Although maximum reduction in the risk of condylomata was seen after three doses of human papillomavirus vaccine, receipt of two vaccine doses was associated with considerable reduction in risk, particularly among women who were younger than 17 at first vaccination, according to a Swedish study.
HPV infection causes condylomata (genital warts) and cervical cancer, and HPV vaccine prevents both, according to background information in the study, which was published in the Feb. 12 issue of the Journal of the American Medical Association. The typical dose schedule requires three doses of vaccine, but small clinical trials have reported measures of vaccine efficacy with fewer than three doses.
Although the primary goal of HPV vaccination programs is to prevent cervical cancer, genital warts related to HPV types 6 and 11 are prevented with a version of the vaccine and are the earliest measurable preventable disease outcome for the HPV vaccine.
Eva Herweijer, MSc, of the Karolinska Institutet, Stockholm, and colleagues assessed the association between the number of doses of HPV vaccination and genital warts among more than 1 million females ages 10 to 24 living in Sweden who were followed up between 2006 and 2010, using the Swedish nationwide population-based health data registers.
Among 20,383 new cases of genital warts, 322 occurred after receipt of at least one dose of the vaccine. Based on a statistical analysis, the researchers determined that maximum risk reductions occurred after three doses, but two doses also were protective to a lesser extent.
There was a small difference in the number of cases prevented by three doses versus two doses: Per 100,000 person-years (e.g. 100,000 women observed for one year), a three-dose regimen prevented 459 cases, compared with 400 cases for two doses.
The authors cautioned the study does not account for HPV disease outcomes other than genital warts, and more studies with longer follow-up are needed to assess whether these observed reductions apply for cervical cancer.