Researchers from the U.S. National Cancer Institute and other organizations looked through trials that were previously conducted on 24,000 young women. The goal was to estimate how much of a protection the women were able to receive from various levels of doses. Vaccine efficacy against incident HPV-16/18 infections for three doses was 77·0% (95% CI 74·7–79·1), two doses was 76·0% (62·0–85·3), and one dose was 85·7% (70·7–93·7).A change in dosage could increase the number of young women vaccinated by reducing the cost of the shots. The study was published in the Lancet Oncology on June 9th, 2015.
Take a glimpse into the study here: http://www.thelancet.com/journals/lanonc/article/PIIS1470-2045(15)00047-9/fulltext