Herd immunity describes the point at which a population is sufficiently immune to a disease to prevent its circulation. Researchers at the University of Manchester first coined the term in 1923, to describe how an entire herd of animal subjects (in that case, mice) could become immune to a disease even though not every member of the herd had been immunized.
Widespread vaccination is the most reliable way to achieve herd immunity. “The whole concept of herd immunity arose from the question: How many people do you need to vaccinate in a population to eradicate a disease,” said Paul Hunter, a professor at the University of East Anglia in the United Kingdom and a member of the World Health Organization’s infection prevention committee.
But achieving herd immunity is often more complicated than that and it’s not always possible — especially when it comes to the novel coronavirus that causes COVID-19, for which there is no vaccine. “Herd immunity involves a variety of factors above and beyond the virus itself,” said Gregory Poland, director of the Mayo Clinic’s Vaccine Research Group.