Once they’re born, children receive a range of vaccinations against seemingly every possible disease. But one vaccination has been severely under-used: the human papillomavirus (HPV) vaccine.

According to the Centers for Disease Prevention and Control (CDC), HPV is the “most common sexually transmitted infection (STI).” The virus affects 14M+ people every year, and will affect almost everyone who is sexually active at some point in their lives. HPV causes 90% of cervical cancers, and other cancers associated with orifices used during sexual activity (think vagina, anus, etc.).

A 2014 study done by researchers at Harvard Medical School and the University of North Carolina (UNC) showed that a “sizable minority” of doctors recommended the vaccine “inconsistently, behind schedule or without urgency.”

Here’s what that translates to numerically:

As of 2014, only 40 percent of girls and 21 percent of boys ages 13 to 17 had received all three doses of the HPV vaccine, whereas 88 percent of boys and girls had been vaccinated against tetanus-diphtheria-pertussis and 79 percent had gotten the meningococcal vaccine.

But why aren’t children getting this vaccination? One reason is that doctors may be reluctant to talk about sexual activity with children, even if it’s future sexual activity. The vaccination does not rank high on the list of children’s immunizations, and isn’t required in many states. There also has not been a public health scare to drive home the importance of this immunization to parents.

The virus was only approved in 2006, and can be cost-prohibitive: the three-shot series can run up to $1K.

Children, both girls and boys, should receive the vaccination around ages 11-12. Boys can get catch-up vaccines until they’re 21, and girls can do the same until they’re 26. But the vaccine has proven less effective when given during the later years.