So shouldn’t all states follow the example of Mississippi and West Virginia, and ban all nonmedical exemptions? The courts have generally upheld such bans, but the political backlash is great, as history shows. After the smallpox vaccine was made compulsory in England and Wales in 1853, there were years of protests, until a commission exempted those with conscientious objections.

Partly because of intemperate comments by politicians, some Americans continue to view vaccines as an intrusion on their personal liberty rather than as a matter of public health. Between 2009 and 2012, 31 bills aimed at making it easier to obtain exemptions were introduced in various states (including 11 in Mississippi and West Virginia). Vaccination advocates who want to make exemptions harder to obtain will have an uphill fight.

But even states like Arizona and Colorado that allow fairly broad exemptions can tweak their rules to make sure parents are as informed as possible — and to make the exemption process difficult.

They can require parents to write a letter elaborating on the reason their child should be exempt. They can require that the letter be notarized. They can insist that parents read and sign a form that discusses the risks of nonvaccination. Better yet, they should mandate in-person counseling so that the decision not to vaccinate is truly informed.