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 smok
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