Eighteen states allow parents to opt their children out of school immunization requirements for nonmedical reasons, with exemptions for religious or philosophical beliefs. And in two-thirds of those states, a comprehensive new analysis finds a rising number of kindergartners who have not been vaccinated.

In a report published Tuesday in PLOS Medicine, researchers from several Texas academic centers tracked the increasing number of children with exemptions in all 18 states from the 2009-2010 to 2016-2017 school years. They characterized many rural counties, as well as urban areas, as “hotspots” because their high exemption rates put them at risk for epidemics of measles, whooping cough and other pediatric infectious diseases.

Idaho has eight of the top 10 counties with the highest exemption rates, the study found. In Camas County in southern Idaho, nearly 27 percent of the kindergarten population opted out of childhood vaccinations in the 2016-2017 school year, according to the study.