Requiring students to get vaccinated against the human papillomavirus, or HPV, to enter school could prevent many cancers linked to the virus, but University of North Carolina at Chapel Hill researchers found that many parents only support such requirements with opt-out provisions that could make the laws less effective.

In a national survey, researchers report only one-in-five parents agreed that laws requiring HPV vaccination for school entry are a "good idea." When added to the number of parents who only backed the requirements if opt-out provisions were included, support increased to nearly 60 percent. The journal Cancer, Epidemiology, Biomarkers & Prevention will publish the findings online on Aug. 19 and in its print edition Sept. 1.

"School entry requirements are highly acceptable to parents, but only when implemented in a way that makes them ineffective," said the study's senior author Noel Brewer, PhD, a member of the University of North Carolina Lineberger Comprehensive Cancer Center and an associate professor in the UNC Gillings School of Global Public Health. "Opt-outs lead to a large number of parents choosing not to vaccinate their children, and that makes requirements ineffective in raising vaccination rates."

The Centers for Disease Control and Prevention estimates that approximately 30,000 new cases of cancers linked to HPV are diagnosed in the United States each year. Most cancers linked to the virus could be prevented, but vaccination rates remain low. Forty percent of girls and 22 percent of boys aged 13 to 17 years of age had completed the three-vaccine series by 2014, the CDC found. The vaccine is recommended for girls and boys aged 11 or 12 years.

All 50 states and Washington D.C. have school-entry requirements for child and adolescent vaccines to protect against tetanus, polio and other diseases. Yet only two states -- Virginia and Rhode Island -- along with Washington D.C., have school-entry requirements for HPV vaccination. Nearly half of U.S. states have proposed school-entry requirements for HPV vaccination without success since 2006.

To study support for school-entry HPV vaccination laws, UNC researchers surveyed a national sample of 1,501 parents of 11-to-17-year-old children. They used a web-based survey between November 2014 and January 2015.

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Twenty-one percent of the parents surveyed agreed that school-entry requirements for HPV vaccination are a good idea. Support increased to 57 percent when parents who disagreed were asked if they thought the laws were OK if parents could opt out.

"It would be hard for law makers to enact a policy that has 21 percent support," Brewer said. "Parents like the idea of generous opt-outs to school entry requirements."

Parents were the least likely to agree with HPV vaccine school-entry requirements if they lived in the Midwest, and they were most likely to agree if they lived in the Northeast. Hispanic parents had higher odds of agreeing with the requirements than non-Hispanic whites.

The researchers also found that 32 percent of parents said they didn't have enough information to decide whether to vaccine their children. Only 40 percent of parents agreed that the vaccine was effective in preventing cervical cancer.

"It is surprising that so few parents understood that HPV vaccine prevents cervical cancer," said the study's first author William Calo, JD, PhD, a postdoctoral researcher at the UNC Gillings School of Global Public Health. "We would expect the number to be much higher given that national organizations have promoted the vaccine for the last decade. We have to continue to communicate that the vaccine is an effective way to prevent cancer."

For health policy makers, Brewer said the findings suggest that school entry requirements for HPV vaccination should be considered once states have implemented other approaches that work, including centralizing vaccination reminders in state health departments, focusing quality improvement visits to providers on HPV vaccination, and training physicians to use announcements to introduce vaccination. He said future studies will look into the potential spillover effect of requiring other adolescent vaccines on HPV vaccination rates.

"Given the many challenges to enacting HPV vaccine requirements, it's unlikely that a large number of states will pass these laws in the near future," Calo said. "Physicians and other health care providers are key to improving HPV vaccination uptake."