How big could the outbreak become?

That’s hard to predict. A measles outbreak is like a forest fire throwing out sparks, Moss says. If a spark lands in a state such as Maryland, which has a 97% measles vaccination rate, it will just fizzle out. But “if the sparks from this initial fire land in communities where vaccination rates are low, then we’re going to have multiple large outbreaks”, Moss says.

“I think it could get to thousands and thousands of cases,” says virologist Paul Offit at the Children’s Hospital of Philadelphia in Pennsylvania.

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