Question

Biology/ Bacteriology:

- What is herd immunity? Give examples.

Answer 1

Herd immunity is a type of immunity that occurs when the vaccination of a portion of population or herd provides protection to the unprotected individuals. Herd immunity theory states that in diseases passed from individual to individual, it is very difficult to maintain a chain of infection where large number of a population are immune. Higher the number of immune individuals lower is the likelihood that a susceptible person will come in contact with an infectious agent.

It provides and immunological barrier to the spread of disease in the human herd. For example when an infectious agent is introduced into a virgin population, i.e. population with very low or no immunity, the attack and case fatality rates tend to be very high involving practically all susceptibles as it had happened in the very severe Measles epidemic in 1854. The Epidemic wave declined with a build up of herd immunity.

Elements which contributes to herd immunity are a) occurrence of clinical and subclinical infection in the herd.b) immunisation of the herd c) herd structure.

Herd structure includes not only the host belonging to the herd species but also the presence and distribution of alternative animal hosts and possible insect vectors as well as those environment and social factors that favour or inhibit the spread of infection from host to host.

if the herd immunity is a sufficiently high the occurrence of an epidemic is highly unlikely.if that high level of immunity is maintained and stepped up by an ongoing immunization program to the point where the susceptible persons are reduced to a small proportion of the population it may lead to elimination of the disease in due course of time. This has been achieved in cases of diphtheria and poliomyelitis. however in the case of smallpox it was not the herd immunity which played a major role. It was eradicated with the help of the vaccines. As the vaccination against smallpox was abolished, herd immunity also declined. Also in the case of tetanus, herd immunity does not protect individual.

It is neither possible normal necessary to have hundred percent herd immunity in an population to halt an epidemic or to control the disease.

The proportion of the Indian individuals in a population above which this is no longer persist is called herd immunity threshold.