Preventing HPV-Caused Cancers

  • By Emily Willingham and Laura Helft
  • Posted 09.05.14
  • NOVA

Cancer cells themselves aren't contagious among humans, as far as we know. But some contagious viruses, including human papillomaviruses (HPVs), can persist in tissues and eventually cause cancer or other diseases. In fact, HPVs are the only known cause of cervical cancer, a leading killer of women worldwide, and they are also a leading cause of oral cancers, which occur in both men and women. A vaccine against HPV can now prevent many of these cancers, but the vaccine itself has proved controversial. What does the HPV vaccine do, and why is there debate about it?

What is HPV?

Although people sometimes refer to HPV as if it were a single virus, it's really a group of viruses, not all of which cause cancer. Of the 120 identified HPVs, about 50 are sexually transmitted—that is, they are passed from person to person by skin-to-skin contact involving the genitalia. The remaining HPVs are not sexually transmitted and cause common warts on other parts of the body, such as the fingers and toes.

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Some types of HPV can cause warts, including on the hands or feet. Enlarge Photo credit: © Getty Images

Some of the 40 sexually transmitted HPVs can lead to cancers of the cervix, vulva, vagina, anus, penis, or throat; the others aren't associated with cancer but can cause genital warts.

Each type of HPV is distinguished by its outward structure, a molecular "face" that interacts with the human cells that the virus infects. These different faces are known as types, and HPV types are designated by numbers.

What diseases does HPV cause?

Two types of the virus, HPVs 16 and 18, cause about 70% of all cervical cancers, at least in populations that have been studied. Another two, HPVs 6 and 11, are responsible for nine out of ten cases of genital warts. The only known preventive is the HPV vaccine, the first of which was approved for use in the United States in 2006.

Genital warts are not life threatening but can prove embarrassing and are uncomfortable to have removed. At any point in time, one in 100 sexually active adults in the United States has visible genital warts, according to the U.S. Centers for Disease Control and Prevention (CDC).

An intrepid researcher named E.V. Ullmann confirmed the infectious nature of HPVs in the 1920s. He experimented on himself by taking extracts from a patient's throat warts, which HPV also can cause, and injecting the fluid into self-made wounds on his arm, triggering wart growth there nine months later.

Confirmation of an HPV-cancer link came many decades later, in the 1980s, when technology finally made it possible to identify HPV DNA in tissue samples from women with cervical cancer. If viruses like HPV persist in the body for a long time, they can cause cancer by disrupting processes that limit cell division, so that cells divide out of control. Warts result from a more controlled and limited overgrowth of cells on the outer layer of the skin.

A loss of cellular control that leads to cancer can be deadly. Cervical cancer kills about 300,000 women worldwide and 4,000 women in the United States annually. Each year, about 12,000 women in the United States learn that they have this cancer.

HPV 16 is also responsible for up to nine out of ten cases of HPV-caused throat cancers and is the most frequently detected HPV so far in anal cancer. These cancers occur in both men and women. HPV-associated oral cancers have been on the increase for the last three decades. Unless preventive measures are taken, the incidence of HPV-related oral cancers is expected to continue to rise.

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Number of cancers caused by HPV worldwide each year Enlarge Photo credit: Courtesy: National Institutes of Health

How are HPVs spread?

HPVs are the most common cause of sexually transmitted infections in the world. At any given time, 20 million people in the United States—about 6% of the population—have some type of HPV infection. Being young is a risk factor; about half the people who contract an HPV are between the ages of 15 and 24. Although many HPVs are transmitted sexually, you don't have to have sex to pick them up; direct skin-to-skin contact with the genitals of an infected person is sufficient to result in transmission. The use of latex condoms may reduce the risk of transmission but doesn't completely eliminate it.

HPVs are so common that, in the United States, most sexually active people acquire an HPV infection at some point in their lives. Many infected people never know they have an HPV because they fight it off without ever showing symptoms. But sometimes, HPV evades the body's defenses and persists for years, leading to cancer.

What are the facts about HPV vaccination?

There are two approved HPV vaccines; both contain protein molecules specific to different HPV types. After they're injected, these proteins trigger the immune system to construct a memory of the molecular "face" of those types. If a real HPV shows up later, the body is able to recognize the protein—on the face of the actual virus this time—and crush the invasion.

Both approved vaccines, Gardasil and Cervarix, target the two most common cancer-causing strains identified so far: HPVs 16 and 18. But Gardasil also targets two more strains—6 and 11—adding protection against the HPVs that cause most genital warts.

