['We investigated the role of antibody responses as potential mechanism for the cross-protective vaccine-efficacies (VE) observed from randomized clinical trials of the HPV16/18 bivalent vaccine. Results HPV31 cases had lower HPV16 antibody levels than controls (OR 4th quartile compared with 1st quartile = 0.63; 95%CI: 0.36-1.08; p-trend = 0.03). HPV31 cases were also less likely to have detectable HPV31 neutralization, and HPV16 avidity than controls. No statistically significant differences by HPV18 antibody or HPV45 neutralization were observed among HPV45 cases and controls. Protection against HPV58 was not associated with any of the markers, confirming the specificity of our findings.', 'Samples are from three-dose HPV vaccine recipients from the Costa Rica HPV16/18 vaccine trial. Women with a new HPV31, HPV45, or HPV58 infections over four years of follow-up were compared with randomly selected control women--with no new infection with HPV31/45/58--with respect to HPV16 and HPV18 antibody, HPV31, HPV45, and HPV58 neutralization, and HPV16 avidity.'] Is cross-protective vaccine efficacy of the bivalent HPV vaccine against HPV31 associated with humoral immune responses : results from the Costa Rica Vaccine Trial?