Only Virginia, Rhode Island and Washington D.C. have laws requiring kids to have the human papillomavirus (HPV) vaccine to attend school, according to a new study.

That lags well behind where similar vaccines, like hepatitis B and varicella, were at the same point in their development, the authors write in a letter in the Journal of the American Medical Association.

“Support for the vaccination has consistently emphasized recently that this should be treated like any other vaccination,” Jason L. Schwartz of Princeton University in New Jersey told Reuters Health by phone. Schwartz coauthored the new research letter.

“We find that the public health community is not following its own good advice, there are clear differences compared to these other vaccines,” he said.

The first HPV vaccine was approved in 2006, and soon after the Centers for Disease Control and Prevention Advisory Committee on Immunization practices recommended that all 11 or 12 year old boys and girls receive the vaccine.

But eight years later, only Virginia and D.C. have any requirement that kids be immunized, and Rhode Island has passed its own regulation which will take effect in August.

Eight years after the CDC had issued the same recommendation for the hepatitis B vaccine and the varicella (chickenpox) vaccine, the hepatitis vaccine was required in 36 states and D.C., and the varicella vaccine was required in 38 states and D.C., the researchers point out.

The HPV vaccine protects against multiple strains of the sexually-transmitted papillomavirus, which can cause cervical, anal, penile and throat cancer.

According to CDC data, less than half of girls and even fewer boys had completed the three dose series of shots in 2013.

An initial push to require the vaccine in 2006 following its approval may have seemed premature and left a “bitter taste” for policymakers, Schwartz said, although this new research cannot explain why HPV vaccines have been treated differently than previous vaccines.

A large body of evidence supports the safety and efficacy of the vaccine, and several studies have found that getting the shots does not lead to kids engaging in more risky sexual behavior.

“That’s been pretty persuasively put to bed,” Schwartz said.

The HPV vaccine works extremely well, according to a CDC spokesperson.

“Since the vaccine was first recommended in 2006, there has been a 56 percent reduction in vaccine type HPV infections among teen girls in the U.S., even with very low HPV vaccination rates,” the spokesperson told Reuters Health by email. “Research has also shown that fewer teens are getting genital warts.”

“We should normalize the way we think and talk about it, and also how we think about requirements,” Schwartz said.

Even Virginia and D.C. have very broad exemptions, so if a parent does not want to vaccinate their child against HPV, they can still send them to school, he noted.

“Those requirements are in name only,” he said.

Hopefully this new study will reopen the conversation about state mandates for the HPV vaccine, he said.