The FDA has warned that if states don't get more school children vaccinated, the agency might step in.
Commissioner Scott Gottlieb says vaccine exemptions allowed by some states are enabling outbreaks which pose national dangers.
Washington state - the starting point for a measles outbreak that's now infected 67 people in 4 states - allows parents to opt out for personal or philosophical reasons.
Experts say America's past vaccination success gives some families a false sense of security.
"There is a complacency that these 'one-time' diseases are no longer around or that they were never severe in the first place. And I think, because of that, parents are more reluctant to vaccinate their kids, and I think we're starting to see some of the adverse effects of that," said Dr. Camille Sabella, Cleveland Clinic.
A number of states - including Washington - are now debating tighter restrictions on vaccine exemptions.
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FDA may step in if states don't require more children to get vaccinated