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.

Here’s what’s known about the virus and herd immunity.

HOW IS THE HERD IMMUNITY THRESHOLD CALCUATED?

It's a formula based on how contagious a virus is — or how many people catch the virus from one infected person, on average.

But the calculation offers only a broad target for when there might be a big drop off in spread. The figure could also vary by region.

“It’s not 64.9 is terrible and 70.1 is fantastic,” said Dr. Walter Orenstein, an infectious disease expert at Emory University.

Orenstein notes vaccination levels and other factors that affect spread could differ even within a city.

HOW DO WE KNOW WE'VE REACHED HERD IMMUNITY?

Proof that we're nearing herd immunity would be a "disruption in the chain of transmission," said Ashley St. John, who studies immune systems at Duke-NUS Medical School at Singapore.

But don't wait for any big declaration that we’ve reached that milestone.

To determine whether to relax restrictions, health officials will be watching infection and hospitalization trends as vaccinations roll out. And...