Delayed dosing intervals for quadrivalent human papillomavirus vaccine do not reduce antibody avidity

Abstract The quadrivalent HPV vaccine (HPV4) is recommended as a 3 dose schedule at 0, 2, 6 months for girls and boys. However, delays in dosing intervals frequently occur. We previously found that delayed intervals in girls resulted in similar or higher antibody (Ab) titers following series completion compared to the recommended schedule. We used archived specimens from that study to determine if delayed dosing intervals affected Ab avidity(a measure of how strongly Abs bind). The importance of avidity is unknown for HPV; however, avidity has been associated with clearance of other pathogens. We tested residual serum from 261 consenting healthy females, ages 9 through 18. Avidity index (AI) was determined just prior to and one month following dose 3 of HPV4 for each participant. Data were analyzed by dosing intervals, classified as:(1) on time dose 2 and 3, (2) delayed dose 2 and on time dose 3, (3) on time dose 2 and delayed dose 3, or (4) delayed dose 2 and 3. Avidity was measured using a modified multiplex ELISA, and AI was calculated as a ratio of IgG Ab bound in the treated and untreated sample. Overall, mean AI was highest for HPV16 and lowest for HPV6. As expected, AI did not differ between groups 1 & 3 or groups 2 & 4 after dose 2, however for most types mean AI was significantly higher for groups with delayed dose 2. Mean AI was higher post-dose 3 in all delayed dosing groups compared to the group 1 for all types. There was a weak to moderate correlation between overall AI and Ab titers for all types (HPV6r=0.27, HPV11 r=0.32, HPV16 r=0.25, HPV18 r=0.44; all p<0.0001). Our findings suggest longer intervals between doses result in higher antibody avidity after a complete 3-doses series, providing further evidence that delayed dosing of HPV4 does not hinder the immune response.