Measles is only considered eliminated in a country if it no longer spreads constantly for a year. This means that if the current outbreaks last for three more weeks, which is highly likely, it may no longer be considered eradicated in the US.

It is prevented by a vaccine administered routinely and safely to children around the world. However, a total of 93% of measles cases this year were among unvaccinated people or those with an unknown vaccination status, the Centers for Disease Control and Prevention said.

Most children in the US are required to get the shot to attend school. But vaccination rates have declined as more parents waive the shots or have fallen behind on recommended vaccination schedules.