Prevention of HPV Infections
Avoiding HPV exposure and limiting HPV progression. Genital HPV infections are important public health problems. Because HPV infections are usually without symptoms and can last for decades, it is not possible to accurately assess the risk of acquiring a genital HPV infection from any specific sexual partner.
Sexual behaviors are the most important risk factors for acquiring genital HPV infection (8), and because of this, delaying sexual intercourse, limiting direct and indirect sexual partners, and using condoms consistently could reduce the risk of HPV infection. Unfortunately, efforts to promote safe sex practices have had mixed results.
Thus, nationally, 7.4% of high school students had their first sexual intercourse before 13 years of age and 61.6% had had sexual intercourse by 12th grade. By ninth grade, 10.8% had had more than four sex partners. (18)
Among HPV infected women, other factors (such as cigarette smoking, long term use of oral contraceptives, and co-infection with other sexually transmitted agents) influence the risk of progression to precancerous changes and to invasive cancer. (7) Therefore, not smoking and consistent use of condoms may reduce the risks of HPV infection progressing to precancer and cancer.
Consistent use of condoms may provide some protection against acquiring HPV, but HPV can also be passed from person to person by skin to skin contact with a HPV infected area. Nevertheless, condoms also protect against other sexually transmitted infections which are associated with progression of HPV infection to precancer and cancer.
Regular Pap screening—and then the treatment of any abnormalities that are detected—prevents HPV progression to cervical cancer.
Preventive HPV Vaccines. A potential tool for preventing HPV infections and thus cervical and anogenital cancers are vaccines. (19) Two vaccines have been under development in the United States and one of these was licensed in June, 2006. The new vaccines consist of the L1 protein of HPV types 16 and 18—the types which cause about 70% of the cases of cervical cancer—have been assembled into “virus-like-particles” (that is, the particles look like the virus to the immune system but do not contain DNA and therefore cannot replicate). Both the licensed and the vaccine under development contain these two “high risk” HPV types.
In clinical studies, both vaccines have been efficacious in a high percentage of women who had not previously been infected. (6, 20, 21) The licensed vaccine also includes components to protect against HPV 6 and HPV 11—which cause genital warts—this vaccine also has been shown to prevent infection with those HPV types and to prevent warts due to these two HPVs.
Although the vaccines may not prevent transient HPV infection, the vaccines have been shown to reduced the acquisition of persistent infections due to the HPV types in the vaccines—as well as the diseases that they can cause.
Because these vaccines will only prevent infection with the two most common types of HPV that cause cervical cancer, they will not eliminate this cancer; however, they should greatly reduce the number of cases.
In the initial vaccine trials, the candidate vaccines have appeared safe, with no vaccine-attributable serious adverse events (hospitalizations or deaths), although the vaccine recipients experienced pain, swelling and redness at the injection sites. (6, 20, 21)
How long the vaccines will protect those who have been immunized is not yet known.
Therapeutic HPV Vaccines. The HPV vaccines that may soon be licensed are not expected to affect pre-existing HPV 16 and 18 infections. However, a number of vaccines that could cause the immune system to target infected cells are being studied and may provide another approach to the management of persistent HPV infection in the future. (8)
HPV vaccine acceptance
Implementation of a preventive HPV vaccination program poses a number of important challenges that will need to be addressed, not the least of which is the fact that many adolescents, young adults, and healthcare personnel are unfamiliar with HPVs and the consequences of HPV infections (11, 21).
HPV vaccines directed at HPV types 16 and 18 have the potential to markedly impact the frequency of cervical cancer. However, they will be effective only if susceptible people accept them. Healthcare personnel, parents, adolescents and young adults will all have a role in determining vaccine acceptance.
Who should receive preventive HPV vaccines?
