Adolescents, Young Adults and HPV Incidence: A Ten-Year Outlook

This paper will attempt to estimate the likely ten-year outlook for head and neck cancers in adolescents and young adults who have received the HPV vaccine. As will be discussed in the introduction, vaccination programs against HPV have focused heavily, although not exclusively, on cancers of the cervix.  This paper will emphasize the potential efficacy of the vaccines on incidence of HPV related head and neck cancers.  It will also attempt to project what the incidence for these cancers is expected to be in adolescents and young adults ten years from now.  

This paper will be divided into three sections.  The introduction will review much of the available knowledge on the epidemiology of HPV.  It will also provide a brief review of the history of vaccination efforts of the disease.  Section two, will discuss HPV-related head and neck cancers (HNCs) in more detail and provide a brief overview of HNC incidence patterns over the past four decades.  Section three will discuss the impact that current vaccination programs are having on the overall health of sexually active individuals.  This section will attempt to use this data to assess the likely incidence of HPV-related HNCs over the next decade. The fourth and final section of the paper is the summary and conclusion. 

Introduction

HPV, the virus that causes genital warts, is reportedly the most common sexually transmitted disease in the US.  Estimates of infection rates range from 50% to 70% of the adult population over the age of 14. It should be noted that not all strains of HPV are transmitted sexually.  The virus only requires the medium of skin to skin contact with an affected area to spread.  HPV infections are sometimes asymptomatic, and this allows it to spread from person to person more efficiently.  HPV infections often resolve on their own and most of the time will not cause serious long-term health problems.  

However, of the approximately 100 strains of HPV, there are a few that cause abnormal changes in the cells of the human body that can lead to cancer.   In particular, HPV strains 16 and 18 are known to cause the majority of HPV-related cancers.  HPV-18 is known to cause an estimated 70 percent of cervical cancers cases.1  However, HPV-16 is known to cause between 85 and 95 percent of oropharyngeal cancers.  HPV-16 is the cause of the most common HPV related cancers in men.2(50)

At an advanced stage, HPV is associated with lesions or masses in the neck or oropharyngeal region.  Symptoms of head and neck cancer manifest as pain, dysphonia or difficulty swallowing, and bleeding.  It is notable that the areas of the mouth affected by HPV are different from the ones affected by oral cancers caused by long-term tobacco or alcohol abuse.2(50-51)   

The disease is spreading more rapidly among white teens and young adults than among other racial groups of the same age.3  It also affects patients from more affluent socio-economic backgrounds. Many studies indicate HPV infection is more coincident in individuals who are HIV positive.4  Oral HPV infections rise with age.  They are prevalent among 3-5 percent of adolescents but among 5-10 percent of adults.  

In 2006 the FDA approved a vaccine called Gardasil for use in girls and women aged 9 to 26. It was later approved for use in males of the same age group to prevent genital warts and the resulting cancers.  In 2009 the FDA approved a second vaccine called Cervarix for girls and women aged 9 to 26.  It is hoped that large-scale vaccination will help prevent future HPV infections and the cancers that may cause.  

Recommendations from the CDC's Advisory Committee on Immunization Practices are that girls between the ages of 11 and 12 take one of the vaccines before they become sexually active.  This recommendation exists because most sexually active adults have had an HPV infection at least once in their lifetime.  The vaccine will have its maximum impact before the onset of regular sexual activity begins. Still the efficacy of giving the vaccine to adult males over the age of 26 is also being debated.

There is some evidence that the vaccines are having the intended effect. This benefit is happening despite political opposition, from conservative organizations and politicians, to vaccinating minors.  There are also some potential warning signs.  Moscicki5 reports that less than 50 percent of targeted youth have gotten at least one dose.  The percent receiving all three doses drops to about one-third of targeted youth.  This improvement is significant because there is evidence that not all patients are taking the required three booster shots.  Another impediment to mass vaccination is the reported cost. The cost is estimated to be around $200 per booster shot.  This is a rather steep expense for populations that are not covered by health insurance.  This issue of vaccine efficacy will be discussed in more detail in the final section.

HPV and HNC incidence 

The increasing attention on head and neck cancers, caused by HPV, has centered around two issues. 1) That the sexual behavior of youth has shifted towards behaviors that raises the risk for HPV related head and neck infections. 2) That medical professionals are just learning of, and evaluating, the connection between HPV related infections and oropharyngeal cancers. 2(52), 6  

There is a large volume of research which finds that the majority of oral cancers are caused by cigarette smoking rather than HPV.  According to surveillance data, the incidence of oropharyngeal cancers in the US increased between 1973 and 2007.  Over the same period, cancer incidence of the head and neck, including oral cancer, declined  and not coincidentally with the decline of tobacco consumption over the same period. 2(50), 3(1191) As noted above, the disease is spreading due to changes in the sexual behavior of adolescents and young adults that involves increased oral-genital contact. Young people engaging in this behavior believe that oral-genital contact is safer sex behavior and an alternative to activities that may also increase the chance of unwanted pregnancy. 

