What Is Herd Immunity?

When you think of a herd, you probably think of cows, wildebeest, or buffalo. In the animal world, there is safety in numbers – more pairs of eyes to look out for predators, for example.

As humans, we don't generally have to worry about predators, but we can gain the protection of the herd in other ways.

Herd immunity — also called population or community immunity — means that enough people in a group or area have immunity to a germ (for instance, a virus) that it no longer spreads easily. That includes people who haven't caught the disease, people who had the illness and recovered, and people who got a vaccine.

History of Herd Immunity

Herd immunity didn't start as a vaccination concept—it came from livestock science in the 1800s.

The term herd immunity was first used in 1894 by Daniel Elmer Salmon, Chief of the Bureau of Animal Industry (BIA), to describe the health and disease resistance of well-fed hog populations. The use of the phrase expanded in 1916 when BIA veterinary scientists used the term to describe immunity in cattle following infection with Brucellosis also known as "contagious abortion"—epidemics of spontaneous miscarriage—in cattle and sheep.

In 1923, British bacteriologists began using the concept in experiments involving epidemics in mice, as part of efforts to better understand patterns of disease transmission in human populations. Unless there was a steady influx of susceptible mice (this is important to remember), the rising prevalence of immune individuals would end an epidemic. In a 1923 article in the Journal of Hygiene, Topley and G. S. Wilson described this phenomenon as "herd immunity."

The idea moved into medicine and by the late 1920s, the concept had gained broader traction—particularly among British scientists—who used it to describe the accumulation of population-level immunity to diseases such as diphtheria, scarlet fever, and influenza. In 1922, Topley suggested a parallel between outbreaks in mice and children: 

"Such a likeness would seem to exist in the case of epidemic diseases affecting children of school age." He also wondered whether measures already "in vogue in dealing with epidemics among live-stock, where methods of segregation are so much more easily enforced than among human populations," might inform decisions about school closings amid epidemics.