Most states also allow exemptions for religious reasons, and 17 states, including Washington and Texas, allow exemptions for both religious and personal or philosophical beliefs, according to the National Conference of State Legislatures. Both Washington and Texas have seen measles outbreaks this year.

Lawmakers in Iowa, New Jersey and Vermont, which already ban personal or philosophical exemptions, are now debating proposals to eliminate religious exemptions.

Proposals in Maine and Oregon would eliminate both exemptions, while measures in Minnesota, Colorado, and Washington state, where there are 66 confirmed measles cases this year, would only eliminate personal exemptions and leave religious exemptions in place.