(RxWiki News) One of the benefits of immunization is that a large enough population of vaccinated individuals can often provide protection even for those not vaccinated in the community.
This principle of "herd immunity" has been shown to occur once more with the use of the vaccine for human papillomavirus (HPV) among teenagers.
"Ask your doctor about HPV vaccines."
Jessica Kahn, MD, MPH, a doctor in the division of Adolescent Medicine at Cincinnati Children's Hospital Medical Center, led a study looking at the rates of HPV infection among young women in a vaccinated and an unvaccinated population.
They first tested 368 unvaccinated girls, aged 13 to 26, in 2006 and 2007. All the girls, recruited from primary care clinics in Cincinnati, had had sexual contact, and 31.7 percent of them tested positive for HPV.
Then in 2009 and 2010, the researchers recruited 409 girls in the same age range from Cincinnati clinics; 59 percent of these girls had received at least one dose of the vaccine.
Overall, the rate of infections had dropped in the second group to 13.4 percent, a 58 percent drop. Among the girls who had received one or all of the vaccine shots, 9.9 percent were infected with HPV, a decrease of 69 percent for the HPV strains targeted by the vaccine.
Yet even the rate of infection for these HPV strains among unvaccinated girls was cut in half: only 15.4 percent of the unvaccinated girls in the later group tested positive for the strains included in the vaccine.
"Two of these HPV types, HPV-16 and HPV-18, cause about 70 percent of cervical cancer," Dr. Kahn said. "Thus, the results are promising in that they suggest that vaccine introduction could substantially reduce rates of cervical cancer in this community in the future."
Meanwhile, the rate of HPV infections for strains not protected against in the vaccine had increased from 61 percent in the first group to 76 percent in the later group.
The first vaccine for HPV, licensed in the US in June 2006, was shortly thereafter recommended by the US Advisory Committee on Immunization Practices for all girls and women aged 11 to 26.
"Four years after licensing of the quadrivalent HPV vaccine, there was a substantial decrease in vaccine-type HPV prevalence and evidence of herd protection in this community," the authors concluded.
The study was relatively small, and the participants were primarily young, black women, many of whom were using Medicaid insurance. The authors said that additional studies should be conducted.
The study was published July 9 in the journal Pediatrics. The research was funded by grants from the National Institutes of Health and the National Institute of Allergy and Infectious Diseases.
Dr. Kahn co-chaired two NIH-funded HPV vaccine trials partly supported by Merck, who also provides support for some of her other work. Two other authors receive research funding and other financial support from Merck.