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Adapted from the NCI Cancer Bulletin, vol. 4/no. 10, March 6, 2007 (see the current issue).
Data from the National Health and Nutrition Examination Survey (NHANES) published in the February 28, 2007, Journal of the American Medical Association (JAMA) have provided the first national estimate of the prevalence of human papillomavirus (HPV) infection among women in the United States aged 14 to 59. Investigators found that a total of 26.8 percent of women overall tested positive for one or more strains of HPV (see the journal abstract).
Overall prevalence included both low-risk and high-risk HPV types. Low-risk types of HPV can cause genital warts or other nonmalignant conditions. High-risk types of HPV can cause cervical cancer, and up to 70 percent of cervical cancers worldwide are caused by two high-risk strains alone - HPV types 16 and 18.
"We think it's important to let women know how common [HPV] is," says Dr. Eileen Dunne from the Centers for Disease Control and Prevention, lead author of the study.
All women aged 14 to 59 selected to participate in the 2003-2004 NHANES, designed to collect health and nutrition measurements from a representative sample of the U.S. population, were eligible to participate in the HPV study. Most eligible women submitted self-collected cervicovaginal swab samples, 1,921 of which could be used for DNA extraction and HPV detection and typing.
Overall, 26.8 percent of women tested positive for one or more strains of HPV. Prevalence of HPV was highest in women ages 20-24. Among all participating women, the prevalence of high-risk types of HPV was 15.2 percent. The prevalence of HPV types 6, 11, 16, and 18 - the types targeted by the HPV vaccine Gardasil - was 3.4 percent overall, translating to an estimated 3.1 million exposed women in the studied age groups.
An important limitation of this study, explains Dr. Philip Castle, an investigator in the National Cancer Institute's Division of Cancer Epidemiology and Genetics, is that "this prevalence study is only a snapshot of HPV in the country, but doesn't tell us anything about total lifetime exposure to HPV or the risk of precancer and cancer. Risk is not testing positive at one time point - it's the persistence of carcinogenic types of HPV."
Persistence of HPV infection - how long the virus remains active in a woman's body - is key to whether exposure to a high-risk type of HPV leads to cervical cancer. "If an infection from specific oncogenic HPV types does not clear within a period of time (about six months), it puts that woman at greater risk for cervical precursor lesions," explains Dr. Dunne.
"There's a lot of misunderstanding about HPV's complex natural history," Dr. Dunne continues. "It's not that if you get the infection, you get the disease. It's a common infection, and a lot of them clear [on their own]. The important thing is that women have routine cervical cancer screening with
Pap tests, and appropriate groups of women receive the preventative vaccine that's now available."
The baseline data provided by this study may help researchers determine the public-health impact of HPV vaccination, explain the authors. However, "This is one piece of the big puzzle," says Dr. Dunne. "Looking at diseases such as genital warts, cervical cancer precursors, and cervical cancer will also be necessary to monitor vaccine impact."
"What we need…is a surveillance program that's linked to HPV vaccination uptake over a long period of time, so we can see the impact, and also any potential adverse effects, of an HPV vaccine," agrees Dr. Castle. "By monitoring benefits and risks of HPV vaccination, we can optimize the use of HPV vaccines to achieve the greatest good for women."
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