Initially, the new vaccine has been licensed for preadolescent girls and young women 9 to 26 years of age. Because young women—once they become sexually active—can quickly become infected with high-risk HPV, initial recommendations focused on routine vaccination for girls 11-12 years of age, although the series can be started as young as 9 years of age—prior to when most children are expected to become sexually active. In addition catch-up immunization is also recommended for adolescent and young adult women 13 to 26 years of age, most of whom may not yet be infected with HPV 16 and 18. Data to permit licensure and recommendations for the catch-up HPV immunization for women older than 26 years of age should become available soon.
In the future HPV vaccines that include additional high risk strains will hopefully become available.
Because little is known about the epidemiology of HPV infection among preadolescent boys and men, additional studies will be required to better understand how men and boys get HPV and transmit it to others. In the future, it is anticipated that HPV vaccines will also be licensed for use in preadolescent boys and young men.
Issues concerning vaccination of preadolescents. There are likely to be concerns about vaccinating preadolescents against predominantly sexually transmitted infections. (23)
Parents’, adolescents’ and young adults’ attitudes. Parents must consent for adolescent immunization and most adolescents over 18 years of age are guided by their parents concerning vaccines. (24, 25) Most studies have found that parents are concerned about safely preventing their children from getting a serious disease and that they are less concerned about how the disease is acquired. However, some parents may hesitate to immunize their children against a sexually transmitted disease because they don’t want to appear to condone sexual risk-taking. A universal recommendation for vaccine use may diminish this concern. Understanding the disease is likely to improve parents’ willingness to accept and recommend the vaccine for their child.
Similarly, adolescents and young adults have been found to accept vaccines that were safe and protected against severe disease, particularly when it was encouraged by their healthcare provider or their parent.
Barriers to the introduction of HPV Vaccines. Currently, there are no public health intervention measures that routinely bring preadolescent girls or adolescent and adult women to a clinic three times over a six-month period—the time needed to administer the three vaccine doses. (19) Recently two new immunizations have been recommended for 11 year old children and adolescents and young adults: tetanus, diphtheria and acellular pertussis (Tdap) and quadravalent meningococcal conjugate vaccine (MCV4), but neither requires a second and third healthcare visit.
A barrier to the widespread use of these vaccines may also be the cost of the vaccines. The vaccines that are now in clinical trials will likely be expensive. Nevertheless, it may be more cost-effective to use the vaccines than to treat invasive cervical cancer and to make up for the loss of productive life. For example, in the United States and other developed countries these types of vaccines may be cost-effective because of a reduction in cervical screening and treatment programs. (22)
A barrier for the immunization of those 18 years of age and older, for all vaccines, has been the absence of an immunization infrastructure similar to that which is available for children. Because these HPV vaccines are recommended for routine, universal administration to 11-12 year old girls, the costs for the immunizations for the underinsured child less than 18 years of age will be borne by the Vaccines for Children Program. However, no such program exists for people 18 years of age and older.
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18. Centers for Disease Control. (2004) Youth behavior surveillance - United States, 2003. MMWR 53:ss-2.
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21. Villa LL, Costa CA, Petta C, et al. (2005) Prophylactic quadrivalent human Papillomavirus (types 6, 11, 16, 18) L1 virus-like particle vaccine in young women: a randomized double-blind placebo-controlled muticentre phase II efficacy trial. Lance Oncol 6:271-8.
22. Goldie SJ, Kohli M, Grima D, et al. (2004). Projected Clinical Benefits and Cost-effectiveness of a Human Papillomavirus 16/18 Vaccine. Journal of the National Cancer Institute, 96(8):604-615.
23. Zimet GD. (2006) Understanding and overcoming barriers to human Papillomavirus vaccine acceptance. Curr Opin Obstet Gynecol 18 (Suppl 1):S23-28.
24. Short MB, Rosenthal SL. (2006) Fostering acceptance of human Papillomavirus and herpes simplex virus vaccines among adolescents and parents. Curr Opin Pediatr 18:53-7.
25. Mays RM, Sturm LA, and Zimet GD (2004). Parental perspectives on vaccinating children against sexually transmitted infections. Social Science & Medicine, 58(7):1405-1413.
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