Posted on July 16th, 2012 by
The human papillomavirus (HPV) vaccine is reducing infections of virus strains associated with genital warts and cancer, even among those who remain unvaccinated, according to the results of a study published in Pediatrics.
HPV is the most common sexually transmitted infection in the United States. There are more than 100 different strains of HPV. Some types of HPV cause warts on the hands or feet; others cause genital warts; and some have been linked with cancer, most notably cervical cancer. In fact, HPV is believed to be the leading cause of cervical cancer. Other HPV-related cancers include vulvar cancer, anal cancer, penile cancer, and certain types of head and neck cancer.
There are currently two FDA-approved vaccines that protect against the types of HPV associated with cervical cancer. These are: Gardasil® (quadrivalent human papillomavirus [types 6, 11, 16, 18] recombinant vaccine), which prevents infection with four types of HPV—types 6, 11, 16, and 18; and Cervarix® (human papillomavirus bivalent [types 16 and 18] recombinant vaccine), which targets HPV types 16 and 18. HPV types 16 and 18 cause roughly 70% of all cases of cervical cancer, and HPV types 6 and 11 account for roughly 90% of genital warts.
Researchers conducted a study to compare prevalence rates of HPV before and after the introduction of the HPV vaccines. The study included two groups of sexually active women, ages 13 to 26, who were seen at two primary-care clinics in Cincinnati, Ohio. The first group included 368 women who were seen at the clinic in 2006 or 2007, before the HPV vaccines were widely available; none of them received the HPV vaccine. The second group included 409 women who were seen in 2009 or 2010, after the vaccines became widely available; about 60 percent of the second group had received the vaccine.
Participants responded to questionnaires regarding demographics and sexual activity and were tested for 37 strains of HPV. The results indicated that between 2006 and 2010, infection with any of the four strains of HPV prevented by Gardasil (HPV 6, 11, 16, and 18) decreased by about 60 percent (from 32 percent to 13 percent). Among the vaccinated, the rates of HPV infection fell 70 percent, from 32 percent to 10 percent. Among the unvaccinated, the rate fell 50 percent, from 30 percent to 15 percent.
The researchers speculated that the drop in HPV rates among the unvaccinated suggests “herd immunity”—which refers to a decrease in infections among the unvaccinated as a result of lower rates of infections among other people in their community who might otherwise be transmitting the disease.
Notably—although the prevalence of the four vaccine strains of HPV fell, the overall rate of HPV infection remained extremely high. In fact, the rate of non-vaccine strains of HPV increased from 60 to 76 percent.
The researchers concluded that four years after the introduction of Gardasil, there was a decrease in the four strains of HPV covered by the vaccine and also evidence of herd protection in the community. Currently, the U.S. Advisory Committee on Immunization Practices recommends that girls and boys get vaccines against HPV at age 11 or 12, before they’ve had sexual contact, or up until age 26 if they missed that window.
Kahn JA, Brown DR, Ding L, et al. Vaccine-type human papillomavirus and evidence of herd protection after vaccine introduction. Pediatrics. Published early online July 9, 2012. doi: 10.1542/peds.2011-3587
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