During this period, the rates of non-medical exemptions were higher in the states with easy opt-out policies, such as California and Maryland, and in those states that allowed philosophical, instead of only religious, exemptions.
"The more relaxed these requirements are, as we and others have shown, the easier it is to get an exemption, the higher the rates of exemptions," said Saad Omer, an infectious disease epidemiologist at Emory University in Atlanta, and lead study author.
"It is common sense to me that it should not be easier to file for an exemption than it is to get your kid vaccinated," Omer said.
Every state requires vaccines for school attendance that protect against diseases including measles, mumps and rubella (MMR), polio, chickenpox, and diphtheria, tetanus and pertussis (DTaP), according to the U.S. Centers for Disease Control and Prevention.
Religious exemptions are permitted in every state except Mississippi and West Virginia, and 20 states also accept philosophical exemptions, the CDC notes.
Image: Boy getting vaccine, via Shutterstock