By Amy Norton
MONDAY, March 18 (HealthDay News) -- Although experts recommend girls and young women be vaccinated against the virus that causes most cases of cervical cancer, parents seem to be increasingly worried about the vaccine's safety, a new U.S. study shows.
Experts say the findings are both worrying and puzzling, because the vaccine -- which guards against the human papillomavirus (HPV) -- has not been linked to any serious side effects.
"It's really concerning that parents think the HPV vaccine isn't safe," said Dr. Dennis Cunningham, an infectious disease specialist at Nationwide Children's Hospital in Columbus, Ohio.
Cunningham, who was not involved in the study, pointed out that about 12,000 U.S. women are diagnosed with cervical cancer each year. And most of those cases are linked to HPV infection.
There are more than 100 strains of HPV, some of which cause genital and anal warts. In most people, the immune system clears the infection fairly rapidly. However, persistent infection with certain HPV strains can eventually lead to cervical cancer in some women.
Because of that, the U.S. Centers for Disease Control and Prevention (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.
The new study, published online March 18 and in the April print issue of Pediatrics, found that between 2008 and 2010, a growing number of U.S. girls were vaccinated against HPV. The percentage of 13- to 17-year-olds who were not up-to-date on the vaccine dipped from 84 percent to 75 percent.
"That's good news," said lead researcher Dr. Paul Darden, a pediatrician at the University of Oklahoma Health Sciences Center in Oklahoma City.
The bad news was that over time, more parents were saying they had no plans to have their daughters vaccinated. And they blamed "safety concerns" more often in 2010 than in 2008.
Among parents whose daughters were not up to date on the HPV vaccine in 2008, 40 percent said they had no intent to have their child get it. In 2010, that number was 44 percent.
As for safety concerns, 4.5 percent of parents said that was one reason their daughter had not been vaccinated in 2008. In 2010, more than 16 percent blamed safety worries.
"I'm not sure what has gone into that increase," Darden said. "Why would safety concerns almost quadruple in a couple years?"
What's more, he noted, the worries seem specific to the HPV vaccine. Parents in the study were also asked about two other vaccines recommended for teenagers: the "Tdap" vaccine against tetanus, diphtheria and whooping cough; and the "MCV4" vaccine against bacterial meningitis -- a potentially fatal inflammation around the brain and spinal cord.
Across the study period, less than 1 percent of parents cited safety concerns over those two vaccines.
The findings come from a CDC survey of U.S. families with 13- to 17-year-old children, done over three years. Parents were asked open-ended questions about their reasons for not vaccinating their child.
Both Darden and Cunningham said it's puzzling that parents' safety worries about the HPV vaccine would grow so much, so fast. It's not clear from the study, but Cunningham said he suspects many parents get misinformation online.
"There's a lot of unreliable vaccine information out there," he said.
According to the CDC, the most common side effects of the HPV vaccine are the same as with other vaccines: pain at the injection site, dizziness, mild fever. There have been cases of fainting reported -- but, Cunningham said, that can happen after any vaccination, especially to teenagers.
The agency has also gotten reports of blood clots in people who received the vaccine. But more than 90 percent of them also had risk factors for blood clots -- such as smoking or using birth control pills, the CDC said.
Parents in this study did have other reasons for not wanting their daughters vaccinated. Just over 17 percent said it was "not necessary," and 11 percent said their daughters did not need the vaccine because they were not sexually active -- an erroneous assumption, Cunningham said, because the HPV vaccine should ideally be given before a girl is sexually active.