Long before herd immunity came to be seen as an elimination threshold guiding mass vaccination campaigns, it explained why epidemics subside, reducing — but not eliminating — an individual’s risk of infection. Much like gravity pulls an object back to earth, herd immunity is the counterweight to sustained epidemic growth.

For SARS-CoV-2, herd immunity should not have been seen as an elimination threshold. Instead, it should have helped us understand that as immunity accumulated in the population, whether from infection or vaccination, the epidemic would recede before everyone was infected. Acknowledging that we couldn’t stop all infections, policy should have focused on minimizing the exposure of those already known to be at enormously increased risk of severe disease, while also limiting the harms caused by prolonged restrictions.

In contrast to the notion of an elimination threshold, which arose relatively recently, herd immunity has been understood as a mechanism of epidemic abatement for nearly a century. In the aftermath of the 1918 flu pandemic, British epidemiologists recreated epidemics in caged mice populations as part of efforts to understand how the shifting ratio of susceptible and immune individuals fueled or restrained epidemics. They hypothesized that epidemic waves “fall because the average resistance of the herd is raised.” But this did not mean that a pathogen disappeared. “Another wave will follow it at a later date,” they continued, when waning immunity causes “the average herd immunity to fall below some critical level.” Herd immunity denoted a turning point in an epidemic that happened before every individual in a population had been exposed, offering the appealing possibility of preventing the infection of those at most risk of severe disease.