What If We Never Reach Herd Immunity?
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Let’s begin by defining our terms. Herd immunity is the hazy, long-promised end of the pandemic, but its requirements are quite specific. Jennie Lavine, a biologist at Emory University, likens it to wet logs in a campfire. If there’s enough water in the logs—if there’s enough immunity in a population—“you can’t get the fire to start, period,” she says. To be more technical about it, a population reaches herd immunity when the average number of people infected by a single sick person falls below one. Patient zero might infect another person, but that second person can’t infect a third. This is what happens with measles, polio, and several other diseases for which vaccines have achieved herd immunity in the United States. A case might land here, but the spark never finds much dry fuel. The outbreak never sustains itself.
For COVID-19, the herd-immunity threshold is estimated to be between 60 and 90 percent. That’s the proportion of people who need to have immunity either from vaccination or from prior infection. In the U.S., the countdown enough people are vaccinated to reach herd immunity has already begun.
