The human papillomavirus (HPV) is the most common sexually transmitted infection in the United States. But although half of HPV carriers are male, most doctors still are not recommending the HPV vaccine to their male patients, according to a new study.

Less than 15 percent of pediatricians and family physicians surveyed reported routinely recommending the vaccine for males ages 9 to 26, whereas about 50 percent said they recommend the vaccine to females in the same age group. This disparity has resulted in particularly low HPV vaccination rates for boys, according to the study.

"While vaccinating males may benefit female partners, it is critical for physicians and parents to recognize the positive health impacts for males and females," said Susan Vadaparampil, a researcher at Moffitt Cancer Center in Tampa, Florida, and senior author on the report, which will be published in an upcoming issue of the journal Cancer Epidemiology, Biomarkers & Prevention. [5 Dangerous Vaccine Myths]

Vadaparampil said HPV can significantly raise men's risk of penile and anal cancers and that the virus also raises the risk of head and neck cancers in men more than it does in women. This is why it is important to vaccinate both boys and girls before they become sexually active, she said.

The Centers for Disease Control and Prevention recommends a three-dose series starting as early as age 11.

HPV is the main cause of cervical cancer, which kills about 4,000 women yearly in the United States and hundreds of thousands of women in countries where gynecologic care is not routine, according to the CDC.

HPV also can cause vaginal and vulvar cancers and a condition called cervical dysplasia, which can lead to excessive menstrual bleeding. For this reason, HPV often is viewed primarily as a female health risk. The U.S. Food and Drug Administration approved the first HPV vaccine for girls in 2006.

Yet males infected with HPV also have a risk of developing cancer, and they are as vulnerable to the virus as females are and equally as likely to spread it to a sexual partner. The FDA approved the same vaccine for boys in 2009.

Studies have shown that up to one-third of sexually active adolescents and more than half of adults are infected with HPV. There are dozens of known HPV strains, and some are more likely than others to cause cancer. Overall, HPV contributes to more than 5 percent of cancer cases worldwide.

In the new study, the researchers said that the lagging rate of doctors' HPV vaccine recommendations for boys could be the result of the later FDA approval date for the boys' vaccine compared with the girls' vaccine, and could also be due to less awareness about the direct benefit of the vaccine to males. But the researchers also said there is reason to believe that more doctors will be starting to recommend the HPV vaccine to males.

"There have been several positive changes that have occurred since we collected our data [in 2011], one of which was the change [based on CDC recommendations] from the vaccination being considered 'permissive' or 'optional,' to 'routine,'" Vadaparampil told Live Science. "In addition, the federal Affordable Care Act requires all new private insurance plans to cover HPV vaccines for the recommended age groups of males and females without consumer cost-sharing."

According to the CDC, 34.6 percent of boys ages 13 to 17 received at least the first dose of the HPV vaccine in 2013, up from 20.8 percent in 2012 and 8.3 percent in 2011. The rate for girls increased to 57.3 percent in 2013, from 25.1 percent in 2007.

The study also revealed that pediatricians were more likely than family physicians to recommend the HPV vaccine to their male patients. This suggests that targeting family physicians with information about the importance of the HPV vaccine could improve vaccination rates, the researchers said.

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