Mathematical Model of HPV Provides Insight into Impacts of Risk Factors and Vaccine

O ver the next few years, we can expect to see the licensing of one or more prophylactic vaccines against human papillomavirus (HPV) infection, considered a necessary precursor to cervical cancer. Among over 40 known types of HPV, 15 have been identifi ed as being " high risk " or oncogenic. The vaccines will likely target at least two of these oncogenic types, HPV types 16 and 18, which have been found to contribute to nearly 70% of cervical cancer cases worldwide—although there is large variation in contribution by geographic region [1,2]. Although cytology-based cervical cancer screening has been successful in decreasing the incidence of invasive cancer in many developed countries, vaccination may be the most promising intervention from a global perspective—reducing the burden of cervical cancer in resource-poor settings, where most cases occur. Mathematical models can be useful tools in exploring disease trends and health consequences of interventions in a population over time. In the case of cervical cancer, in which the time from acquisition of HPV infection to development of invasive cancer can be two decades or more, models can be used to translate short-term fi ndings from vaccine trials into predictions of long-term health outcomes [3–9]. In a new study in PLoS Medicine , Barnabas and colleagues present a particular type of mathematical model known as a dynamic model to directly assess the effect of sexual transmission of HPV type 16 in a Finnish population [10]. This model is the fi rst to capitalize on empirical data on sexual history and HPV seroprevalence from a single population to estimate the transmission of HPV 16 between men and women, and to explore the impacts of risk factors on observed changes in cervical cancer incidence over time. The model itself, although technically complicated, is conceptually straightforward when broken down into its vital parts. Adult men and women were modeled separately, and in the model they formed partnerships over time based on sexual history data from Finnish adults within the period of 1971–1999. HPV type 16 is transmitted between men and women in sexual relationships at a rate that is dependent on the prevalence of HPV 16 in the population, as well as on age and sexual activity level; women infected with HPV 16 can further develop precancerous lesions and invasive cervical cancer (ICC). As a general rule, not all parameters in a model are known, which requires adjustment (or …