Signs that HPV vaccination may be reducing infections, even among unvaccinated

Issue 21, Summer/Fall 2013

Study finds a 49% decline in infections with cancer-linked strains for non-immunized girls


One of the great benefits of any successful vaccination campaign is what's known as 'herd immunity' -- the fact that as more people get immunized against a particular infection, the likelihood that non-immunized people will encounter the germ also declines.

Now, a study is hinting that herd immunity may be developing for human papillomavirus (HPV) 16 and 18 -- the two strains of the sexually transmitted virus that are most closely linked to cervical cancer.

In the study, led by Dr. Jessica Kahn of Cincinnati Children's Hospital Medical Center, rates of infection among vaccinated girls and young women fell by more than two-thirds during the four years since the vaccine was introduced.

But most significantly, rates of infection also dropped by nearly half among girls who had never received the vaccine.

"I was surprised at the decrease in the prevalence of HPV and the effectiveness of the vaccine in a realworld setting," lead author Jessica Kahn told ABC News at the time of the study's publication in the journal Pediatrics.

However, herd immunity is never a guarantee against infection with HPV 16 or 18, which are thought to cause 70 percent of cervical cancers.

"Although the study shows evidence of early herd immunity, the results cannot be generalized to imply you shouldn't get vaccinated," Kahn stressed. Immunization is "still very important... there is no way of knowing you are one of those protected unless you actually get the vaccine," she explained.

Gardasil® was the first HPV vaccine to gain approval for use in the United States, starting in 2006. Approval of a second vaccine, Cervarix®, soon followed. The CDC has recommended that both girls and boys get vaccinated against HPV at age 11 or 12, before sexual debut, or up until age 26 if they have missed that window. The vaccine is given in three doses spread over six months.

The new study involved two groups of young females aged 13 to 26, primarily urban blacks seen at two health clinics in Cincinnati. All of the participants had already become sexually experienced by the time they joined the study.

One group of 368 individuals was seen at the clinics during 2006-2007, before the HPV vaccine had become widely available; while the second group of 409 individuals was seen in 2009-2010, after availability had become widespread.

While no one in the 2006-2007 group had been vaccinated, 59 percent of girls/women in the 2009-2010 cohort had gotten at least one dose of the vaccine.

All of the participants were tested for 37 strains of HPV, including the four strains covered by the vaccines -- strains 16 and 18, linked to cervical cancer, and strains 6 and 11, thought to cause almost all cases of genital warts.

Comparing the two groups, Kahn and her colleagues found that rates of infection for the four HPV strains covered by the vaccine dropped by 69 percent among vaccinated females over the four years of the study.

Perhaps more remarkably, rates of infection also fell by 49 percent among participants who had never gotten the shot.

The trend was all the more significant considering that overall rates of HPV infection (infection with any strain of the virus) actually went up slightly for the participants between 2006-2010. In fact, "nearly one in four unvaccinated study participants was already positive for at least one high-risk HPV type," Kahn noted in a university news release.

She stressed however, that more and larger studies are needed to confirm this result. The study involved a particular population of girls and young women from one urban center; it's possible they might not be replicated nationally. The study was funded by the U.S. National Institutes of Health.