What happens when some people don’t get vaccinated?

Even a small number of unvaccinated people can make it much easier for a disease to spread and survive.

There is a concept in science called “herd immunity,” which refers to the idea that a lot of people need to get a given vaccine, whether it’s for the flu or measles, to stop a disease from spreading. Vaccinated people essentially act as barriers to outbreaks, since diseases can’t pass through them and infect others.

This barrier helps protect some of the most vulnerable populations: infants under 12 months of age, who can’t get vaccinated and are more susceptible to infection; the elderly, who have a higher risk of death if they contract vaccine-treatable illnesses; and people with compromised immune systems, who can’t get vaccines and are more likely to die from the diseases they protect against.

The threshold for herd immunity depends on the disease and how it’s transmitted. In an analysis of several vaccine-treatable illnesses, the Centers for Disease Control and Prevention set the lowest threshold of vaccine coverage at 75 percent (for mumps) and the highest at 94 percent (for measles). Even the low-end threshold requires at least three in four people to get vaccinated.

The public health goal is to vaccinate everyone who is able so there’s herd immunity for all vaccine-treatable illnesses. Over time, this would eradicate dangerous diseases — specifically, those that are exclusive to humans — by stopping them from spreading and developing in human hosts.