In California, by contrast, parents simply check a box on the school immunization form to opt out of vaccines, Omer said.
Other states with easy exemption policies are Arizona, Colorado, Connecticut, Hawaii, Maryland, Missouri, North Dakota, Oregon, Rhode Island, Vermont, Washington and Wisconsin.
While some states with easy policies have already started to toughen them up, possibly because they have had outbreaks of vaccine-preventable diseases, other states with difficult policies are relaxing them, Omer said.
Commenting on the findings, Dr. Mark Schleiss, a professor of pediatrics at the University of Minnesota Medical School, said, "The data speak for themselves -- the more alternatives and options that are provided to decline immunizations, the greater the likelihood that immunizations will be declined."
And there are sometimes tragic results of not getting adequate vaccinations, such as the five children in Minnesota who got sick from Haemophilus influenzae type b in 2007, Schleiss said. One child died.
However, Schleiss thinks that the answer is to address public perception of vaccines instead of rewriting policies. "It has to come from our political leaders. I think they need to articulate [the importance of getting vaccinated]," he said.
Schleiss said that spending time talking openly with parents about the safety of vaccines -- and how reports in the early 1990s about the relationship with autism were completely fraudulent -- usually helps parents understand that they should get their children vaccinated.
To learn more about childhood vaccinations, visit the U.S. Centers for Disease Control and Prevention.
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