Herd Immunity Is Humanity’s Great Hope, and It’s Proving Elusive

Long before herd immunity became humanity’s shared obsession, the phrase referred to sick cows. More than a century ago, veterinarians observed that outbreaks of a highly contagious bacterial infection menacing cattle died down once they’d burned through a certain percentage of a herd, so long as new animals weren’t introduced. Soon the concept was extended to a variety of human outbreaks, where it became a staple of epidemiology.

Since the beginning of the pandemic, exactly when the U.S. might reach herd immunity for Covid-19 has been furiously debated in congressional hearings, on TV shows, and among the many armchair epidemiologists on Twitter. In the popular imagination, the phrase has become shorthand for the end of the pandemic—a finish line that will suddenly cause the virus to subside and allow maskless normalcy to resume.