A decision-theory based model of HPV vaccination and infection dynamics

HPV, or ​human papillomavirus​, is the most prevalent sexually transmitted infection (STI) in the United States, with an estimated 79 million Americans infected [1]. There are over 150 different types of HPVs, some which affect the skin and cause common warts, and others which affect the genital area and can be transmitted only through sexual contact. Within the genital HPVs there is a further breakdown of types into low-risk and high-risk. Most HPV infections are harmless and clear on their own; in fact, most sexually active people will become infected at some point and not even know it. But in some cases HPV can manifest itself and cause symptoms such as genital warts, which are caused by low-risk types, or it can lay dormant and later develop into cancer, which is primarily caused by high-risk types [2]. Cervical cancer is the most common HPV-related cancer in women. Each year in the U.S., about 11,700 new cervical cancers are diagnosed, and it is estimated that 90% of these cancers are caused by an HPV infection. Several other genital cancers are often caused by HPV, including anal, rectal, penile, vaginal, and vulval. In addition, 70% of cancers of the oropharynx (back of the throat) are caused by HPVs. Oropharyngeal cancer is the most prevalent HPV-related cancer overall, and these cancers actually appear predominantly in men [3]. In total, about 40,000 cancers are diagnosed every year in sites where HPV is found, and HPV causes about 31,500 of these [4]. A vaccine for HPV was approved in 2006, with 2 or 3 doses required for it to be fully effective [5]. The vaccine protects against the most common high-risk HPV types, which together account for about 90% of HPV-related cervical cancers, as well as the majority of other HPV-related cancers and genital warts [6]. Initially the vaccine was only recommended for girls, beginning at ages 11-12. Then in 2009 the FDA began recommending that boys also receive the vaccine at the same age [5]. Unfortunately, the HPV vaccine has seen lower uptake rates than other child and adolescent vaccines in the United States. As of 2016, the CDC reports that 60% of teens ages 13-17 had received at least 1 does of the HPV vaccine, and only 43% were fully vaccinated. The numbers are slightly higher among females (65% received ​>​1 dose and 50% were fully vaccinated) compared to males (56% received ​>​1 dose and 38% fully). In contrast, 88% of teens have received a tdap (tetanus, diphtheria, and acellular pertussis) vaccine, and 82% have received a meningococcal vaccine, both of which are also recommended at ages 11-12. Childhood vaccines have even higher uptake rates, with over 90% of teens vaccinated against chickenpox, hepatitis B, and MMR (measles, mumps, and rubella) [7]. Experts have speculated about the reasons for this discrepancy in vaccination rates. Proposed explanations include the relative newness of the vaccine, the association with sexually-transmitted disease, and the general anti-vaccine movement, among others [8]. We believe that many factors play a role in the vaccination decision, and that these factors are largely influenced by parents’ perceptions of the disease and the vaccine itself. Thus instead of studying one possible reason, in this paper we aim to construct a more holistic model of the