>> P-678: HPV Antibody Responses Among Subjects Enrolled in a 2- vs. 3-dose HPV Vaccine Trial

19:00 PM - 19:00 PM

Mel Krajden1,2, Simon Dobson2,3, Ron Chow1, Amanda Yu1, Darrel Cook1, Shelly McNeil4, Deborah Money2,5, Monika Naus1,2, Gina Ogilvie1,2, Martin Petric1,2.

1British Columbia Centre for Disease Control, Vancouver, BC, Canada; 2Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada; 3Vaccine Evaluation Centre, Children's & Women's Hospital, Vancouver, BC, Canada; 4Centre for Vaccinology, Dalhousie University, Halifax, NS, Canada; 5Women's Health Research Institute, Children's & Women's Hospital, Vancouver, BC, Canada.

Objective: To compare pseudovirus neutralizing antibody (PsV NAb) and Merck Competitive Luminex Immunoassay (cLIA) titres in subjects enrolled in a 2- vs. 3-dose HPV vaccine (Gardasil®) trial.

Methods: 828 females were enrolled in 3 dosing regimens: 1) 9-13 yrs: 2 doses at 0 & 6 months; 2) 9-13 yrs: 3 doses at 0, 2 & 6 months; 3) 16-26 yrs: 3 doses at 0, 2 & 6 months. Baseline and 7 month sera were available from 798 women. PsV NAb was reported as the geometric mean titre (GMT) and the Merck cLIA as log Merck units. GMT and cLIA results were compared by study group.

Results: At baseline 15/798 (1.9%) and 9/798 (1.1%) subjects were seropositive for HPV 16 and 18 respectively; these were excluded from analysis. At 7 months all subjects had seroconverted. The correlation coefficients for the two assays were 0.669 for HPV 16 and 0.903 for HPV 18. Antibody levels at 7 months were:
Study Arm                                          Log PsV NAb GMT (Range)                        Log Merck Units (Range)
HPV 16     9-13 yr; 2 dose                             9.5 (6.5-12.4)                                              8.9 (4.2-11.7)
                 9-13 yr; 3 dose                             9.4 (7.2-12.0)                                              9.0 (5.1-11.7)
                 16-26 yr; 3 dose                           8.7 (5.1-11.3)                                              8.2 (4.7-11.2)
HPV 18     9-13 yr; 2 dose                             8.2 (4.4-10.6)                                              7.1 (3.7-9.0)
                 9-13 yr; 3 dose                             8.5 (5.1-11.7)                                              7.5 (4.3-11.0)
                 16-26 yr; 3 dose                           7.5 (3.7-10.6)                                              6.5 (2.8-10.2)

Conclusions: NAb GMTs and Merck cLIA signals correlated well, but for HPV 16, PsV NAb GMTs were often higher for samples with lower cLIA signals. The 2- vs. 3-dose regimen resulted in similar post-vaccination titres for 9-13 yr olds, whereas 16-26 yr olds, who had only 3 doses, demonstrated significantly lower titres (p <0.001), consistent with published reports. Titres for HPV 16 were higher than for HPV 18 for all subjects.

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