Jul 13, 2012

HPV Vaccination Offers Herd Protection

July 13, 2012 — Vaccination against the human papillomavirus (HPV) has shown positive results in reducing the prevalence of targeted viral types among adolescents and young women who received the vaccine, as well as some susceptible unvaccinated individuals who benefited from herd protection.

Jessica A. Kahn, MD, and colleagues from the Cincinnati Children’s Hospital and University of Cincinnati, Ohio, reported results of a 2-phase study in an article published online July 9 in Pediatrics. After propensity weighting, the overall prevalence rate for the 4 vaccine types of HPV fell substantially, from 31.7% to 13.4%, respectively (= .0001), in sexually active girls and women between ages 13 to 26. This change represents a 58% decline in prevalence, according to a Pediatrics journal news release.

Before widespread availability of the HPV vaccine, 368 unvaccinated teens and young women were recruited from 2 different primary care clinics between 2006 and 2007 — the prevaccination surveillance period. For the second, or postvaccination, surveillance study, 409 adolescent girls and young women were recruited between 2009 and 2010; 59.2% had received the vaccine. The mean age for both surveillance groups was roughly 19 years; most of the participants were black, non-Hispanic, and Medicaid recipients without health insurance.

For the postvaccination surveillance participants (2009 to 2010), the reduced rate of vaccine-type HPV was seen in both those who had received the vaccine (31.8% to 9.9%; = .0001) and those who had not received the vaccine (30.2% to 15.4%; = .0001). Per the journal news release, these represent 69% and 49% declines in infection rates, respectively.

Real-World Study Implications

The study’s real-world findings, drawn from observations of girls in their early teens to early adulthood, impressed Mark C. Poznansky, MD, PhD, director of the Vaccine and Immunotherapy Center at Massachusetts General Hospital in Boston, who was not involved in the study. He considers the findings to be intriguing, although preliminary.

“The data tend to support the concept of herd protection,” noted Dr. Poznansky in an email toMedscape Medical News. “This is an important first-of-its-kind study for HPV vaccination and clearly needs repeating rigorously at a larger scale,” he continued. “The implications for HPV vaccination are [worthwhile] and may support more emphasis on HPV vaccine uptake in the broader community.”

A nearly 50% decline in the rate of vaccine-type HPV infection among unvaccinated women suggests that herd immunity came into play after vaccines were widely available, Dr. Kahn noted in an interview with Medscape Medical News. The data suggest that the vaccines had disrupted the previous pattern of transmission from men to women to men to other unimmunized partners.

“Even young women who are unvaccinated can benefit,” Dr. Kahn said.

However, she expressed concern that such information could lead parents to forego vaccination for their children.

“The only way to maximize the personal benefits of vaccination is to maximize the rates of vaccination,” Dr. Kahn stated. In fact, added Dr. Poznansky, clinicians and the community should be “[s]upporting vaccination of males in addition to females.”

Potential Misinterpretation

Dr. Kahn also pointed out that she and her colleagues revealed an increase in the overall HPV infection rate from 68.3% before vaccines were introduced to 76.8% afterward (< .003). When broken down into vaccine vs nonvaccine HPV types, the latter increased by 23%, according to the researchers. It would be easy, said Dr. Kahn, to misinterpret the implications of a 23% increase in nonvaccine HPV infections in the postvaccination group. The finding may suggest type replacement, in which HPV strains unaffected by the vaccine could be filling a biological niche created by the elimination of some viruses, according to the study authors.

However, the authors argued that type replacement was unlikely on a biological basis because papillomaviruses have been genetically stable for thousands of years. A mutation allowing an HPV virus to fill a niche is unlikely, they note.

The finding is more likely a reflection of human behavior and a lack of study control patients to account for differences between vaccinated and unvaccinated women in the postvaccination surveillance study, Dr. Kahn proposed.

Women who received vaccinations at a younger age were probably also younger than other patients during their first sexual intercourse, Dr. Kahn explained. In fact, from results of this trial, which are consistent with other studies, the mean age at onset of sexual intercourse for vaccinated individuals was 14.6 vs 15.3 years for those who were unvaccinated (P = .0007). Younger onset of sexual intercourse, the authors explain, has been associated with higher rates of HPV infection in previous research.

“More study is necessary,” said Dr. Kahn, “but I think this is a statistical snafu. The results reflect the facts that women who are vaccinated practice riskier behaviors than women who don’t seek vaccinations.”

Study Limitations

The study’s limitations include its small sample size and a lack of consistency during patient recruitment, with some originating from clinical physicians and others from the women themselves, Dr. Poznansky noted.

Demographic and behavioral differences between the pre- and postsurveillance groups could have affected findings. Errors in the assessment of vaccination status were possible because of data gaps in the statewide registry.

Dr. Kahn is cochair of 2 HPV vaccine trials sponsored by the National Institutes of Health for which Merck Inc is providing vaccine and immunogenicity testing. She received funding from the Society for Adolescent Health and Medicine, which is supported by Merck. One author is an investigator on HPV vaccine clinical trials for which she received support from Merck, and another author serves on Merck’s Women’s Health Advisory Board, ad his laboratory receives direct funding from Merck. The other authors and Dr. Poznansky have disclosed no relevant financial relationships.

Pediatrics. Published online July 9, 2012. Abstract

— James Brice and Jacqueline A. Hart, MD

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