In the 19th century, a curious observation lead to the discovery of what we now know as herd immunity- epidemics often ended before involving all susceptible people. At the time, two possibilities were considered:
- epidemics ended due to changes in the infectious agent (loss of virulence, etc.)
- it reflected the dynamics between susceptible, infected and immune segments of society.
The latter point of view (mass action principle) became widely accepted, as its uncomplicated mathematical formulation provided one of the simplest logical arguments.
Ronald Ross, from his work on mosquito dynamics, deduced that it was not necessary to get rid of all mosquitoes to eradicate malaria. His “mosquito theorem” provided a mathematical means of calculating the required threshold.
Experiments conducted among rats supported the idea that disease transmission could be halted by ensuring a large proportion of rat population was immune. This supported previous observations that vaccinating a proportion of the population stopped the transmission of smallpox.
These gave rise to the notion of herd immunity- a situation wherein the presence of a large proportion of immune individuals in a population protects susceptible individuals from acquiring disease. Thus, herd immunity refers to indirect protection of susceptible persons from disease. The term ‘herd immunity’ was first used in 1923 by Topley and Wilson in a paper titled “The spread of bacterial infection: the problem of herd immunity”.