At least 12 states considered bills last year that would follow California's in requiring that nearly all children receive childhood vaccines, a new analysis finds.

The analysis also finds that Mississippi and West Virginia have the highest childhood vaccination rates in the country.   They were also the only two states in the U.S. that forbid nonmedical exemptions to mandatory school vaccination laws for religious or philosophical reasons, until they were joined by California last year.

While all 50 states allow exemptions for children who have a valid medical reason, and almost all states allow nonmedical exemptions for parents with either religious or philosophical objections, the political climate has recently shifted in favor of making exemptions more difficult to obtain, according to analysts at Columbia University’s School of Public Health, who conducted a new study that appears in this month’s Public Affairs.

Last year, California, long known as a state with lenient provisions and high rates of opting out, followed the example of Mississippi and West Virginia and eliminated nonmedical exemptions.  The state’s action was a response to the widely publicized outbreak of measles in June 2015, traced to two Disneyland theme parks. That year, at least twelve states considered bills on nonmedical exemptions, the study finds.

The study also shows that the bans result in high immunization rates. In the 2014-15 school year, vaccinations in Mississippi for measles-mumps-rubella and diphtheria-tetanus-pertussis stood at over 99 percent for kindergarteners, the highest rate in the country. Only 17 children of kindergarten age received medical exemptions.

"Contrary to conventional wisdom, it may be politically feasible to limit exemptions to only medical reasons," writes James Colgrove, PhD, a professor who co-authored the study.  He suggests such alternatives as retaining nonmedical exemptions but making them more difficult to obtain, requiring parents to receive educational counseling from a licensed health care provider, or to renew the exemption annually.