More doctors are reporting encounters with parents refusing to vaccinate their kids than a decade ago, but parents’ reasons for skipping immunizations have shifted in the past decade.

A new survey published in Pediatrics on Monday reports that 87 percent of pediatricians in the United States say they encountered parents refusing to vaccinate their children in 2013. A decade earlier, 75 percent of doctors reported they had experienced vaccine refusals.

The most common reason parents gave, according to doctors? The vaccines weren’t necessary.

“Vaccines have been so good at taking diseases away that when we don’t see diseases, we don’t think they’re important,” said Dr. Kathryn Edwards, a pediatrician and director of the Vanderbilt Vaccine Research Program.

It’s the paradox of vaccines—they do away with infectious diseases to the point that they no longer seem necessary. The uptick in parents feeling that vaccines aren’t needed is concerning, Edwards said. Recent outbreaks of diseases such as measles and mumps have been blamed on a decline in vaccination rates.

“If you have a four-way stop and everybody stops at the stop sign, we’re fine. If one person doesn’t stop, we’re fine,” Edwards explained. “But if more people aren’t stopping, we’re going to have an accident.”

The survey also showed a shift in the number of parents who cite worries about the unfounded claim that vaccines or thimerosal—a preservative formerly used in childhood vaccines—can cause autism as the reason why they didn’t want their children to be vaccinated. The proportion of doctors who had encountered autism concerns fell from 74 percent in 2006 to 64 percent in 2013. It’s the first documented drop in that concern, the authors said.

“I’m not surprised by that,” said Dr. Paul Offit, the head of the infectious diseases department at Children’s Hospital of Philadelphia. “I hear from pediatricians that’s been happening less.”

Offit, who was not involved in the Pediatrics study, said that scientific research and subsequent media coverage have helped to dispel the myth. The 2014 measles outbreak in California also served as “an education” about the importance of immunization, he said.

Many parents are concerned about giving their children too many vaccines in a short period of time, a nod to the idea of an alternate vaccine schedule that’s gained steam in recent years. Nearly 73 percent of pediatricians say that parents delayed vaccines because of scheduling concerns—something public health officials discourage because delays increase the time unvaccinated children are at risk of becoming sick. It’s not clear whether this concern increased or decreased—the 2006 survey didn’t ask doctors about vaccine schedules.

The retrospective survey asked pediatricians to recall encounters with parents. Doctors weren’t asked to keep hard numbers on those encounters, so the study could have introduced recollection bias.

More and more, doctors don’t want to put up with vaccine refusal. The rate of doctors who reported always dismissing families who refuse vaccinations from their practices nearly doubled between surveys. Their efforts to convince parents to vaccinate their kids are sometimes fruitless—pediatricians reported that they were able to persuade only about one-third of parents who had refused vaccines to change their minds.

The American Academy of Pediatrics is trying to give doctors some added ammunition in those conversations with parents. The organization released a new guide called “Countering Vaccine Hesitancy” to accompany the survey.

“Generally, doctors aren’t prepared. It’s hard to counter,” Offit said. “We need to arm physicians with info to answer those questions.”

Republished with permission from STAT. This article originally appeared on August 29, 2016