Herd Immunity

Herd immunity was recognized as a naturally occurring phenomenon in the 1930s. It was observed that the number of new infections temporarily decreased after a significant number of children had become immune to measles. Mass vaccination to induce herd immunity has become common and has been successful in preventing the spread of many infectious diseases.

Herd immunity occurs when a significant percentage of a population becomes immune to a disease, either through vaccination or previous infections. The spread of disease from person to person becomes unlikely when herd immunity is achieved. This way the whole community is protected including those who lack immunity to that disease.

 

A certain percentage of the population must be capable of getting a disease in order for it to spread. This is called a threshold proportion.

Herd immunity threshold is achieved when the proportion of the population that is immune to the disease is greater than this threshold. The spread of disease declines when herd immunity threshold is achieved.

What percentage of a community needs to be immune in order to achieve herd immunity?

It varies from disease to disease and depends on how contagious the disease is. A more contagious disease requires a greater proportion of the population to be immune in order to stop the disease from spreading.

Herd immunity is achieved in 2 ways

1. Natural infections

2. Vaccinations

1. Natural infections

Herd immunity can be achieved through natural infections. This occurs when sufficient percentage of people in the population suffered from a disease and  recovered. Thus they have now developed protective antibodies which will protect them from future infection.

2. Vaccinations

Herd immunity can also be achieved when sufficient percentage of people have been vaccinated against a disease and have developed protective antibodies against future infection. 

Unlike the natural infection method, vaccines create immunity without causing illness or resulting in complications.

The immune system does not distinguish between natural infections and vaccines, so immunity induced via vaccination is similar to immunity induced through natural infection.

How does herd immunity prevent disease spread?

Individuals who are immune to a disease act as a barrier in the spread of disease thus slowing or preventing the transmission of disease to others.

Role of Herd Immunity

Herd immunity protects individuals in the community who lack immunity, like those who  cannot be vaccinated for medical reasons or those who cannot develop immunity after vaccination. Herd immunity also protects individuals who are immunodeficient like those who are receiving organ transplants or chemotherapy.

Role of herd immunity in eradication of diseases

If herd immunity has been established and maintained in a population for a sufficient time, endemic transmissions of the disease stop occurring. That means the disease is eliminated from that population. If the elimination is achieved worldwide and the number of cases is permanently reduced to zero, then a disease can be declared eradicated.