Head and neck cancers (HNC) are the fifth most common form of cancer diagnoses worldwide. HNCs place eighth among the causes of cancer related deaths.3(1191)  HNCs refer to a group of malignant growths that affects the following areas of the head and neck region: the lip, oral cavity, nose and paranasal sinuses, nasopharynx, oropharynx, hypopharynx and larynx. The majority of the malignancies are of squamous cell carcinoma histology.  

There is some agreement, among medical professionals, that vaccines may provide protection against a wider array of HPV infections than is presently indicated. But, at the present time, the efficacy of HPV vaccines as a preventative measure against oropharyngeal cancers is still unknown.2(52), 7  

Ten Year Outlook

This section will discuss the ten year outlook for incidence of HPV related head and neck cancers in adolescents and young adults who have been vaccinated against HPV.  Evaluating the ten-year impact for the HPV vaccine Gardasil is difficult to do at the present time.  This difficulty stems from the time horizon from HPV infection to onset of HNC diagnosis.  The time period can be as long as two decades or more.2(52) 

The recent implementation of the vaccine, combined with the long-time horizon for the development of cancers associated with HPV-16, mean that much more time is needed for a full evaluation.  Also, the epidemiology of oropharyngeal cancers is still being determined by medical professionals. D'Souza and Dempsey8 state that the only way to determine the effectiveness of HPV vaccination is by way of longitudinal ecological studies.  In order to draw conclusions, these studies will compare rates of HNCs before and after the introduction HPV vaccines. 

Two obstacles to vaccine efficacy may be the previously mentioned difficulties in distributing the vaccine to youth.  These are the cost of the vaccine and the opposition of parents.  In Australia both obstacles are overcome, at least for girls, by having the government pay for the vaccination.  Australia also makes vaccination virtually unavoidable by using school-based programs for distribution.  These methods have allowed vaccination coverage to reach 73 percent of the target population of girls aged 12-17.9  It may help that the vaccination program was accepted by parents in contrast to the US experience.  A significant decline in genital wart diagnoses in both women and men has already been observed in Australia. 5(54) It's notable that the vaccination program was not free for boys.  Therefore, a very low rate of coverage has been reported for males. The noticeable decline in HPV diagnoses is thus thought to be due to herd immunity, and confidentiality rules within hospitals and clinics.

Different effects may occur in the US because coverage rates are lower for US girls than for Australian girls. According to D'Souza and Dempsey,8(59) estimates projecting future lower incidence of HPV infection are based on highly optimistic studies.  These studies rely on high rates of immunization.  But in the US, only 27 percent of 13-17-year-old females were fully vaccinated as of 2009.4(63) D'Souza and Dempsey8(59) also report several studies indicating that individuals most at risk for contracting HPV are also the least likely to receive the vaccine. In addition, low female immunization rates raises the cost-effectiveness of immunizing males. The problem is that current policies of HPV immunization also depend on high female vaccination rates.  Therefore, more aggressive vaccination of males is needed.10 

Despite these difficulties, there may be some evidence of the direction of future trends for HNC incidence.  Indeed, data from short term trends can be somewhat illustrative of future incidence rates.  D'Souza and Dempsey8(59) report research on quadrivalent trials that have reached the five-year point following patients who have received the vaccine.  The data from the trials indicates that while HPV vaccine-type antibody titiers may decline, the protection provided by the vaccine remains quite strong.  Also, a group of women immunized with a mono-valent precursor of HPV-4 demonstrate very high sustained protection after 8 years. Bi-valent vaccine studies demonstrated similar efficacy after 6 years. This data seems to point to a period in which cancer-causing HPV prevalence will decline substantially. One can only infer from this data the likely impact on HPV-related HNC incidence.  But it does appear there may be a promising health benefit to the target population ten years from now.

Conclusion

In sum, the incidence of HPV-related HNCs has been growing in the US over the past four decades. This increasing prevalence appears to be due to changes in the sexual behavior of adolescent and young adult populations in response to the HIV-AIDS epidemic.  Although, vaccines have been deployed to reduce the incidence of HPV causing cancers it is unclear at this time what the future trends for coincident HNCs will be.  But some short-term preliminary evidence shows the efficacy of the vaccines even with limited distribution.  In light of this evidence the decline of HNC incidence is quite likely over the next decade.  However, in order to verify how much efficacy, the vaccines are producing, pre-and post-test studies are needed at the ten-year point. 

References

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