Monday, August 17, 2009

Prevalence of genital HPV - HPV Vaccine

United States

See also: HPV: Prevalence of Genital HPV

According to the Centers for Disease Control and Prevention, by the age of 50 more than 80% of American women will have contracted at least one strain of genital HPV. Both men and women can be carriers of HPV. HPV is the most common sexually transmitted infection in the US. A large percentage of the American population is infected with genital HPV because HPV is highly communicable. As a result, American public health experts recommend widespread HPV vaccination.

Only a small percentage of women with HPV develop cervical cancer. Each year, between 250,000 and 1 million American women are diagnosed with cervical dysplasia, which is caused by HPV and is a potential precursor to cervical cancer. About 11,000 American women are diagnosed with cervical cancer every year, and about 3,700 die per year of the disease. Most cancers occur in those who have not had Pap smears within the previous five years.


HPV-induced cancers

There are 19 "high-risk" HPV types that can lead to the development of cervical cancer or other genital/anal cancers; some forms of HPV, particularly type 16, have been found to be associated with a form of throat cancer.[28] Studies have found that human papillomavirus (HPV) infection is responsible for virtually all cases of cervical cancer.

Condoms protect against HPV, but do not completely prevent transmission. College freshmen women who used condoms consistently had a 37.8% per patient-year incidence of genital HPV, compared to an incidence of 89.3% among those who did not.

No data is kept by the U.S. government on genital wart infection rates,[33] however it is estimated that 20 million people are presently infected with genital warts, and there are six million new cases of genital warts every year in the United States.


Worldwide

Worldwide, cervical cancer is the fifth most deadly cancer in women. There are an estimated 470,000 new cases of cervical cancer, and 233,000 deaths per year.


Vaccination and public health

Gardasil is available poster for a university health clinicAccording to the American National Cancer Institute, "Widespread vaccination has the potential to reduce cervical cancer deaths around the world by as much as two-thirds, if all women were to take the vaccine and if protection turns out to be long-term. In addition, the vaccines can reduce the need for medical care, biopsies, and invasive procedures associated with the follow-up from abnormal Pap tests, thus helping to reduce health care costs and anxieties related to abnormal Pap tests and follow-up procedures."

Current preventive vaccines protect against the two HPV types that cause about 70% of cervical cancers worldwide. Because of the distribution of HPV types associated with cervical cancer, the vaccines are likely to be most effective in Asia, Europe, and North America. Vaccines that protect against more of the types common in cancers would prevent more cancers, and be less subject to regional variation.

Only 41% of women with cervical cancer in the developing world are able to access medical treatment for their illness. Therefore, prevention of HPV by vaccination may be a more effective way of lowering the disease burden in developing countries than cervical screening.[citation needed] However, individuals in many resource-limited nations, Kenya for example, are unable to afford the vaccine.


Vaccine target populations

Gardasil and Cervarix are preventative vaccines and do not have efficacy for treating active HPV infection. They are recommended for women who are 9 to 25 years old who have yet to contract HPV. However, since it is unlikely that a woman will have already contracted all four viruses, and because HPV is primarily sexually transmitted, the U.S. Centers for Disease Control and Prevention has recommended vaccination for women up to 26 years of age.

When Gardasil was first introduced, it was recommended as a prevention for cervical cancer for women that were 25 years old or younger. New evidence suggests that all Human Papillomavirus (HPV) vaccines are effective in preventing cervical cancer for women up to 45 years of age.

In November 2007, Merck presented new data on Gardasil. In an investigational study, their HPV vaccine reduced incidence of HPV and 18-related persistent infection and disease in women through age 45. The study evaluated women who had not contracted at least one of the HPV types targeted by the vaccine by the end of the three-dose vaccination series. Merck planned to submit this data before the end of 2007 to the U.S. Food and Drug Administration (FDA), and to seek an indication for Gardasil for women through age 45.


Vaccination during pregnancy

In the Gardasil clinical trials, 1,115 pregnant women received the HPV vaccine. Overall, the proportions of pregnancies with an adverse outcome were comparable in subjects who received Gardasil and subjects who received placebo. However, the clinical trials had a relatively small sample size. Currently the vaccine is not recommended for pregnant women.The long-term effects of the vaccine on fertility are not known, but no effects are anticipated.


Vaccination of males

Gardasil can also be used in males to reduce their risk of genital warts and precancerous lesions caused by HPV. The reduction in precancerous lesions is expected to reduce the rates of penile and anal cancers in men. Since penile and anal cancers are much less common than cervical cancer, HPV vaccination of young men is likely to be much less cost-effective than for young women. From a public health point of view, vaccinating men as well as women might be useful if it decreased the virus pool within the population. Gardasil is in particular demand among gay men, who are at significantly increased risk for genital warts, penile cancer, and anal cancer.

As with females, the vaccine must be administered before infection with the HPV types covered by the vaccine occurs. Vaccination before adolescence makes it more likely that the recipient has not been exposed to HPV.

In the UK, HPV vaccines are licensed for boys aged 9–15. Merck, the maker of Gardasil, is expected to ask the U.S. Food and Drug Administration (FDA) for permission to market the vaccine in the United States for boys and men ages 9–26. The vaccine is already available in the United States and can be used off label for the vaccination of males.


Mechanism of action

The latest generation of preventive HPV vaccines is based on hollow virus-like particles (VLPs) assembled from recombinant HPV coat proteins. The vaccines target the two most common high-risk HPVs, types 16 and 18. Together, these two HPV types currently cause about 70 percent of all cervical cancer. Gardasil also targets HPV types 6 and 11, which together currently cause about 90 percent of all cases of genital warts.

Gardasil and Cervarix are designed to elicit virus-neutralizing antibody responses that prevent initial infection with the HPV types represented in the vaccine. The vaccines have been shown to offer 100 percent protection against the development of cervical pre-cancers and genital warts caused by the HPV types in the vaccine, with few or no side effects. The protective effects of the vaccine are expected to last a minimum of 4.5 years after the initial vaccination.

While the study period was not long enough for cervical cancer to develop, the prevention of these cervical precancerous lesions (or dysplasias) is believed highly likely to result in the prevention of those cancers.

Although a 2006 study suggests that the vaccines may offer limited protection against a few HPV types that are closely related to HPVs there are other high-risk HPV types are not affected by the vaccines. Ongoing research is focused on the development of HPV vaccines that will offer protection against a broader range of HPV types.[47] There is also substantial research interest in the development of therapeutic vaccines, which seek to elicit immune responses against established HPV infections and HPV-induced cancers.


Vaccine implementation

Main article: vaccination policy

In developed countries, the widespread use of cervical "Pap smear" screening programs has reduced the incidence of invasive cervical cancer by 50% or more. Current preventive vaccines reduce, but do not eliminate the chance of getting cervical cancer. Therefore, experts recommend that women combine the benefits of both programs by seeking regular Pap smear screening, even after vaccination.

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