The most common sexually transmitted infection in the U.S. has a bit of a twist.
It’s what’s known as a “subclinical infection” — those infected suffer no immediate consequences, but years later have vastly increased risks for at least six different types of cancer.
That sleeper agent is, of course, human papillomavirus or HPV, the sexually transmitted virus responsible for causing nearly all cervical and anal cancers, as well as a high number of other genital cancers and some oral and head and neck cancers.
Several vaccines are available that can prevent HPV infection — and its associated cancers — but those vaccines aren’t being widely used in this country.
Now, a new study published Thursday in the journal JAMA Oncology points to how common that common STI really is — in men. Approximately 45 percent of U.S. adult males aged 18 to 59 are carrying active genital HPV infections, the researchers found.
That’s nearly 35 million adult men currently infected with the virus in the U.S.
HPV infection in men is quite a bit higher than in women — about 27 percent of U.S. women age 14 to 59 test positive for genital HPV. And, unlike in women, adult men seem to have high levels of HPV infection no matter how old they are, said Dr. Jasmine Han, a gynecologic oncologist at Womack Army Medical Center in Fort Bragg, North Carolina, and senior author of the new study. HPV infection rates peak for women in their early 20s and decrease in later years.
And, despite the availability of the HPV vaccine and its approval for adolescent boys since 2009, very few adult men are vaccinated. Only 11 percent of the men in their study who could have been vaccinated had received even one dose of the multi-dose vaccine series, Han and her colleagues found. (Children 14 years old and younger can receive two HPV shots, while those 15 and older still need three doses, according to the Centers for Disease Control and Prevention.) Nearly a third of eligible women have received at least one dose of the vaccine.
The new results in men aren’t completely surprising to those in the HPV research field — a previous large study of men in Florida, Mexico and Brazil also found high levels of the infection, even higher than the current study. But, together with the low vaccination rates and the rise of HPV-associated cancers in men, the high prevalence of HPV in men sends a clear signal for Han.
“We need to think about the HPV vaccine as a cancer vaccine and not [just] as a sexually transmitted infection vaccine,” she said.
The study results come from the latest cycle of the National Health and Nutrition Examination Study, or NHANES, a broad epidemiological survey conducted by the CDC to study a variety of aspects of health and nutrition in the U.S. NHANES combines self-reported data with clinician-collected medical information and specimens. In the latest iteration — the 2013-2014 cycle — NHANES added a genital swab, allowing researchers to collect new information on HPV prevalence.
Looking at more than 1,800 adult men from that study, Han and her colleagues asked how many had tested positive for an active genital HPV infection, as well as how many reported getting one or more doses of the HPV vaccine.
Men in the youngest age group, those 18 to 22 years old, actually had the lowest rates of infection at 29 percent. This is likely a reflection of more young men and adolescent boys getting vaccinated, Han said. But every age group 23 years and older had much higher levels of infection — some groups even higher than half.
This difference in age distribution between men and women is likely due to biological differences between the sexes, said Dr. Denise Galloway, a Fred Hutchinson Cancer Research Center virologist whose work linking HPV to cervical cancer and development of virus-like particles helped pave the way to the vaccine.
“Men are clearly different. We think this is related to their ability to mount a good immune response to HPV infection,” said Galloway, who holds a Fred Hutch 40th Anniversary Endowed Chair.
While the NHANES study can distinguish active genital infections from prior presence of the virus, it can’t say whether these infections are new or not, Han said. That is, they don’t know whether the men are getting newly infected with new viruses, have chronic active infections, or may have periods of latent and active infection from the same virus. More detailed studies following the same men over time would be required to tease out those possibilities.
The virus’ devastating effects are clear though, for both men and women. HPV is linked to more than 30,000 new cases of cancer every year in the U.S., according to the CDC, and many of these are preventable by vaccines.
Men and boys should get vaccinated not just to protect women from the more than 10,000 new cases of cervical cancer caused every year by HPV, but to protect themselves, Han said. Recently, cases of HPV-linked oropharyngeal cancer, or cancer of the mouth and throat, surpassed cases of cervical cancer. And most of the 11,000 new cases of HPV-linked oropharyngeal cancer per year occur in men.
For now, the HPV vaccine is only approved for men 26 years old and younger. The vaccine works best in people who haven't yet been exposed to the virus, Han said. But her group's results indicate that studies looking into raising the maximum age for vaccination could be warranted, given the high level of HPV infection in older men.
Not all of the estimated 35 million HPV-infected men could have avoided infection with vaccination. Nor will most of these men go on to get cancer. The vaccines protect against either four or nine different strains of HPV, depending on the vaccine, and cover the majority of the cancer-causing strains. Of the 45 percent of men with HPV, about a third carry the strains the vaccines protect against, Han said.
And yet, so few are getting the vaccine.
“HPV vaccination in men is as important, or even more important, than in women. We as a society are not doing it well,” Han said.
That disparity may be due in part to the fact that the vaccine was approved by the FDA for boys several years later than it was for girls, Galloway said. Marketing of the vaccine was also initially focused on girls, and parents of boys were urged to get the vaccine for the sake of protecting girls and women.
But it’s clear from this and other studies that boys and men have more reasons than pure altruism to get vaccinated, Galloway said.
“There’s a perception that men aren’t at risk of getting HPV-associated diseases, which is not true,” she said. “Men should be getting vaccinated not just to protect women, but to protect themselves.”
Rachel Tompa is a former staff writer at Fred Hutchinson Cancer Center. She has a Ph.D. in molecular biology from the University of California, San Francisco and a certificate in science writing from the University of California, Santa Cruz. Follow her on Twitter @Rachel_Tompa.