EXPLAINER: How will we know we’ve reached herd immunity?

Health officials around the world are racing to vaccinate enough people to stop the spread of COVID-19, but what qualifies as “enough” is still an open question.

The goal is to get to “herd immunity,” which is when enough people have immunity, either from vaccination or a past infection, to stop uncontrolled spread.

Herd immunity doesn’t make any one person immune, and outbreaks can still flare up. It means that a virus is no longer easily jumping from person to person, helping to protect those who are still vulnerable to...