The widening measles outbreak has prompted public health officials to take extraordinary measures to limit the spread of the highly contagious virus. But what about people who can’t – or shouldn’t – get vaccinated?

New York City will make unvaccinated people in high-risk Brooklyn ZIP codes get immunized or pay a fine. Schools in Clark County, Washington, have barred hundreds of unvaccinated students from class and school events during an outbreak there.

And hospitals in some communities are screening kids who have a fever or rash in the nation’s second-worst outbreak since measles was declared eliminated in 2000.

Widespread vaccination creates herd immunity that protects both the healthy and the vulnerable who are unable to protect themselves – perhaps a “more morally compelling reason” to vaccinate, said David Kimberlin, co-director of the division of pediatric infectious diseases at the University of Alabama-Birmingham.