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Safe and effective vaccines against anogenital human papillomaviruses (HPV) are now available. These vaccines, composed of virus-like particles (VLPs) made from the L1 major capsid protein of specific HPV types, induce a polyclonal antibody response directed against specific conformational and linear epitopes displayed on the VLP. Numerous studies indicated the importance of neutralizing antibodies in protection from infection. However, our understanding of the antibody responses to these vaccines is not complete, and there is no established immune correlate of protection nor antibody threshold that correlates with protection against HPV infection or disease. In the current study, antibody responses of young women to Gardasil®, the quadrivalent HPV 6, 11, 16 and 18 L1 VLP vaccine (qHPV), were assessed through 48 months (M) in total IgG and competitive Luminex immunoassays (total IgG LIA and cLIA). The total IgG LIA was developed as a research assay to evaluate preclinical multivalent HPV VLP vaccine formulations. The cLIA simultaneously evaluates the antibody response to a unique conformational, neutralizing epitope on each of the four HPV types present in the quadrivalent vaccine; HPV 6, 11, 16 and 18. The same sera from women vaccinated with the qHPV vaccine were tested in both the total IgG LIA and the cLIA assays. The proportion of vaccinated women achieving seropositivity and the anti-HPV VLP total IgG and cLIA geometric mean titers (GMTs) were summarized at M7, M24, M48 based on the serostatus cut-points defined for each immunoassay. Overall, greater than 99% of subjects seroconverted to all four vaccine types in both assays; GMTs peaked at M7. For all four HPV types, regardless of the immunoassay used, the most significant decline in GMTs was observed between M7 and M24. By M24, the antibody titers had reached a plateau and minimal declines in antibody titers were observed between M24 and M48 for all four HPV types in both immunoassays. Testing the same sera, seropositivity for M48 HPV18 remained high (96.7%) in the total IgG LIA, but was 64.8% in the cLIA. The current study illustrates potential important differences in serologic assays utilized in the clinical trials of the two currently available HPV VLP vaccines (quadrivalent and bivalent). Differences in seropositivity status are attributed to the measurement parameters and sensitivity of the individual immunoassays and do not indicate reduced anti-HPV18 protective antibodies.
Brown et al. (Tue,) studied this question.