(CNN) In California, the kindergarten students most likely to be exempt from mandatory vaccinations based on their parent's personal beliefs are white and wealthy, according to a recent study.

The percentage of kindergartners with state-issued personal belief exemptions doubled from 2007 to 2013, from 1.54% to 3.06%. That's about 17,000 children, out of more than half a million, opting out.

Vaccine exemption percentages were higher in mostly white, high-income neighborhoods such as Orange County, Santa Barbara and parts of the Bay Area.

The study , which was published in the American Journal of Public Heath, looked at more than 6,200 California schools and found vaccine exemptions were twice as common among kindergartners attending private institutions.

"If you live in a rich, white community where lots of people don't vaccinate their kids, that could be dangerous," said Tony Yang, a health policy professor at George Mason University and author of the study.

According to a 2011 study in Public Health Reports, when parents refuse vaccines it's usually due to concerns about children receiving too many shots or developing side effects, including autism. This despite an exhaustive review last year of 20,000 scientific titles and 67 papers that concluded childhood vaccines are safe, and a complete retraction of the study that spawned the fear that vaccines cause autism.

Yang's study didn't investigate why wealthier, white families in the state are more likely to reject vaccines. One reason may be that some parents are trying to protect their children's immunity from diseases by insisting on specialized diets and natural living practices instead of vaccines, according to a different study

In June, California lawmakers outlawed personal belief exemptions , starting next summer. A "grandfather" clause will continue to exempt some children.

JUST WATCHED Unvaccinated kids pose health threat? Replay More Videos ... MUST WATCH Unvaccinated kids pose health threat? 06:33

"It's still a step in the right direction. Fewer people will be able to opt out," Yang said. "Vaccines are becoming the victims of their own success. Most people have never witnessed the infections that vaccines prevent."

Vaccines trick the body into thinking it's had a disease when it really hasn't. If enough people get vaccinated you achieve "herd immunity" -- the bodies of so many people have been tricked that there's little chance of a widespread outbreak. But when enough families in a community fail to vaccinate their children, diseases can spread just like a rash of measles spread earlier this year in California and several other states.

"It's a life-threatening problem. Some people could die because you're not vaccinating," Yang said.

But some vaccine refusers remain unswayed. In February, Dr. Jack Wolfson, an Arizona cardiologist, told CNN he did not vaccinate his two sons and that he could live with himself if his unvaccinated child got another child gravely ill.

"It's an unfortunate thing that people die, but people die. I'm not going to put my child at risk to save another child," Wolfson said.