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. While all of these analyses have been presented, there has been little discussion regarding their respective significance. In this methodological review, we show that an ITT analysis does not preserve an unbiased comparison of treatment groups in relation to estimating prophylactic HPV vaccine efficacy. Furthermore, ITP is more suitable at preserving an unbiased comparison of treatment groups in relation to estimating prophylactic HPV vaccine efficacy.