In response to the measles outbreak that has infected more than 100 people, California lawmakers have proposed legislation that would make it harder for parents to opt out of vaccinating their children.

The law would end the personal belief exemption that allows parents to cite personal grounds, rather than medical reasons, for refusing to immunize children. The strict measures would bring California in line with just two other states in the country.

Mississippi, a state that's often on the nation's worst list for poverty and obesity, has a near-100 percent vaccination rate. 

Daniel Salmon, deputy director of the Institute for Vaccine Safety and associate professor at Johns Hopkins Bloomberg School of Public Health, said that's because the state doesn't offer anything besides a medical exemption. 

The process of getting an exemption varies from state to state.

"In some states, it's incredibly easy. You simply check a box and sign your name. Other states have requirements like notarization, writing a personal letter, annual renewal and counseling with either a physician or health department," said Salmon. 

But simply clamping down on exemptions might not be the answer. According to Salmon, exemptions serve a lot of purposes.

"Among others they serve an important policy purpose, which is it allows an escape vowel or pressure release vowel for those people who are really strongly opposed to vaccination and it may not be in the public's interest to force these parents to vaccinate so while that may seem like simple solution, it may actually cause more problems in the long run," he said.