The licensure of a vaccine against human papillomavirus (HPV) types 6, 11, 16, and 18 represents a major advance in the armamentarium for preventing cervical cancer, genital warts, and other anogenital diseases related to infection with this virus. HPV infection is known to be ubiquitous,1 but it was not until recently that national prevalence estimates, at least for females, were available for the United States.2 These estimates indicate that 27% of women between the ages of 14 and 59 are infected with one or more genital types of HPV at a given point in time, with values as high as ∼45% for female adolescents and young adults aged 14 to 24 years.2 The frequency of cervical disease in the US population reflects this high prevalence of infection, with more than 20% of adult women reporting ever having had an abnormal Pap smear,3 resulting in an annual expense of more than 3.8 billion dollars for the follow-up and treatment of cervical dysplasia and cancer.4
In contrast to the recent influx of information on the national burden of HPV infection and cervical dysplasia/cancer, there has been a paucity of information about another important HPV-related condition, genital warts. Though not life threatening, genital wart diagnoses exact a significant financial toll on the health care system with more than 200 million US dollars spent each year on managing this disease.4 The psychological stress and shame associated with genital warts5 represents another important facet of this illness with pervasive effects in the population. Because the HPV vaccine currently licensed in the US provides protection against genital warts and cervical cancer,6 understanding the national burden of genital warts is critical to being able to evaluate the population-level impact of national HPV vaccination campaigns. Two previous national surveys, and a third reported in this issue (by Hariri et al.), use various methodologies to estimate the national prevalence of HPV-6 and/or -11, the 2 HPV types responsible for most cases of genital warts.2,7,8 Though these estimates indicate a prevalence of HPV 6 or 11 infection of ∼2% to 5% among sexually active adults in the US, information on the prevalence of clinically apparent genital wart disease has been lacking.
In this issue, a study by Dinh et al. provides for the first time national estimates of the prevalence of genital warts in the US adult population, providing a critical piece of information about HPV-related diseases that has previously not been described. This study is based on data from the 1999 to 2004 National Health and Nutrition Examination Surveys, a survey conducted annually by the National Center for Health Statistics at the Centers for Disease Control and Prevention that uses information collected from a random sample of the US civilian noninstitutionalized population and probability sampling weights to derive nationally representative data. From this, Dinh et al. report an estimated 5.6% of sexually active US adults between the ages of 18 and 59 years have ever been diagnosed by a medical provider with genital warts.
Only 2 other studies have evaluated the prevalence of genital warts in a broad (though not nationally representative) population. These studies generated annual prevalence estimates of 170 and 205 cases per 100,000 adults (0.17% and 0.21%) in years 2000 and 2001, respectively.9,10 These estimates are significantly lower than that reported by Dinh et al., because they are annual rather than cumulative prevalence estimates. Also, the 2 previous studies identified cases of genital warts using billing claim information and therefore estimated this disease specifically among the insured population who received a genital warts diagnosis from a medical provider. In contrast, the study by Dinh et al. derived data from a nationally representative population and identified cases of genital warts using self-report. It is likely that this latter approach identified more cases because individuals without insurance and those who self-diagnose and self-medicate would not be excluded.
In addition to providing national cumulative prevalence estimates for genital warts, the study by Dinh et al. also identified several demographic and behavioral characteristics associated with an increased likelihood of disease. Two of these associations were notable for their divergence from previous findings. First, this study found that female gender was associated with an increased likelihood of genital warts diagnosis across all age groups. This finding contrasts previous studies showing an increased prevalence of this disease among males in the older adult age ranges.9,10 Second, Dinh et al. also reported that non-Hispanic whites had a higher prevalence of genital warts when compared with other racial/ethnic groups. Few previous studies have investigated racial/ethnic variations in genital warts diagnoses, but one national survey indicated that, at least among women, the point prevalence of HPV infection due to any HPV type was highest for non-Hispanic blacks (39%) and lower for non-Hispanic whites (24%) and Mexican Americans (24%).2
An important question that could not be addressed in the study by Dinh et al. is the prevalence of genital warts in children less than 18 years. The lack of knowledge on this issue represents a significant gap in our understanding of the burden of HPV-associated disease—especially given that adolescents and young adults are the population at highest risk of acquiring HPV,11 and that the Advisory Committee on Immunization Practices has identified adolescents as the primary target population for HPV vaccination.4 Finally, as Dinh et al. acknowledge, reliance on self report of genital warts as the basis for estimating disease prevalence has limitations. Any associations identified in this study could be explained by sociodemographic-based differences in knowledge or recognition of genital warts, willingness to report this diagnosis, or health care seeking behavior, rather than actual differences in disease prevalence.
Despite these limitations, the findings of Dinh et al. raise several interesting questions that might warrant further investigation as we develop and refine policies regarding HPV vaccination. For example, are females actually more likely to develop genital warts than males, and if so, what are the behavioral and/or biologic reasons for this? How much of the racial differences in disease prevalence can be explained by differences in access to medical care? Do changing sexual practices by birth-cohort explain the higher cumulative prevalence estimates for genital warts among those in their 20s or 30s versus those in their 40s and 50s or is this difference due to recall bias? Answering these questions will improve our understanding of which populations or practices place individuals at highest risk of HPV-associated illnesses. This in turn could help to identify target groups where additional efforts at maximizing HPV vaccine usage might be warranted.
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