Why do we call it herd immunity? Syringes with the Pfizer-BioNTech vaccine await use a University Health Service vaccination clinic. The idea of herd immunity — brought about my mass vaccination — has evolved in the past century. Photo: Jeff Miller As the world begins vaccinating more and more people against COVID-19, we’ve all heard about herd immunity in the news — the idea that if enough people receive the vaccination, even if it’s not the entire population, the virus won’t be able to find enough infectable people to sustain its spread. But where did the term “herd immunity” come from, and how has its meaning changed over the years?