Herd immunity occurs when a sufficient proportion of a population is immune to an infectious disease, providing indirect protection to individuals who are not immune. As more individuals are immune, susceptible people have less contact with infectious agents. Vaccination contributes to herd immunity by increasing the number of immune individuals, creating an immunological barrier that makes disease less likely to spread through the human population. Maintaining high levels of herd immunity through ongoing immunization programs can lead to the elimination of diseases like diphtheria and polio.

1. HERD IMMUNITY

2.  “Immunity that occurs when the vaccination of a
portion of population (herd) provides protection to
unprotected individuals”.
 Higher the number of immune individuals, the lower
a susceptible person will come in contact with an
infectious agent.
 Provides an immunological barrier to the spread of
disease in the human herd.

3.  Elements which contribute to herd immunity are:
 Occurrence of clinical and subclinical infection in the
herd
 Immunization of herd
 Herd structure (new births, deaths and population
mobility)

4. • If high level of herd immunity is maintained and
stepped up, by an on-going immunization program,
to the point where the susceptible persons are
reduced to a small proportions of the populations, it
may lead to elimination of disease.
• Achieved in such disease as diphtheria and
poliomyelitis.