Definition: Herd immunity refers to a reduction in the probability of infection of susceptible members of a population when a significant proportion of the individuals are immune. This reduction is observed because the chance of coming in contact with an infected individual is less. In other words, disease transmission is blocked when a critical percentage of the population has been vaccinated. When this occurs, herd immunity levels have been reached.
The concept of herd immunity describes the impact of indirect protection of susceptible members of a population from infection on the protection of the population as a whole. This is brought about by the presence of immune individuals. It is therefore the general aim of those involved in public health to establish herd immunity in the populations for which they are responsible.
Vaccinated people act as a firewall that blocks the spread of the disease, slowing or preventing the further transmission of the disease to others. Since the protection offered by vaccines is rarely 100%, the vaccine will be more effective if more people have been vaccinated. This is because the disease may be able to jump from one unvaccinated person to another person who has not been vaccinated, but is unlikely to jump from one unvaccinated person to one who has been vaccinated.
Example: In sustained transmission, a transmitting case (A) infects a susceptible person (B). Person B then turns into a transmitting case and has the opportunity to infect another susceptible person (C).
In diseases that can be passed from person to person, it is more difficult to pass a disease with the presence of immunity in the population. The more immune individuals there are in a population, the less likely it is that a susceptible person will come into contact with someone who has the disease.
For example, if Person A had smallpox and exposed Person B, who was immune because of vaccination, Person B would not get ill and could not pass on the disease to Person C upon contact with him. Therefore, even if Person C is not vaccinated, he is indirectly protected from the disease.