Effectiveness of preventive human papillomavirus vaccination.

Infection with human papillomavirus (HPV) is one of the major preventable causes of cancer world-wide. Rapid increase in cervical cancer incidence also in some western countries with cervical cancer screening programmes is probably due to increase in background exposure to HPV in the young. HPV vaccines are in clinical trials and the results have been promising, but due to assortative transmission of the infection and multiple HPV types the effect of large-scale immunization on their spread will vary between different populations and by HPV type. Thus, it is difficult to predict the effect of vaccination on cancer incidence on the basis of efficacy trials only. In the following evaluation of population level, effectiveness of vaccination on cervical cancer incidence (1) and HPV prevalence (2) by combined cluster/individually randomized trials (1) and cluster/community randomized trials (2) are described.