 -- The HPV, or Human Papilloma Virus, vaccine has continued to be seen as controversial. Despite years of recommendations and support from leading medical institutions, parents remain wary about requiring children to receive an HPV vaccination for school admissions, according to a new study.

While just 21 percent of parents thought laws requiring the vaccine for school were a "good idea," that number rose significantly -- to 57 percent -- if there was an "opt-out" provision offered, according to the study published today in the journal Cancer Epidemiology, Biomarkers and Prevention.

"We were expecting a higher number of parents supporting vaccine requirements," study author, William Calo of the Department of Health Policy and Management at the Gillings School of Global Public Health at the University of North Carolina told ABC News. "21 percent is a lot lower than we expected."

Calo added that he feared an opt-out provision could make the laws less effective.

HPV infects approximately 80 million people, about 14 million per year, in the U.S., according to the U.S. Centers for Disease Control and Prevention (CDC). The virus can have serious effects on both men and women, causing genital warts, as well as vaginal, vulvar, penile, and anal cancers. The HPV vaccine was approved in 2006 and is now available starting at the age of nine and up to age 26.

The majority of states have introduced legislation on HPV vaccination. Half of those have enacted regulations around funding, public health messaging, or school requirements and three jurisdictions have made HPV vaccination required for school attendance: Rhode Island, Virginia, and Washington, D.C.

Controversy over the HPV vaccine, which was developed to help combat a hugely growing rate of infection, has persisted for a few reasons. The survey aimed to assess parents' current views on the vaccine in light of states considering it as a requirement for school admission. Researchers from multiple institutions, including the University of North Carolina, surveyed an ethnically diverse group of more than 1,500 parents of 11- to 17-year-olds from around the country.

They found that parents often did not trust there was a need for the vaccine or did not feel they knew enough about it. Approximately 23 percent were concerned the vaccine could cause long-lasting health problems and 32 percent thought the vaccine was a ploy for drug companies to make money.

Calo said many parents were also unaware of the ways the vaccine could safeguard health.

"One of the most surprising findings of the study is that 60 percent of people don’t believe the vaccine is effective in preventing cervical cancer," Calo said.

HPV is a virus that is often passed through sexual relations, which is one reason some parents have been hesitant to vaccinate their children.

But, it is "important to start vaccinating at age of 11 or 12 as opposed to waiting until people become sexually active because that's when there is the most benefit of the vaccine," Dr. Henry Bernstein, pediatrician and member of the American Academy of Pediatrics (AAP) Committee on Infectious Disease told ABC News.

Because HPV protects against preventable cancers, Bernstein said, it is most effective when it is administered earlier, before the conditions exist for those cancers to develop.

The side effects of the vaccine have also raised some concern, particularly in the first years after the vaccine was introduced. Stories about severe side effects traveled the internet, creating fear about lasting complications or even death. From June 2006 through September 2015 there were 117 deaths reported out of the 80 million doses of HPV vaccine given, according to the CDC. However, the agency said that, upon further review, none of these cases had evidence to show the vaccine caused the death.

In most cases, "the side effects are quite mild –- a little redness or soreness at the site," Bernstein said, adding that the vaccine is exceptionally safe. "Otherwise, it has been very well tolerated by teenagers of both sexes."

Vaccination rates remain low, with only 40 percent of girls and 22 percent of boys aged 13 to 17 finishing the HPV vaccination series, according to a 2014 study. The Department of Health and Human Services has set a goal of an 80 percent vaccination rate by 2020.

"I’m not surprised," Dr. Georges Benjamin, the executive director of the American Public Health Association, told ABC News. "There has been a terrible job of promoting it. The funds have not been there to promote the vaccine as part of a comprehensive vaccination program."

While many parents remain suspicious of the vaccine, the vast majority of medical experts support giving children and teens the vaccine. The CDC, American College of Obstetricians and Gynecologists and the American Academy of Pediatrics (AAP) are among the institutions that recommend routine vaccination of both boys and girls for HPV.

The vaccine has been extensively studied since it was officially recommended in 2006 and studies have shown major gains in protecting the health of teens. Four years after the vaccine was recommended in the U.S., related HPV decreased by 56 percent in teen girls, according to the CDC.

"The HPV vaccine is the first vaccine that is an anti-cancer vaccine and we have not really made that case effectively," Bernstein said. "That’s the real tragedy here."

He said that it’s important for boys to also get the vaccination since they can develop cancers from HPV. Among young people, the most common cause of head and neck cancer is HPV.

"I think immunizations are one of the number one public health achievements in the last century," Bernstein added. "Both my daughter and son have received the three-dose series, among all of the other vaccines."

Jennifer Chevinsky, MD is a medical resident at Lehigh Valley Health Network in Allentown, PA and will be continuing on to Loma Linda University Medical Center for her Preventive Medicine Residency. She is a resident in the ABC News medical unit.