Epidemiology and natural history of human papillomavirus infections and type-specific implications in cervical neoplasia.

Worldwide human papillomavirus (HPV) prevalence in women with normal cytology at any given point in time is approximately 10% indicating that HPV is one of the most common sexually transmitted infections. HPV-16 is consistently the most common type and HPV-18 the second with some minor regional differences. Furthermore, across the spectrum of cervical lesions, HPV-16 is consistently the most common HPV type contributing to 50-55% of invasive cervical cancer cases strongly suggesting that this viral type has a biological advantage for transmission, persistency and transformation. The same phenomenon is observed albeit at a lower level for HPV-18 and HPV-45. Sexual behavioral patterns across age groups and populations are central to the description of the HPV circulation and of the risk of infection. The concept of group sexual behavior (in addition to individual sexual behavior) is important in exploring HPV transmission and has implications for defining and monitoring HPV and cancer prevention strategies. In natural history studies, the pattern of HPV DNA prevalence by age groups is similar to the patterns of HPV incidence. Rates of exposure in young women are high and often include multiple types. There is a spontaneous and rapid decrease of the HPV DNA detection rates in the middle-age groups followed by a second rise in the post-menopausal years. This article reviews: 1) the evidence in relation to the burden of HPV infections in the world and the contributions of each HPV type to the spectrum of cervical cellular changes spanning from normal cytology to invasive cervical cancer; 2) the critical role of the patterns of sexual behavior in the populations; and 3) selected aspects of the technical and methodological complexity of natural history studies of HPV and cervical neoplasia.