The human papillomavirus vaccine is the only vaccine invented explicitly for the purpose of preventing cancer. Yet a decade after its approval, the vaccine continues to struggle from a PR problem, as shown in a study that finds just one in five parents would support making the HPV vaccine a requirement for school.

But this first national study of parents' attitudes found that those attitudes shifted if the requirement allowed parents an opportunity to opt out.

"It's interesting; if you had opt-out provisions, it tripled their support, but we know from previous research that if you add the opt-out provisions, you will make the law less effective," says the study's lead author, William Calo, a postdoctoral research associate at the University of North Carolina, Chapel Hill.

Such is the conundrum faced by public health officials and policymakers in attempting to improve the coverage of a vaccine that could save an estimated 30,000 lives a year, according to Calo.

The HPV vaccine, recommended by the Centers for Disease Control and Prevention for boys and girls ages 11 and 12, boasts one of the strongest safety records of any vaccine and is highly effective in preventing the infection that causes nearly all cervical cancer, 95 percent of anal cancer, 70 percent of throat and neck cancers and substantial proportions of vaginal, vulvar and penile cancers.

Yet despite the clear medical consensus supporting the CDC recommendation, hesitancy among parents and even providers has hindered its uptake over the past decade.

"Now we have the chance to prevent cancer. This is a disease people care a lot about," says Anna Giuliano, director of the Center for Infection Research in Cancer at Moffitt Cancer Center in Tampa, and a researcher who led some of the HPV vaccine clinical trials. "We have a simple intervention to prevent cancer, but the message has gotten diluted and contorted by the general public that we don't need prevention interventions, whether it's to prevent measles or pertussis or HPV, so this is the struggle."

Every state requires public school children to be up to date with some, if not most, of the vaccinations recommended by the CDC. But only two states, Rhode Island and Virginia, and the District of Columbia require students to receive the HPV vaccine.

Calo's study, published Friday in the journal Cancer Epidemiology, Biomarkers & Prevention, sought to quantify national support for such requirements, which have already failed in several dozen states.

In an online survey, his team asked 1,501 parents of 11- to 17-year-old children whether they agreed that laws requiring HPV vaccination for sixth-grade school entry were "a good idea." The parents came from all 50 states and represented a range of incomes and educational levels, though most were mothers and 70 percent of them were white. Their children were an average age of 14, and 46 percent of them had received at least one dose of the HPV vaccine.

The survey also assessed parents' beliefs about the value of the HPV vaccine, the harms of it, challenges to getting it for their children and possible psychological reasons parents might oppose HPV requirements, such as whether they tend to resist things more when they're forced to do them.

Even though half the parents believed the HPV vaccine was at least as important as the Tdap and meningococcal vaccines, the other two recommended for adolescents, only 40 percent believed the vaccine prevented cervical cancer. Nearly a quarter of parents inaccurately believed the vaccine might cause long-term health problems and one-third thought drug companies were pushing it to increase profits.

One-third felt they lacked enough information to decide whether to vaccinate their children, and just 21 percent of parents believed laws requiring the HPV vaccine were a good idea, though that figure jumped to 57 percent if parents could opt out of the requirement.

Calo cautions that the study did not describe specific types of opt-out provisions, so parental support may wax and wane according to what an exemption process might require.

Trying to impose mandates without broad public support can backfire, says Saad Omer, a professor of global health, epidemiology and pediatrics at Emory University Schools of Public Health & Medicine whose research focuses on state exemptions for vaccines required for school. "We're living with the legacy of going for a mandate a little too early," he says. Getting a particular vaccine needs to be a social norm before a law requires it, he adds.

Take the case of Texas, where Gov. Rick Perry issued an executive order in 2007 mandating that all girls entering sixth grade receive the HPV vaccination. That requirement included an opt-out provision that required a signed affidavit for religious or philosophical objections, but it still raised such ire from both sides of the aisle, and especially Perry's own Republican allies, that it was reversed in veto-proof legislation.

But Perry also issued the order just one year after the vaccine had been approved by the FDA, before any baseline immunization coverage existed for that vaccine and with only the clinical trial data, rather than the additional years of safety and effectiveness data available today.

"It was a public health decision that was really sound, but it was a political decision that was disastrous," Giuliano says.

Compare that to Rhode Island, where HPV immunization rates were already well above the national average when the Rhode Island Department of Health began requiring incoming seventh-graders to receive the vaccine in September 2015.

"They pounded the pavement and did due diligence, and then they said let's just codify it," says Giuliano.

Even in Rhode Island, however, the requirement is facing scrutiny and a legislative challenge. That's something to be expected with any attempts to require vaccination, suggests Dorit Reiss, a professor of law at the University of California Hastings College of Law who specializes in vaccine policy.

Political battles about mandates usually arise from the mobilization of an extremely small but very active minority that oppose vaccines, says Reiss, who testified during the SB 277 hearings for California's mandatory vaccination law. "And that minority of antivaccine activists does mobilize against lesser things than mandates." That includes requiring parental education or adding vaccines to existing rosters, even with personal belief exemptions, she says. "So I'm not sure we can go from here to whether a political battle will be easier with opt-outs."

Mandates are also best suited for vaccines whose coverage already exceeds 75 or 80 percent, Omer says. That makes for "an effective policy tool as opposed to a draconian tool." For proponents of mandates, that means there are other goals to focus on first, such as closing the gap between those who start HPV vaccine series and those who complete the third dose, Omer says.

Other approaches include using effective reminders for parents about needed vaccines, making it easier for parents to get their children vaccinated and giving providers the tools to effectively talk about vaccines.

"We need to do a better job informing parents about the real benefits of these vaccines, not only for school requirements but for potentially saving the lives of their kids in the future," Calo says. "I think as parents know and understand the benefits of HPV vaccination, they will support introducing laws requiring it for school entry."

His research bears this out: Parents were more than twice as likely to believe mandates were a good idea if they believed the HPV vaccine was at least as important as other vaccines given to teens or if they believed it effectively prevented cervical cancer.

The trick is waiting for the right moment to propose such legislation and crafting exemptions so that opt-outs aren't the path of least resistance.

"You're using mandates for the last couple of miles of your journey," Omer says. "They work best as nudges" making it more convenient to vaccinate than not to.

Omer's research has revealed that the most effective exemptions — those least likely to be used widely or to erode the herd immunity that keeps young children and the immune-compromised protected — require counseling from a doctor or the health department and an annual renewal process. Most importantly, parents choosing to opt out should be required to receive information about the disease and about the vaccine's safety and effectiveness, Omer says.

"That's where you find the balance between individual freedoms and societal responsibility, and parents' responsibility to their children," Omer says.

In the meantime, it's up to physicians and other health care providers to better educate parents about HPV vaccination, both Calo and Giuliano say.

"This vaccine is an unprecedented opportunity to prevent multiple male and female cancers, and it's covered almost entirely through insurance," Giuliano says. "If you could prevent breast cancer with a vaccine, wouldn't you take the vaccine?"

Tara Haelle is the co-author ofThe Informed Parent: A Science-Based Resource for Your Child's First Four Years. She's on Twitter:@tarahaelle

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