What is definition of herd immunity
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Here is a thorough explanation of herd immunity drawn from authoritative medical textbooks.
Herd Immunity (Community Immunity)
Definition
Herd immunity (also called community immunity or population immunity) describes a type of protection that occurs when the vaccination of - or prior infection in - a sufficient proportion of a population provides indirect protection to unprotected individuals. The core principle is that when large numbers of a population are immune, it becomes difficult to maintain a chain of infection, because susceptible persons are less likely to come into contact with an infectious agent.
"Herd immunity theory proposes that in diseases passed from individual to individual, it is difficult to maintain a chain of infection when large numbers of a population are immune. The higher the number of immune individuals, the lower the likelihood that a susceptible person will come in contact with an infectious agent."
- Park's Textbook of Preventive and Social Medicine, p. 115
How It Works
Herd immunity provides an immunological barrier to the spread of disease within the human herd. A classic historical example: when measles was introduced to the Faroe Islands in 1854 (a "virgin" population with no prior immunity), attack and case fatality rates were extremely high. The epidemic wave only declined with the build-up of herd immunity following natural infection.
Elements That Contribute to Herd Immunity
According to Park's Textbook, three main elements contribute:
- Occurrence of clinical and subclinical infection in the herd (natural immunity)
- Immunization of the herd (vaccine-induced immunity)
- Herd structure - the composition of the population, which is never constant; it varies due to new births, deaths, and population mobility. An ongoing immunization programme keeps herd immunity at a high level.
Herd Immunity Threshold (HIT)
It is neither possible nor necessary to achieve 100% immunity to halt an epidemic. The minimum proportion of the population that must be immune to prevent sustained disease spread is called the Herd Immunity Threshold (HIT). This threshold is tied directly to a disease's basic reproduction number (R₀) - how many people, on average, one infected person infects in a fully susceptible population:
HIT = (R₀ - 1) / R₀
Different diseases require very different coverage levels to achieve herd immunity. From Harrison's Principles of Internal Medicine (2025):
| Disease | Estimated Herd Immunity Coverage Needed |
|---|---|
| Measles (highly transmissible) | ~95% |
| Polio | ~80% |
| Smallpox | Lower than measles |
Important Limitations
- For tetanus, herd immunity does NOT protect the individual, because the toxin comes from environmental soil bacteria, not from person-to-person spread.
- Diseases that mutate frequently (like influenza, RSV, COVID-19), spread without symptoms, or confer only short-lived immunity may be very difficult or impossible to achieve lasting herd immunity against.
- Herd immunity protects those who cannot be vaccinated - newborns, the elderly, and immunocompromised individuals.
Why It Matters
Some immunizations reduce not only individual illness but also transmission from immunized to non-immunized persons. This indirect benefit is at the heart of herd immunity. As stated in Harrison's (2025):
"Some immunizations also reduce transmission of infectious disease agents from immunized persons to others, thereby reducing the impact of infection spread. This indirect impact is known as herd immunity."
- Harrison's Principles of Internal Medicine, 22E (2025), p. 1045
Sources:
- Park's Textbook of Preventive and Social Medicine - Herd Immunity section, p. 115
- Harrison's Principles of Internal Medicine, 22E (2025) - Vaccine Impact, p. 1045
- Fine P, "Herd Immunity: A Rough Guide," CID 2011 - the landmark epidemiological reference (cited 1,800+ times)
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