PHOENIX - A rise in non-medical exemptions by families to opt out of vaccinations for their kids have created hotspots in the U.S. where the risk of contracting diseases like measles are growing, a study finds.

The Phoenix area is one of those hotspots, topping a list of places where more than 400 kindergarten-aged children aren't vaccinated, according to the study. In Maricopa County, there were almost 3,000 kindergarten non-medical exemptions to vaccinations in the 2016-17 school year.

That's more than three times the amount of kindergarten non-medical exemptions in Salt Lake County, which is at No. 2 on that list.

The study, published in the Public Library of Science journal PLOS Medicine, found 12 of 18 states allowing exemptions to vaccines for religious or philosophical reasons have shown an increase in the number of kindergarten-age children enrolled in school with non-medical exemptions since 2009.

In Arizona, parents and guardians can submit a signed statement to schools that exempt a child from required vaccinations if the parent or guardian, due to personal beliefs, does not consent to immunization.

The rise in unvaccinated children stems from a discredited belief vaccines are linked to autism. In recent years, multiple outbreaks of measles — once considered eliminated by the CDC in 2000 — have popped up.

"As larger unvaccinated populations grow, particularly in highly mobile cities, the potential for vaccine-preventable disease outbreaks grows," said Peter Hotez, professor at Baylor College of Medicine and co-editor-in-chief of PLOS Neglected Tropical Diseases; and Melissa Nolan, an assistant professor of epidemiology at the University of South Carolina, in a joint statement.

A 2016 study published by JAMA from researchers at Emory University links refusals of vaccines to increases in cases of measles and whooping cough.

The 12 states showing an increase in exemptions are Arkansas, Arizona, Idaho, Maine, Minnesota, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Texas, and Utah. Six more states — Colorado, Louisiana, Michigan, Missouri, Washington and Wisconsin — also allow the exemptions.

According to the most recent Centers for Disease Control and Prevention data from 2015, nearly 92 percent of kids ages 19 to 35 months received a vaccination for measles, mumps and rubella (MMR). Most states require children to have up-to-date vaccinations to enroll in public schools.

"Our concern is that the rising NMEs linked to the anti-vaccine movement in the U.S. will stimulate other countries to follow a similar path," reads an excerpt from the study.