Herd

Herd Immunity is a form of indirect protection from infectious diseases that occurs when a large percentage of a population has become immune to an infection, whether through vaccination or previous infections, thereby providing a measure of protection for individuals who are not immune.

Overview

In a population where a large number of individuals are immune, chains of infection are likely to be disrupted, which stops or slows the spread of disease. The greater the proportion of immune individuals in a community, the smaller the probability that non-immune individuals will come into contact with an infectious individual.

Mechanism

Herd immunity does not apply to all diseases, just those that are contagious, meaning that they can be transmitted from one individual to another. Tetanus, for example, is infectious but not contagious, so herd immunity does not apply.

Types of Immunity

There are two types of immunity in a population: active and passive:

• Active immunity is immunity generated by the individual's own immune system. This can be acquired in two ways, by contracting the disease or through vaccination.
• Passive immunity is acquired through transfer (injection or infusion) of antibodies or activated T-cells from an immune individual; it is short lived—usually lasting only a few months.

Herd Immunity and Vaccination

Vaccination acts to reduce the number of susceptible individuals and to increase the number of immune individuals. For diseases where the vaccine mimics the disease (this is true for most vaccines), this increases herd immunity.

Limitations and Criticism

Herd immunity has limitations, since it may not provide complete protection to those who are unable to be vaccinated. These individuals can still be infected by individuals within the population who are carriers of the disease.

See Also

• Vaccination
• Infectious Diseases
• Immune System

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