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Heard immunity from park textbook

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Here is the complete coverage of Herd Immunity from Park's Textbook of Preventive and Social Medicine:

Herd Immunity (Community Immunity)

Definition

Herd immunity (also called community immunity) describes the type of immunity that occurs when vaccination of a portion of the population (the "herd") provides protection to unprotected individuals. The theory proposes that in diseases passed from individual to individual, it is difficult to maintain a chain of infection when large numbers of the 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.

How It Works

Herd immunity provides an immunological barrier to the spread of disease in the human herd. A classic example:
  • When an infectious disease is introduced into a "virgin" population (very low or no pre-existing immunity), attack rates and case fatality rates tend to be very high.
  • Historical example: The severe measles epidemic in the Faroe Islands in 1854, where the population had no previous experience of measles. The epidemic wave eventually declined as herd immunity built up through natural infection.

Elements Contributing to Herd Immunity

Three main elements:
  1. Occurrence of clinical and subclinical infection in the herd
  2. Immunization of the herd
  3. Herd structure
Herd structure is never constant - it varies continuously because of:
  • New births and deaths
  • Population mobility
An ongoing immunization programme keeps herd immunity at a very high level.
The herd structure includes:
  • The host population (herd species)
  • Presence and distribution of alternative animal hosts
  • Insect vectors
  • Environmental and social factors that favour or inhibit spread of infection

Significance & Epidemiological Impact

  • If herd immunity is sufficiently high, occurrence of an epidemic is regarded as highly unlikely.
  • If that high level is maintained and stepped up by an ongoing immunization programme - reducing susceptibles to a small proportion - it may lead (but not necessarily) to elimination of the disease in due course.
  • Examples of diseases controlled this way: diphtheria and poliomyelitis
  • Smallpox eradication: Herd immunity was important but NOT the crucial factor - elimination of the source of infection by surveillance and containment measures played the decisive role. With abolition of smallpox vaccination, herd immunity to smallpox naturally tends to decline over time.
  • Tetanus: Herd immunity does NOT protect the individual (because tetanus is not a communicable disease - it comes from environmental spores, not person-to-person spread).

Herd Immunity Threshold

Studies have shown it is neither possible nor necessary to achieve 100% herd immunity to halt an epidemic or control disease (as demonstrated by smallpox and polio eradication).
Herd immunity threshold = The proportion of immune individuals in a population above which a disease may no longer persist.
Its value varies with:
  • Virulence of the disease
  • Efficacy of the vaccine
  • Contact parameter (transmission dynamics) for the population

How to Measure Herd Immunity

Herd immunity may be determined by serological surveys (serological epidemiology) - measuring the proportion of the population with protective antibody levels.
Source: Park's Textbook of Preventive and Social Medicine, p. 115
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