written by Tara Haelle

Do you have teenagers? If you do, I hope you’ve already gotten them the HPV shots because there’s some great recent news about the vaccine — one of only two vaccines* in existence that can actually prevent cancer.

According to a CDC-funded study this week in The Journal of Infectious Diseases, among teenage girls, infections from the four strains of HPV that the vaccine protects against have been cut in HALF in just the past decade. It’s hard to emphasize how remarkable this is, especially given that only about a third of girls have received the full three-shot series of the vaccine.

HPV stands for human papillomavirus, the sexually transmitted infection that approximately 80 percent of sexually active adults have had or carried at some point in their lives. The CDC estimates that 79 million Americans, mostly in their late teens and early 20s, are infected with HPV. Most HPV strains have no outward symptoms and clear up on their own with no long-term effects, but four strains (HPV-6, -11, -16 and -18) are responsible for the majority of cancers that can be caused by HPV.

The best known of these is cervical cancer: wiping out all the cancer-causing HPV strains would just about wipe out all of cervical cancer. But thanks to Michael Douglas’s recent admission, there’s also been more awareness that HPV can cause throat and neck cancers in men (and women) too. HPV has also been linked to penile and anal cancers. In fact, the CDC reports that women suffer about 19,000 cancers caused by HPV each year, primarily cervical; men suffer about 8,000 HPV-caused cancers annually, primarily of the throat.

The most common HPV vaccine is Gardasil, which was introduced in late 2006 and is recommended for 11- and 12-year old girls. Some critics have expressed concern that this is too young for a vaccine aimed at preventing an STI, but the whole point is to make sure girls (and boys) get the vaccine *before* they are sexually active. The vaccine is not as effective if the recipient has already had one of the four strains the vaccine protects against. But if a person gets the vaccine before becoming sexually active, even getting one dose of the vaccine is 82 percent effective at preventing all four strains.

Teens and young adults (age 13-26) are also advised by the CDC to get the shots on a catch-up schedule. Yet, four years after the vaccine’s introduction, the 2010 vaccination rate among girls aged 13 to 17 was just 32 percent. With such a relatively low vaccination rate, the findings of the recent study on HPV prevalence are all the more remarkable.

The researchers compared cervical/vaginal swab test results for 4,150 women from the pre-vaccine time period of 2003-2006 to the results for 4,253 women during 2007-2010, after the vaccine had been introduced. The participants ranged from age 14 to 59. Among girls aged 14 to 19 in the pre-vaccine era (2003-2006), 11.5 percent had one of the four HPV strains the vaccine protects against. But during 2007-2010, just 5.1 percent of girls aged 14 to 19 had one of those four strains.

Even with a vaccination rate below 50 percent, this study shows a 56 percent reduction of HPV prevalence in female teens within half a decade. The authors note that the most likely explanation for this decrease is that the HPV vaccine is working:

“We investigated factors that could have accounted for the decrease in vaccine type HPV prevalence. There were no differences in sexual behavior that we measured or in race/ethnicity between the two periods. We also found no downward trend in vaccine type HPV prevalence in the two 2-year National Health and Nutrition Examination Surveys cycles prior to vaccine introduction. Furthermore, there was no change in herpes simplex virus type 2 seroprevalence among females in this age group between the periods (CDC, unpublished data). While other factors that we did not measure could have contributed to the decrease in prevalence in 2007–2010, our findings suggest an early impact of HPV vaccination. Early vaccine impact in the United States has also been suggested by investigation of genital warts trends.”

It also appears that herd immunity is working too, though the evidence is a less clear on this possibility. If herd immunity is contributing to this decrease, as more girls (and boys) are vaccinated, fewer are being infected with HPV, which means fewer are passing it around as well, thereby offering greater protection to the whole community. The authors reported that the decrease in HPV prevalence among those vaccinated was 88 percent. Among those not vaccinated, the authors found a decrease of 28 percent, though this reduction was not statistically significant (the researchers could not rule out that it was a result of chance). It’s hard to tell the extent to which that 28 percent can be attributed to herd immunity because unvaccinated girls in 2007-2010 had fewer lifetime partners than their counterparts before the vaccine was introduced. (The authors controlled for sexual behavior in their analysis of the vaccinated girls, but this was statistically difficult to do for the unvaccinated girls.)

Past research findings have found similar (though not quite as dramatic) results, but those studies tended to compare different age groups, which can lead to bias in the results. It makes sense that younger girls would have a lower prevalence of HPV than slightly older women because a smaller percentage of teens are likely sexually active compared to young adults. But in this study, a group of teen girls was compared against another group in the same age range only a couple of years after the vaccine had been introduced.

Meanwhile, HPV prevalence among older age groups of women did not change much from 2003-2006 and 2007-2010. This finding is less surprising since most women have already become sexually active by the time they’re 20. The majority of them have therefore likely been exposed to at least one of the HPV strains in this study.

I am hoping these findings will motivate more parents to vaccinate their kids against HPV. This vaccine has encountered some resistance even among some people who follow the CDC immunization schedule. Most that I have heard express fears about the safety and side effects of the vaccine, and there is no shortage of inaccurate, misleading horror stories online which have no scientific evidence to back them up.

In reality, the HPV vaccine is one of the safest ones out there, with the fewest and rarest number of serious possible adverse events. Keep in mind that the HPV vaccine was approved very recently, which means two things. First, people often fear newer vaccines because they are unsure about something that hasn’t been around for as long (even though the vaccine had been tested for many years before approval). Second, the vaccine was approved in the midst of heightened scrutiny of vaccine safety due to the MMR scare, Jenny McCarthy’s unscientific scare tactics, relatively new vaccine courts and the switch to the acellular pertussis vaccine in the DTaP from the older whole DTP vaccine that could cause high fevers.

Not many vaccines have been tested as thoroughly as the HPV in the time since it was introduced. There were early reports that blood clots had been linked to the vaccine, but further research revealed that the blood clots were actually linked to birth control pills that vaccine recipients were taking. (Hormonal birth control bills are a known risk factor for blood clots.) After controlling for birth control, the link between HPV vaccination and blood clots vanished. There is still a link between HPV vaccination and fainting, but this primarily has to do with the fact that teen girls are getting an injection at all; they also faint when given other injections.

As the vaccination rate (hopefully) increases, HPV incidence should continue to decrease, as should HPV-caused cancers. While the decrease in HPV seen in this study is great news, the low vaccination rate means thousands more girls will still get cervical cancer in the coming years, not to mention the men and women who will get throat and neck cancers.

In a prepared statement, CDC director Tom Frieden gave more specifics on the potential to prevent even more cancer: “This report shows that HPV vaccine works well, and the report should be a wake-up call to our nation to protect the next generation by increasing HPV vaccination rates,” Dr. Frieden said. “Unfortunately only one third of girls aged 13-17 have been fully vaccinated with HPV vaccine. Countries such as Rwanda have vaccinated more than 80 percent of their teen girls. Our low vaccination rates represent 50,000 preventable tragedies – 50,000 girls alive today will develop cervical cancer over their lifetime that would have been prevented if we reach 80 percent vaccination rates. For every year we delay in doing so, another 4,400 girls will develop cervical cancer in their lifetimes.”

The bottom line is that the HPV vaccine, which has been shown to be safe and effective, is working. That means less HPV and less cancer for those coming of age today. That’s some darn good news — but it could get even better.

*Corrected: The original read “the only vaccine in existence,” but I edited it to reflect the correction that the hepatitis B vaccine can also prevent liver cancer, per the comment by Dorit Reiss.