Achieving herd protection can stop the spread of an infectious disease within a population, but as experts from the Johns Hopkins Bloomberg School of Public Health explain, the U.S. is nowhere near that point with SARS-CoV-2, and getting there could prove difficult

When the coronavirus that causes COVID-19 first began to spread, virtually nobody was immune. Meeting no resistance, the virus moved quickly through communities, and ultimately around the world. In the absence of an effective treatment or vaccine, stopping it will require a significant percentage of the population to acquire immunity, a state that epidemiologists refer to as herd immunity.

What is herd immunity? In short, the term describes a condition in which most of a population is immune to an infectious disease, thus conveying indirect protection to those who are not immune. This indirect protection is called herd immunity, also sometimes referred to as herd protection.

For example, if 80% of a population is immune to a virus, four out of every five people who encounter someone with the disease won't get sick, and thus won't spread the disease any further. In this way, the spread of infectious diseases can be kept under control. Depending on how contagious an infection is, typically 50% to 90% of a population must be immune to achieve herd immunity, according to Gypsyamber D'Souza and David Dowdy, infectious disease experts at the Johns Hopkins Bloomberg School of Public Health.

But reaching that level of immunity, particularly with a disease as deadly as COVID-19, can come with significant risks and costs. In this Q+A, D'Souza and Dowdy examine what we know—and what we don't know—about COVID-19 immunity, describe multiple paths to herd protection, and explain why the fastest option is not the best one.

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