The schedule for administering the vaccines is the same: a three-shot series over the course of six months, usually beginning at age 11 or 12. The rationale behind the target age is to get people vaccinated before they start engaging in sexual behaviors, including skin-to-skin genital contact, that can lead to an HPV infection.

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Dr. Amy Middleman and patients Enlarge Photo credit: © WGBH Educational Foundation

But people who don't receive the three-dose series as scheduled can often get it later. The CDC's recommendations for individuals who haven't completed the series by the time they turn 13 vary slightly depending on whether they receive Gardasil or Cervarix, but the agency still recommends either vaccine for girls and women aged 13 to 26 and Gardasil for some groups of boys and men. In teens and young adults who have become sexually active, the vaccine can't stem an infection they've already acquired, but in those who have not yet encountered the types of HPV the vaccine covers, immunization still offers some protection. The HPV vaccine can be given at the same time as other vaccines.

What are side effects of the HPV vaccine?

The HPV vaccine has proved controversial because of rumored side effects, including blood clots, strokes, and seizures. The vaccine even came in for comment during the 2011 political season, when Republican presidential hopeful Michele Bachmann erroneously asserted that it was linked to intellectual disability. Subsequent reports in the popular press reinforced the impression that the vaccine can cause alarming side effects.

These claims commanded the attention of researchers, but studies have shown no cause-and-effect relationship between the vaccine and such side effects. A huge 2014 study of more than 1.6 million women, for example, found no link between Gardasil and a dangerous clotting disorder.

Evidence has shown that HPV vaccinated girls are no more likely to be sexually active than unvaccinated girls.

There is one known side effect of the HPV vaccine, however—fainting. The CDC says that adolescents are more prone than other age groups to fainting after a vaccination. The fainting itself doesn't indicate anything serious but carries a risk of falls and injury, so resting for 15 minutes after the vaccine is administered is suggested for tweens and teens.

The just-vaccinated preteen or teen who faints might be reacting to one of the most widely known negative features of the HPV vaccine: It hurts, possibly because it has a higher concentration of salt than other vaccines. Fortunately, the pain doesn't seem to deter adolescents from completing the shot series.

Does HPV vaccination affect sex-related behaviors?

Because HPV infection is a sexually transmitted disease, some have expressed concern that the vaccine might affect sexual behaviors—especially that girls who've received the vaccine would be more likely to engage in sex at an early age. Evidence has shown these worries to be unfounded. Vaccinated girls are no more likely to be sexually active than unvaccinated girls, and vaccination is not associated with riskier sexual behavior.

What is worrisome, however, is that many girls incorrectly believe that the vaccine protects them against sexually transmitted diseases other than HPV, which it does not.

How effective are the HPV vaccines?

Both vaccines effectively prevent most cervical, anal, and oral cancers, and Gardasil prevents most genital warts as well. Both vaccines have even been shown to protect unvaccinated girls and young women against HPV infections, thanks to a concept known as "herd immunity." In a population of young women in Cincinnati, for example, within four years after the vaccine was introduced, rates of infection with an HPV type covered by the vaccine had dropped by 70% among vaccinated women and had dropped by 50% among unvaccinated women. Nationally, the vaccine has shown similar results, with a 56% drop in vaccine-type HPV infection rates among teen girls in the four years after the vaccine debuted, even though only a about third of such girls had received the full series of shots at that time.

Yet unfortunately, despite this documented success, the U.S. vaccination rate has remained well below the CDC's goal; the agency's target is to see 80% of girls aged 13 to 17 vaccinated, but as of 2013 just under 38% had received the full series.

But not all populations may benefit equally from the existing two vaccines. The different types of HPV vary in their geographic incidence, and some strains cause cancer more often in certain populations. HPV 16 is relatively common worldwide, for example, but HPV 18 is twice as prevalent in cervical lesions diagnosed among women in North America as among women South and Central America.

What's next for the HPV vaccine?

There is the possibility of new vaccines that will cover more strains of HPV. Maximum coverage from the current vaccines is four strains of the virus, two of which cause cancer. New vaccines that are in development could cover up to nine strains, including seven that cause cancer.

And for those with anxiety about the pain associated with the HPV vaccines, there is more potentially good news: The shot series may be reduced, thanks to evidence that two shots can be just as effective as three.

Emily Willingham is a Northern California-based writer who covers many topics, including vaccines and autism. She has written for the New York Times, Discover, and Forbes and is co-authoring an upcoming book on evidence-based parenting.

Laura Helft is the senior researcher for Tangled Bank Studios, a production company of the Howard Hughes Medical Institute. She produces supporting web content for the science programming produced by Tangled Bank.

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