(CNN) The head of the US Food and Drug Administration says that if states don't require more schoolchildren to get vaccinated, the federal government might have to step in.

Nearly all states allow children to attend school even if their parents opt out of vaccines. These vaccine exemptions are especially popular in Washington state, where a measles outbreak started last month that has now sickened at least 67 people in four states. And New York has been working to contain its largest outbreak in decades, which began in October and has sickened more than 200 people.

"Some states are engaging in such wide exemptions that they're creating the opportunity for outbreaks on a scale that is going to have national implications," FDA Commissioner Dr. Scott Gottlieb said Tuesday in an interview with CNN.

If "certain states continue down the path that they're on, I think they're going to force the hand of the federal health agencies," he added.

Gottlieb's suggestion about the federal government and vaccines was first reported by Axios

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