In the decade since the U.S. Food and Drug Administration approved the vaccine for the human papillomavirus (HPV), it’s been a tough sell for states, students and their parents.

“It’s a tricky issue to raise. Most places don’t like to think about teens having sex,” said Dorit Reiss, a professor at the University of California, Hastings, who specializes in vaccine law.

As of 2014, only 40 percent of teenage girls and 22 percent of teenage boys have completed the three doses necessary to be protected against HPV, a sexually transmitted infection that most people contract at some point in their lifetime. While it doesn’t cause long-term health problems for most, some strains of the virus can cause cervical cancer.

Only Rhode Island, Virginia and the District of Columbia require the vaccine for students. By comparison, eight years after the meningitis vaccine was approved, 29 states and D.C. had approved school requirements.

The slow adoption isn't for a lack of trying, though. According to the National Conference of State Legislatures, 41 states have introduced legislation that would either require the vaccine or educate students about its benefits.

In Rhode Island's case, it wasn't legislation that required students to get the vaccine. Instead, the health department added the vaccine to the list of mandatory immunizations for middle school students.

So far, the mandate has been successful: 88 percent of teen girls and 80 percent of teen boys received their first dose in 2015.

Rhode Island lets families opt out for religious and medical reasons. So does Virginia, but there, the opt-out option is partially why the mandate hasn’t had much of an impact.

“Opt-outs have been more the rule than the exception,” according to a news release from the University of Virginia.

Virginia also only requires girls to get the vaccine, and in 2014, just 28 percent of teenage girls got all three doses.

Experts blame the low immunization rates, in part, on the fact that the vaccine has to be given in three rounds (unless you're younger than 13). Sometimes, it’s tough to get people back to the doctor’s office that many times in a roughly one-year period.

Despite the low immunization numbers across the nation, Nicole Alexander-Scott, the director of Rhode Island’s health department, is optimistic that states are at a tipping point. She’s been in talks with her health counterparts in New England who are “thrilled with the results we’ve obtained."

"It used to be controversial to give the hepatitis B shot to infants," said Alexander-Scott. "The more we can normalize it for families, I’m confident in time [that] rates will increase.”

But Reiss, the law professor, thinks it will be difficult to raise immunization rates -- especially in socially conservative states.

“When you wage the battle on sexual nature," she said, "it’s going to be problematic."