By Erin A. Mordecai, Mallory J. Harris and Marc Lipsitch

Dr. Mordecai is an assistant professor of biology at Stanford University. Ms. Harris is a Ph.D. candidate at Stanford University, where she studies infectious disease. Dr. Lipsitch is a professor of epidemiology at the Harvard T.H. Chan School of Public Health.

Half of Americans have now received at least one dose of a Covid-19 vaccine, and with children ages 12 to 15 now eligible for immunizations, the United States appears to be getting the coronavirus pandemic under control. But despite the tremendous progress, it’s still not clear that the nation will ever truly reach the herd immunity threshold — the point at which enough people in a population are immune to a pathogen to limit its spread.

More important, we may not need to achieve that goal in order to escape the pandemic.

Reaching the herd immunity threshold doesn’t guarantee that people cannot be infected by a pathogen. But the closer a community gets, the more transmission slows down, which benefits everyone. The current rate of new Covid-19 cases in the United States right now shows this phenomenon in action. Forty percent of the U.S. population is fully vaccinated, and the number of new cases of Covid-19 reported each day is now one-tenth of what it was at the pandemic’s apex in January.

Last year many scientists suggested the herd immunity threshold would be reached when 60 to 70 percent of the population was immune, either because of vaccination or exposure to the virus. Scientists have now revised this number upward, to at least 80 percent. But there is no single, universal herd immunity threshold. The number depends on the transmissibility of a disease, its variants and the characteristics of the population it’s invading.

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