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Public health officials are heralding the availability of a vaccine that prevents a high percentage of cervical cancers, and Michigan lawmakers recently debated whether to require girls entering the sixth grade to be inoculated. A basic understanding of the new vaccine is necessary when considering public health policy.
Cervical cancer is the leading gynecological malignancy in the United States, causing an estimated 9,710 new cases and 3,700 deaths in 2006. It is a serious problem worldwide, causing about 275,000 deaths annually.
In a breakthrough for cancer prevention, Merck & Co. Inc. on June 8, 2006, received approval from the U.S. Food and Drug Administration for the use of Gardasil® to prevent the majority of cervical cancers. It is the first vaccine to prevent a cancer. The Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices has recommended inoculation for girls and women aged 11 to 26. Proper inoculation requires three injections over six months. A second cervical cancer vaccine, developed by GlaxoSmithKline, is awaiting review by the FDA.
Cervical cancer is most often caused when particular strains of the human papillomavirus are transmitted sexually. HPV is considered the most common sexually transmitted disease, with about 60 percent of women acquiring the infection within five years of becoming sexually active. However, only a very small proportion of women persistently infected with high-risk strains of HPV actually develop cervical cancer.
There exist more than 100 types of HPV, only a few of which cause cervical cancer. Many types of HPV simply produce common warts. Others may produce genital warts and lesions in the larynx (which may become malignant on rare occasions). A group of about 15 HPV types, characterized as "high-risk," can cause changes to cells of the cervix that can lead to cervical cancer.
HPV are small, very simple viruses composed of an outer layer of two proteins and a core of DNA (the genetic code of the virus). Infection by high-risk HPV strains can interfere with the normal function of the cells in the cervix. Consequently, these cervical cells may divide abnormally and result in cancer.
Although the presence of HPV is necessary for cervical cancer to develop, it is not sufficient to cause the disease. Other risk factors — including smoking, multiple sex partners and infection from other sexually transmitted diseases — increase the likelihood of cancer. Genetics probably also play a role in the body’s susceptibility to HPV infection.
Unlike some vaccines, the HPV vaccine does not contain the live virus and, therefore, is not infectious. Instead, the vaccine is produced from the outer layer of HPV proteins.
Clinical trials involving more than 20,000 women have shown the HPV vaccine to be about 99 percent effective in blocking HPV infection. The vaccine does not cure an existing HPV infection, but instead protects against new infection. It is not known precisely how long the vaccine protects against infection by high-risk HPV, although antibodies were present in women five years after inoculation. Other than temporary soreness at the injection site, there appear to be no serious side effects from the vaccine.
Annual PAP tests are still recommended for vaccinated women because the vaccine protects against only two of the HPV strains that cause cervical cancer. About 30 percent of cervical cancers caused by other high-risk HPV will not be prevented by the vaccine.
Development of an HPV vaccine involved contributions by scientists at Georgetown University in Washington, D.C., the University of Rochester in Rochester, N.Y., Queensland University in Brisbane, Australia, and the National Cancer Institute in Bethesda, Md. It took 10 years of legal wrangling before the U.S. Patent Office ruled on overlapping claims. The dominant patent for the technology was granted to Georgetown University. Merck and GlaxoSmithKline have cross-licensed the technology.
There are no recommendations for vaccinating men at present. However, high-risk HPV strains are associated with cancers of the male genitals, oral cavity and tonsils. The inoculation of men might also help to reduce transmission of HPV and thus the incidence of cervical cancer.
Since the vast majority of cancers are not caused by viruses, the HPV vaccine cannot be used as a model for preventing most other forms of cancer. Nevertheless, Gardasil® and GlaxoSmithKline’s Ceravix® represent the remarkable progress being achieved in cancer research. These are unique vaccines — the first ever developed to protect against infection by viruses that can lead to cancer.
