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Study Shows First Statistical Evidence for Herd Protection from HPV Vaccine

APR 24, 2017 | CAROLE ELLIS
A study conducted over the course of 11 years, and designed to evaluate the effectiveness of the human papillomavirus (HPV) vaccine in females, provided the first statistical evidence for herd protection as a result of HPV vaccination, said lieutenant commander Sara Oliver, MD, MSPH, during the Stephen B. Thacker Opening Session at the 2017 Epidemic Intelligence Service (EIS) Conference in Atlanta, Georgia, today, April 24, 2017. The study results also added further support for the belief that the HPV vaccine is “highly effective.”
 
Dr. Oliver and her team used National Health and Nutrition Examination Survey (NHANES) demographic and self-reported vaccination data along with HPV DNA gleaned from self-collected cervicovaginal specimens to estimate HPV prevalence during three distinct eras: 2003 to 2006 (pre-vaccine era), 2007 to 2010 (early-vaccine era), and 2011 to 2014 (recent-vaccine era). “Although our main objective was to assess HPV prevalence, we also wanted to assess herd protection,” Dr. Oliver stated in reference to using specimens from the designated eras.
 
The team used a series of data collections and “home interviews” to collect medical history, information on sexual behavior, and demographic information that allowed them to use weighted logistic regression models to adjust for race and poverty. “Sexually active” was defined as self-reported oral, vaginal, or anal sexual activity.
 
Cervicovaginal swabs were collected in 83% of the pre-vaccine era subjects, 86% of the early-vaccine era subjects, and 87% of the recent-vaccine (referred to in the presentation as “later-vaccine”) era subjects. When asked about the acceptability of self-collected cervicovaginal swabs, Dr. Oliver responded that although there can be issues with this type of collection, “There was a relatively high acceptability rate because they were not collected at home but in a mobile examination center (MEC).” She added that those who did not choose to submit cervicovaginal swabs said that they either had never had sex or that they had more than 3 encounters, thereby somewhat “balancing out” the non-submitting population.
 
Among the 18-24-year-old women in the recent-vaccine era group, HPV prevalence was 7.1%, compared to 18.8% in the pre-vaccine era and 16.9% in the early-vaccine era. In vaccinated females in the recent-vaccine era alone, HPV prevalence was 1.9%, and in unvaccinated females alone, HPV prevalence was 11.8%.
 
“That vaccine type prevalence decreased among non-vaccinated females indicates herd protection,” said Dr. Oliver. She also noted that the team did not note significant changes or differences in sexual behaviors between the vaccinated and unvaccinated populations.
 
When colleagues suggested that the decline in HPV prevalence could have been due to the virus’ declining presence in the population before the introduction of the vaccine, Dr. Oliver was skeptical. She said that the relatively higher prevalence of HPV in the unvaccinated population indicated that “HPV was not just going away anyway;” although she did note that the tests used in the study could not necessarily distinguish between vaccine-induced and wild-type HPV.
 
“We have shown that the vaccine is safe and effective. It is reducing prevalence. If you are a parent, get your adolescent vaccinated. If you are a provider, continue to make a strong recommendation that adolescents get vaccinated,” she said.
 
For the future, the team will continue to examine the data considering one-dose and multiple-dose vaccines and the addition of the male vaccination in recent years.
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