By Lisa Rapaport

(Reuters Health) – Although a vaccine for the cancer-causing human papillomavirus (HPV) has led to a dramatic decrease in infections, a new study suggests that half of U.S. teens are still not fully vaccinated against this virus.

Last year, nearly 66 percent of youth ages 13 to 17 received the first dose to start this vaccination series, the U.S. Centers for Disease Control and Prevention (CDC) reports. But just 49 percent of teens received the two or three doses needed to be fully vaccinated against HPV.

“Even with lower vaccine uptake, HPV infections and cervical precancers (abnormal cells on the cervix that can lead to cancer) have decreased significantly since the HPV vaccine was introduced more than 10 years ago,” said lead study author Tanja Walker, a researcher at the CDC in Atlanta.

“While it will take decades to assess the impact of HPV vaccination on decreasing cancers caused by HPV, the latest estimates show that HPV vaccination could prevent 31,200 cases of cancer every year,” Walker said by email.

Vaccination rates are steadily moving in the right direction.

On average, the percentage of adolescents who started the HPV vaccine series grew 5 percentage points each year between 2013 and 2017, researchers report in the CDC’s Morbidity and Mortality Weekly Report.

HPV is among the most common sexually transmitted diseases. Most infections don’t cause symptoms and go away on their own, but the virus is still a leading cause of cancer deaths among women worldwide. It can also cause genital warts and lesions in the upper respiratory tract.

In the U.S., the HPV vaccine is recommended for both boys and girls at age 11 or 12, with the goal of protecting them against the virus before they become sexually active. It’s also recommended for teens and young adults who may not have previously been vaccinated.

Many girls and boys don’t receive the vaccine at least in part because their parents may question whether it’s necessary to protect them against a sexually transmitted disease at an age when they think children shouldn’t be having sex, researchers say.

“Originally, the vaccine was prescribed only for girls as a means to protect against a sexually transmitted infection, when in reality the vaccine is important for all young boys and girls to prevent several types of cancer,” said Dr. Lois Ramondetta of the University of Texas MD Anderson Cancer Center in Houston.

“Mixed messages from the early days of the vaccine have resulted in confusion on the part of many physicians and parents, and it’s important that we educate everyone as to importance of this vaccine to prevent cancer,” Ramondetta, who wasn’t involved in the study, said by email.

Beyond resistance to the vaccine from some parents, some pediatricians may not prioritize this vaccine as much as inoculations against other childhood diseases.

“The main reason parents report for not getting the HPV vaccine for their child is that it wasn’t recommended by their doctor,” Walker said.

“We also know that it can be difficult to get adolescents in for multiple office visits,” Walker added. “That’s why it’s important to remind parents that their child only needs two doses if they get the first dose before their 15th birthday.”

From 1999 to 2015, cervical cancer rates decreased by 1.6 percent a year, researchers note in a separate CDC report by Dr. Elizabeth Van Dyne of the CDC and colleagues. This is the most common type of cancer caused by HPV.

Over that same period, however, anal cancers tied to HPV increased 2.9 percent a year in girls and 2.1 percent annually for boys. Throat cancers related to HPV also increased 0.8 percent a year in girls and 2.7 percent annually for boys.

This may be due to changing sexual behaviors, and more people having unprotected oral and anal sex, the study authors note.

“We anticipate that vaccinated individuals will be protected against infections that cause almost all cervix cancers, anal cancers and cancers of the tonsil or back of tongue and many of the infections that cause vulvar, vaginal and penile cancer,” Ramondetta said.

SOURCE: and Morbidity and Mortality Weekly Report, online August 23, 2018.