Close to half of U.S. girls ages 13 and 17 have gotten at least one dose of the vaccine against human papillomavirus (HPV), but there is still a way to go to improve those numbers, according to researchers at the Centers for Disease Control and Prevention (CDC).

There are more than 100 strains of HPV, some of which cause genital and anal warts. In most people, the virus is sexually transmitted but the immune system clears the infection fairly rapidly. However, persistent infection with certain HPV strains can eventually lead to cancer.

Persistent HPV infection is the primary cause of cervical cancer. For that reason, the CDC and other groups recommend that all girls ages 11 and 12 receive the HPV vaccine, and that teenagers and young women up to age 26 get “catch-up” vaccination.

But not all girls are getting the vaccine. The CDC estimates that in 2010, 48 percent of girls ages 13 to 17 had gotten at least one dose of the HPV vaccine, which is given in three doses over six months.

That’s higher than in the two years before, but still too low, the agency says.

In the new study, reported in the journal Pediatrics, CDC researchers looked at the factors that seemed to influence whether girls got the vaccine or not.

They found that getting a doctor’s recommendation for vaccination made the biggest difference.

Among more than 18,000 U.S. girls ages 13 to 17, those whose parents had a doctor’s recommendation were 2.6 times more likely than their peers to have gotten at least one HPV dose.

The findings are based on responses to a 2008-2009 CDC survey of parents of more than 18,228 teenage girls. Overall, 37 percent of the girls had gotten at least one HPV dose in 2008, while 44 percent had in 2009.

The 2010 figure of 48 percent was slightly higher but not by much. “We’re concerned about that,” said Dr. Christina Dorell of the CDC, who led the current study.

Based on these findings, she told Reuters Health, doctors and other health providers play a big role in HPV vaccination decisions.

“Of the girls who started vaccination, most had a provider recommendation,” Dorell said. “We want providers to make the recommendation, and make it a strong one.”

But in this study, only 53 percent of parents said their doctor had recommended that their daughter get the HPV vaccine.

It’s not clear why many providers would not promote the vaccine. But Dorell said one potential reason could be cost: the vaccine is expensive and some doctors may not want to stock it.

There are two vaccines that can prevent infection with certain cancer-related strains of HPV: Merck’s Gardasil and GlaxoSmithKline’s Cervarix, both of which cost about $400 for three doses.

The study also found some other factors linked to a girl’s likelihood of getting the HPV vaccine, including a lack of health insurance. That’s despite the fact that uninsured girls are eligible for the federal Vaccines For Children program, which provides free vaccines for families who cannot afford them.

In many of those cases, parents might not be aware that they are eligible for the program. Dorell said that raising awareness among uninsured families - through local health-department campaigns, for instance - might help boost HPV vaccination rates.

Another issue is the fact that while about half of teenage girls have started the HPV vaccine series, many may not finish it. In 2008 and 2009, Dorell’s team found, only 53 percent of girls who had started the series received all three shots.

Again, it’s not clear why that is, according to Dorell. In some cases, doctors may not make it clear that three doses are needed, she noted. In others, families may just not return for the follow-up visits.

“Parents should be aware that (HPV vaccination) is a measure to protect their daughter from cervical cancer in the future,” Dorell said.

She added that the vaccine has a “strong safety record.” The most common side effects, according to the CDC, are pain at the injection site, fever, dizziness and nausea.

As of this past June, about 35 million doses of Gardasil had been dispensed in the U.S., and about 18,700 “adverse events” - which may or may not be caused by the vaccine - had been reported to the CDC’s Vaccine Adverse Event Reporting System.

Of those, 92 percent were considered mild - meaning they were not life threatening and caused no persistent damage. The rest were serious, and included blood clots and 56 deaths.

However, there were no patterns to suggest that the vaccine itself was responsible for those serious medical problems or deaths, the CDC says.

Cervical cancer, the disease HPV vaccination is designed to prevent, is diagnosed in more than 12,000 U.S. women each year. Most of those cases are linked to HPV infection.

SOURCE:  Pediatrics, online October 17, 2011.