On the risk of vaccine exemptions, the science is clear — it would take a relatively small decline in immunization rates to produce big jumps in disease and health care spending. The trick is keeping communities above the danger threshold.

In 2011, the Centers for Disease Control and Prevention reported that Washington state topped the nation in both overall vaccine exemptions and nonmedical vaccine exemptions, at 6.2% and 5.8% respectively. A few months after that study published, the state passed a law requiring parents who were seeking an exemption to submit a certificate signed by a physician and issued after health-provider counseling. (Physicians are often tapped as the most-trusted sources of vaccine information.) This month in the journal Pediatrics, researchers published a study on the impact of Washington’s parental counseling law, reporting a more than 40 percent decrease in the state’s exemption rates since implementation.

“I do believe that this is an intervention that could be translated to other states,” said study co-author L. Beryl Guterman, a public health program associate within the Omer Research Group at Emory’s Rollins School of Public Health. “With other states looking to change their policies to increase vaccination coverage, we believe this is a viable option they should consider.”

In examining data from the Washington State Department of Health, Guterman and her colleagues documented a 40.2% decrease in exemption rates and an absolute reduction of 2.9 percentage points. Vaccine coverage for all vaccines required for school entrance either went up or remained the same after the new law, and the number of students out of compliance went down. The study found a slight increase in conditional entrants — kids who aren’t fully vaccinated — but described it as “small and insignificant.”

Researchers also found a decrease in the probability that exempted kindergarteners would encounter non-exempted ones or that exempted kindergarteners would encounter each other. The new law also pushed down the geographic clustering of vaccine exemptions — a particularly important key to reducing the chance of a disease outbreak. Guterman noted that prior to the 2011 law, parts of northern Washington were reporting up to 10 exemptions per 100 kids at the school level.

Overall, the outcomes were “impressive to see,” Guterman said. She and her study co-authors Saad Omer, Kristen Allen, D.H. Chang, Robert Bednarczyk, Alex Jordan, Alison Buttenheim, Malia Jones, Claire Hannan, M. Patricia deHart and Daniel Salmon write:

Given the concerns of a potential backfire effect for information-based intervention, it is reassuring to see that parental counseling was not associated with a net increase in exemption rates or net decrease in vaccination rates. This could be caused by many factors. First, the so-called backfire effect has not been consistently replicated. Second, the clinical interaction between a health care provider and a parent revolves around not only correcting misinformation but also, among other things, increasing disease salience, which may be more relevant to parents.

Prior research had described Washington’s pre-2011 exemption criteria as “easy” — in other words, parents could easily obtain a vaccine exemption for their kids. Post-2011, Guterman said it’s difficult to tease out exactly what mechanism of the new law resonated loudest with families — plus, the law doesn’t mandate exactly what doctors have to include in their exemption counseling — but she said it’s probably a mix of factors.

“The bill has two potential effect mechanisms,” she told me. “Increasing the steps or barriers to getting an exemption is one part, but physicians are also the most trusted sources of information. Our group believes it’s a combination of both.”

According to recent data from CDC, the median percentage of kindergartners with a vaccine exemption remains low at 2 percent, ranging from less than 1 percent in Mississippi to nearly 7 percent in Alaska. Currently, all states grant vaccine exemptions for medical reasons, more than half grant religious exemptions and 18 allow philosophical exemptions. In 2015, California became the third state to eliminate all nonmedical vaccine exemptions and the first state to do so in more than 30 years. Guterman and her colleagues will be studying the effect of the California law as it unfolds.

“Broadly, vaccine refusal is not a benign choice,” Guterman said. “It impacts everyone.”

For a full copy of the new vaccine study, visit Pediatrics.