While a vaccine has existed for human papilloma virus (HPV) — the most common sexually transmitted disease in the United States — for seven years, researchers and public health officials are trying to figure out why fewer than one-third of parents are getting it for their children.

The first HPV vaccine, which entails three shots over six months and protects against the four most common strains of the virus, was approved by the FDA in 2006 and is recommended for girls and women between the ages of 9 and 26. But while two different vaccines are now available, researchers have been frustrated with how few young girls are getting protection from the virus, which can cause genital warts and cervical and other cancers.

Studies suggest that parents’ reasons for not getting their children vaccinated include language barriers, concerns over vaccine safety and speculation that the vaccine would lead to promiscuity.

“Having a vaccine isn’t always enough. Delivering that vaccine is critical, and in the United States our delivery rates have been lower than what most of us hope they would be,” said Dr. Larry Pickering, an associate professor of pediatrics at Emory University School of Medicine, at a press conference Friday in San Francisco, part of a meeting of many of the nation’s leading experts on infectious diseases.

In several studies presented this week, researchers discussed low rates of HPV vaccination in the United States. Pickering said only 20 to 30 percent of girls get all three shots in the series. But as researchers investigate, they find there is no single reason why rates of HPV vaccination in the U.S. lag far behind the goal of 90 percent coverage by 2020.

“You can’t assume the reasons that a parent would not want to have their child immunized, because there are five or six different reasons,” Pickering told Al Jazeera.

In one study presented at the IDWeek conference, Dr. Sean O'Leary, an assistant professor of pediatrics at Children’s Hospital Colorado, and his colleagues spoke at length to 41 parents with lower socioeconomic status, whose children either hadn’t received or hadn’t finished the HPV vaccine series. The researchers found that among Spanish-speaking parents whose daughters weren’t vaccinated at all, concerns were mainly that the vaccine would encourage their daughters to experiment with sex, while those who started but did not finish the series said they had not been informed their daughters would need three shots. Meanwhile, parents who spoke English were more likely to feel they didn’t know enough about the vaccine, and if their daughter had started and not finished the series, they were more likely to have concerns about safety.

The study did not go further into family background, and O’Leary said it’s not clear whether language indicates Spanish-speaking families are more conservative. There’s also a strong possibility, he said, that there was a language barrier, as many seemed to feel their health care provider didn’t promote the vaccine. O’Leary is looking into doctors’ thoughts on the HPV vaccine for a future study.

Difficulties with communication were also found by Dr. Tina Tan, a professor of pediatrics at Northwestern University’s Feinberg School of Medicine, who looked at what parents thought of having their teenage sons vaccinated against HPV, which is recommended at age 11 or 12 because of the disease’s link to cancers in men. Her research was sponsored by Merck, which manufactures Gardasil, one of the two HPV vaccines on the market, and the only one approved for use in males.

Tan, who along with colleagues spoke with parents of boys who got primary care at public health clinics and private doctors’ offices, reported that many parents said they had not been informed of the vaccine's importance. Over the two years of her study, Tan said, the rate of HPV vaccination went up, as health care providers started encouraging use of the vaccine more. While parents in these clinics said they trusted their health care providers, many felt the benefits of the HPV vaccine weren’t spelled out.

Pickering said one reason this may occur is that pediatricians are pressed for time, with the need to discuss many health issues, and they ultimately do not see cases of cervical cancer, which occurs in older women — with a result similar to the way vaccination rates for measles may have dropped slightly because younger pediatricians have never seen that disease.

But there are other mistaken beliefs that must be countered, Pickering said. For example, while parents may be worried about promiscuity, three studies have shown getting the vaccine has no effect on promiscuity.

Ultimately, he said, the benefits need to be spelled out better.

While many parents may be uncomfortable with vaccinating their daughters for an STD at the ages of 9 to 11 — the period recommended for starting the three-shot series — tests on recipients have shown that in addition to protecting girls from HPV infection earlier, getting the shot at a younger age provokes a stronger immune response, so a girl vaccinated at 11 is better protected in the long run than a girl vaccinated later on. Tan said a similar benefit is found in boys who are vaccinated at 11 or 12.

“The younger you get it, the better the immune response,” Pickering said. “Let’s go ahead and give it, and that’s one thing you don’t have to worry about.”