope-based inhibition immunoassays, and direct enzyme-linked immunosorbent assays (ELISAs). Neutralization assays based on multiple epitopes and independent of vaccine material are considered the ‘gold standard’ for unbiased assessment of the protective potential of vaccine-induced antibodies. However, their use in large clinical trials is challenging. Here, we compare both the direct ELISA and the single epitope-based inhibition ELISA with the pseudovirion-based neutralization assay (PBNA) for HPV-16/18 antibody responses in vaccinated women enrolled in trials of Cervarix™, GSK's cervical cancer vaccine. The direct ELISA, which is based on multiple epitopes, was shown to have a higher degree of sensitivity and correlation with the PBNA when compared with the single epitope-based inhibition ELISA. Among double-positive results, high correlations were observed between the PBNA and the direct ELISA (0.70-0.88 for HPV-16 and 0.82-0.94 for HPV-18) and also with the single epitope-based inhibition ELISA (0.60-0.89 for HPV-16 and 0.57-0.96 for HPV-18) in women aged 15-25 years. The correlation persisted up to 6.4 years after primary vaccination. Similar levels of correlation were observed for adolescents aged 10-14 years and women aged 46-55 years. Therefore, the direct ELISA appears to be an excellent surrogate for neutralizing activity and can be used to evaluate antibody response induced by L1 VLP-based cervical cancer vaccines, regardless of time elapsed after vaccination (up to 6.4 years) and the age of the vaccine recipient.