In a 2012 study of vaccine exemption policies across the country, a team of researchers led by Saad Omer, a professor of public health at Emory University, found that of the 20 states that allowed personal belief exemptions for enrollment in a public school or child-care program, less than a third made it "difficult" to do so (for instance, by making parents re-apply for one each year, explain their beliefs in writing, or get a notarized letter of approval from a health care provider). In the nine "easy" states identified in the study, the rules required only signing a form. Indeed, Omer suspects that some parents sign vaccine exemption forms not because they actually hold anti-vaccine beliefs, but simply because it's easier than juggling the doctors' appointments, missed work, and other inconveniences of getting kids vaccinated. (More about that here.)
Personal belief exemptions aren't the only option available to vaccine-averse parents. Every state allows for medical exemptions for reasons such as an anaphylactic allergic response to a previous vaccine. Forty-eight states (all but West Virginia and Mississippi) allow exemptions on religious grounds. In many states, obtaining a religious exemption isn't any harder than getting a personal belief exemption. But according to Omer, religious exemptions aren't as popular as personal belief exemptions. He's found that opt-out rates in states that allow personal belief exemptions are 2.5 times as high as rates in states that only permit religious exemptions. In one analysis he found that whooping cough rates in states with personal belief exemptions are more than double those in states that allow only religious exemptions.