What is herd immunity?

Herd immunity is the phenomenon that once a particular proportion of a population is immune to a particular disease (usually 90%+), susceptible individuals are also better protected from the specific disease. When a high threshold percentage of a population is vaccinated, it is highly difficult for infectious diseases to spread from individual to individual.

Picture this; you have measles, and you are surrounded by people who are all vaccinated against measles. The disease cannot be passed on to anyone around you, and it will quickly disappear again as there are too few susceptible hosts for the pathogen to be transmitted to.

The lack of susceptible hosts provides protection to vulnerable individuals in a community such as newborns, individuals with compromised immune systems and the elderly.

*It is important to note, that not all vaccine-preventable diseases are not protected by herd immunity. Tetanus for example, which as you may know has its reservoir in soil, is not preventable by herd immunity. It is infectious, but not contagious.

What are the main principles of herd immunity?

1. A high proportion of the population becomes immune to the disease

2. Immunity through vaccines or catching disease naturally causes the formation of specific antibodies and memory cells

3. Leads to few susceptible people catching the disease, lowering the pathogen population

4. Infected individuals are more likely to have contact with immune individuals, reducing transmission risk and suceptible people risk

5. The higher the proportion of immune people, the greater the protection for all individuals

6. Those who cannot be vaccinated are protected (e.g. those who may go into anaphylaxis following a vaccine)

7. The proportion of immunity required to achieve herd immunity depends on the virulence of the pathogen

Image: Community Immunity image, Image by National Institute of Health (NIH), Sourced Under a Creative Commons 4.0 License from Wiki Commons