Herd immunityHerd Immunity: Why the Figure Is Always a Bit Vague

Published 24 July 2020

Nearly 100 years ago, two British researchers, William Topley and Graham Wilson, were experimenting with bacterial infections in mice. They noticed that individual survival depended on how many of the mice were vaccinated. So the role of the immunity of an individual needed to be distinguished from the immunity of the entire herd. Adam Kleczkowski writes in The Conversation that, fast-forwarding a century, and the concept of “herd immunity” is now widely discussed in government dispatches and newspaper articles. But what does it actually mean? “When a disease such as COVID-19 spreads through the population, it leaves some people immune, at least in the short term,” he writes. “The people who become infected later will increasingly have contact with these immune people and not with the susceptible ones. As a result, the risk of infection is reduced and eventually the disease stops spreading. This might happen even if some people in the population are still susceptible. Vaccination can be used to protect susceptible people and thereby hasten the decline of the epidemic. It can also be used to stop the virus from spreading in the first place.”