Nearly half of all men in the US have some type of genital human papillomavirus infection—and about 25 percent have a type linked to cancer, according to a study appearing Thursday in JAMA Oncology.

The study is the first to look at the prevalence of HPV among American men. Much of the past attention paid to HPV has focused on women, because the virus is the cause of nearly every case of cervical cancer in the US. For this reason, health experts have been recommending since 2006 that girls and young women get vaccinated against HPV.

Experts updated that recommendation to include boys and young men in 2011, but the new study shows that few are actually getting those shots—just 10 percent were vaccinated. But with the new prevalence data, the authors suggest that vaccinating men may not only prevent thousands of cancer cases, but it could also be key to stamping out cancer causing-HPV transmission overall.

In the US, HPV is the most common sexually transmitted infection, with 79 million estimated cases. The virus infects skin cells and mucus membranes around the mouth, throat, genitals, and anus. It’s known to cause skin warts and several types of cancer. These include, as mentioned, nearly all cases of cervical cancers. But it also causes vaginal, penile, anal, throat, and mouth cancers.

The tricky thing about HPV and studying its prevalence is that not all types cause cancer. In fact, many cases cause no symptoms at all and clear up on their own—usually between six to 18 months. A lot of people won't even know that they're infected.

Scientists have found more than 100 strains of the virus and only around 18 or so are known to cause or are associated with cancers. That bunch is designated as “high-risk” strains. The “low-risk” strains are those that either come and go without notice or stick around and show up as warts. The latest vaccine can protect against nine strains, seven behind the vast majority of cancers and two that cause 90 percent of all warts.

To get a better grasp at HPV prevalence and types among American men, researchers analyzed penile swabs from a representative group of 1,868 men between the ages of 18 and 59. The researchers, led by cancer experts at the Womack Army Medical Center at Fort Bragg, North Carolina, found that about 45 percent had at least one type of HPV. That represents 34.8 million men nationwide.

DNA tests showed that 25 percent were infected with at least one type of high-risk strain.

The men also showed an unexpected pattern of infection. Among girls and women, those under the age of 20 are the most likely to be infected, with cases dropping off in the older age groups. But for men, incidence increased with age. In fact, the youngest group, 18 to 22 year olds, had the lowest prevalence of just 28.9 percent. This suggests that older men, as well as younger men, would benefit from vaccination.

The study comes on the heels of reports of increases in incidence of mouth and throat cancers among men, which are largely attributed to HPV infections. Although the study only looked for genital infections (not mouth or throat infections), the new data could offer a useful baseline for gauging the success of vaccination campaigns.

Overall, the study authors conclude that “male HPV vaccination may have a greater effect on HPV infection transmission and cancer prevention in men and women than previously estimated." Additionally, “only when vaccination rates are significantly increased will progress be made in eradicating most HPV-related cancers in the United States.”

JAMA Oncology, 2017. DOI: 10.1001/jamaoncol.2016.6192 (About DOIs).

This post has been updated to add information about the current HPV vaccine.