An increasing number of parents say they’re not going to vaccinate their adolescent daughters against human papilloma virus, a new study shows.

“The large and increasing proportion of parents who do not intend to immunize their adolescent daughters [against] HPV is troubling,” researchers from the Mayo Clinic, the Medical University of South Carolina and the University of Oklahoma Health Sciences Center wrote in a paper published on Monday in the journal Pediatrics.

HPV is the most common type of sexually transmitted infection, with an estimated 79 million Americans currently harboring the virus, according to the U.S. Centers for Disease Control and Prevention. There are more than 40 different types of HPV that can causes a range of ill effects, from genital warts to cervical cancer. Each year, HPV is thought to cause 360,000 cases of genital warts, more than 10,000 cases of cervical cancer, 2,100 cases of vulvar cancer, 500 cases of vaginal cancer and 600 cases of penile cancer.

The researchers examined data from three years’ worth of the National Immunization Survey of Teens, an annual survey conducted by federal health agencies. They focused on three vaccines routinely given in adolescence: Tdap, which guards against tetanus, diphtheria and acellular pertussis; the meningococcal conjugate vaccine, or MCV4; and the vaccine that protects against HPV.

While immunization rates rose for all three vaccines, the proportion of girls that got the full regimen of the HPV vaccine -- three shots over six months -- was substantially lower than the proportion of adolescents that received the other two vaccines. In 2010, 80 percent of teens receive the Tdap vaccine, and 63 percent got the MCV4 shot. Meanwhile, only about 33 percent of girls had received the HPV vaccine.

The number of parents vocal about avoiding the vaccine is increasing. In 2008, 40 percent of parents surveyed said they wouldn’t vaccinate their daughters against HPV. By 2009, that figure was 41 percent, and, by 2010, it was 44 percent.

"That's the opposite direction that rate should be going," senior author and Mayo Clinic pediatrician Robert Jacobson said in a statement.

More parents are worried about the safety of the HPV vaccine than about the safety of the Tdap or meningococcal vaccine.

Gardasil, which was approved for use in women ages 9 to 26 in 2006, protects against four strains of HPV, two of which are implicated in nearly three-quarters of cervical cancer cases. But the vaccine came under fire in 2009 when a government report linked it to 32 unconfirmed deaths, along with a high rate of fainting and blood clots compared to other vaccines.

In October, the Archives of Pediatrics & Adolescent Medicine published a study on Gardasil’s safety, conducted by the Kaiser Permanente Vaccine Study center and funded by the vaccine’s maker, Merck. Researchers examined data on 190,000 women and girls that got at least one dose of the three-dose regimen between August 2006 and March 2008, 44,000 of whom received all three shots.

While the study found that Gardasil could cause fainting on the day the shot was given and some skin problems weeks after inoculation, there was no established link to any more serious adverse effects. Fourteen of the patients examined did die over the course of the study, but the causes were unrelated to Gardasil: car accidents, suicide and heart problems, among other issues.

According to the Pediatrics paper, parent surveys show that doctors are recommending HPV vaccines at an increasing rate. But even though this rate has improved, physicians still only recommend the vaccine about half the time.

"The vaccine works better the younger the child is, and it doesn't work after the child is grown up and is exposed to the virus, so our message should be: 'Give this vaccine now to your child while your child is young and responsive to it,'" Jacobson said.

SOURCE: Darden et al. "Reasons for Not Vaccinating Adolescents: National Immunization Survey of Teens, 2008-2010." Pediatrics published online 18 March 2013.