Intention-to-prevent analyses for estimating human papillomavirus vaccine efficacy in clinical studies.

HPV vaccine efficacy trials have been conducted in populations exposed to HPV infection (i.e., sexually active individuals); participants were not excluded from participating in the trials based on their HPV status at baseline. Thus, some participants could have been infected at baseline with 1 or more vaccine HPV types. Because HPV vaccines are prophylactic and do not affect existing HPV infections, prophylactic efficacy was assessed in a per-protocol population (those not infected at enrollment to the HPV type being analyzed who also completed the 3-dose regimen of vaccine and had no protocol violations). Supportive intention-to-treat (ITT) and modified ITT, were also conducted to include those with prevalent HPV infection. ITT analyses included those who received ≥1 dose of vaccine and had efficacy follow-up regardless of whether or not they were infected with HPV prior to vaccination. Efficacy in the ITT population simply reflects the amount of prevalent infection in a particular population of study subjects. Intention-to-prevent (ITP) analyses included those who received one dose of vaccine, had efficacy follow-up, and were not infected at enrollment to the HPV type being analyzed.