Once the herd immunity threshold is reached, the virus just doesn’t go away. | arthobbit/Getty Images/iStockphoto

There are two ways to reach herd immunity for Covid-19: the slow way, and the catastrophic way.

How will the Covid-19 pandemic end? And when?

These have been the biggest questions since the pandemic began earlier this year. The answer likely depends on one routinely misinterpreted concept in public health: herd immunity.

“Herd immunity is the only way we’re going to move to a post-pandemic world,” says Bill Hanage, an epidemiology researcher at Harvard. “The problem is, how do you get to it?”

Typically, the term herd immunity is thought of in the context of vaccination campaigns against contagious viruses like measles. The concept helps public health officials think through the math of how many people in a population need to be vaccinated to prevent outbreaks.

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