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 catching it.

Nobody knows for sure what the herd immunity threshold is for the coronavirus, though many experts say it’s 70% or higher. And the emergence of variants is further complicating the picture.