The belief that vaccines cause autism -- which has been debunked by the U.S. Centers for Disease Control and Prevention, among others -- is no longer the primary reason parents are refusing vaccines for their children, according to a new study today from the American Academy of Pediatrics (AAP). Instead, a growing number of parents believe vaccinations are "unnecessary," researchers found.

The AAP surveyed more than 600 pediatricians in 2006 and then again in 2013, asking them to estimate the percentage of parents they encountered who wished to refuse or delay vaccines and the parents' rationale for doing so. Pediatricians were also asked how often they dismissed patients from their practice for continuing to refuse vaccines.

By 2013, nearly 9 out of 10 pediatricians (87 percent) say they were asked by at least one parent in their practice to alter their child’s immunization schedule. Pediatricians in the study responded that 73 percent of parents refusing or delaying vaccines for their children in 2013 were doing so because the parents believed the vaccines were unnecessary, whereas that number was 64 percent in 2006.

Dr. Lolita McDavid, medical director of child advocacy and protection at the University Hospitals Rainbow Babies & Children’s Hospital in Cleveland, Ohio, said the findings did not surprise her.

“In the past, people were scared of polio and whooping cough, but parents aren’t now because they don’t see it anymore,” McDavid, who was not involved in this study, told ABC News. “It’s a very uninformed way to approach a child’s health.”

Dr. Catherine Hough-Telford, lead author on the paper and a pediatrician with the Pediatric Health Care Alliance in Tampa, Florida, told ABC News that “it will be interesting to look in future studies at how parents’ perceptions change after the Disneyland measles outbreak.” Disneyland is owned by Disney, the parent company of ABC News.

Vaccinations have prevented an estimated 322 million illnesses, 21 million hospitalizations and 732,000 deaths for Americans born between 1994 and 2013, according to the U.S. Centers for Disease Control and Prevention. Communicable diseases that previously afflicted the population are now largely a distant memory due to the development of vaccines.

Pediatricians reported in 2013 being able to persuade hesitant parents to allow scheduled vaccinations only one-third of the time, according to the study. Understanding why parents are refusing or delaying vaccines is critical for doctors so they can tailor their counseling accordingly, researchers said.

“When I talk to families ... I always emphasize vaccines are safe, effective and they save lives,” Hough-Telford said. “We all want the same thing -- for the child to be healthy.”

Other reasons for vaccine delay were parental concern about causing discomfort for the child or concern for burdening the child’s immune system. The fear of autism as the reason for delaying vaccines fell to 64 percent in 2013 from 74 percent in 2006.

The study did not mention the specific vaccines that parents were refusing, which will be important for health care providers to explore. A study published this month in the journal Cancer Epidemiology, Biomarkers and Prevention reported that many parents wanted to opt out of the Gardasil vaccine for HPV -- approved by the U.S. Food and Drug Administration in 2006 -- because they thought the vaccine was unnecessary for their sexually inactive child. It is unclear if parents refusing the HPV vaccine would feel similarly about vaccines that protect against measles, whooping cough and tetanus.

Pediatricians in suburban and rural settings were more likely to see children whose parents wished to delay or avoid vaccinations than parents in urban settings, according to the AAP study, and the percentage of pediatricians "always" dismissing patients from their practice for continued vaccine refusal nearly doubled from 6 percent to 11 percent between 2006 and 2013.

McDavid said she is one of those physicians.

“[The parents and I] are partners, from the child’s newborn exam until they are 18 [years old],” she said. “I can’t treat a child in a way that I feel is not best for that child.”

To help inform parents about the importance of vaccination, McDavid said she gives parents an assignment to go on a field trip to a graveyard.

“I want you to go to an old cemetery, walk through, look at the headstones of the babies that died at age 1, 2, 3 years of age," she said, but thanks to vaccines, "people don’t see this anymore, so people don’t know what to be afraid of.”

The study was published as the AAP called on public health authorities today to universally eliminate all vaccine exemptions, unless they are medically necessary. While public school students in the U.S. are required to receive various vaccinations before attending classes, in most states the requirement can be waived by "non-medical" vaccine exemption forms because of a parent's beliefs or religion.

Dr. Geoffrey Simon, board-certified pediatrician and lead author of AAP’s policy statement today, and chairperson of the AAP's Committee on Practice and Ambulatory Medicine, explained the decision as necessary to protect people via herd immunity, where the majority of people being protected helps guard against an outbreak.

“We disenfranchise children and adults who are medically unable to receive the vaccine," Simon said of diminishing herd immunity from lowered vaccine rates. "Who’s to say that creating a risk of developing preventable disease is fair. They deserve a safe school and work environment."

Dr. Kathryn J. Horton is a senior internal medicine resident at the University of Washington in Seattle and is currently working in the ABC News Medical Unit. Dr. Shailja Mehta, a resident in the ABC News Medical Unit, also contributed to this report.