Immunity Background

In this section, you'll learn how individuals and populations can gain immunity, which reduces the spread of an infectious disease.

Immunity Can Slow the Spread of a Pathogen

As a pathogen spreads throughout a population, individuals who become infected and recover typically develop immunity against the pathogen.

The body’s ability to effectively recognize and destroy certain pathogens. When an individual has immunity against a pathogen (also called being immune), they cannot be infected by that pathogen.

Another way individuals can develop immunity against the pathogen is by receiving a vaccine. Vaccinated individuals go directly from the susceptible group to the removed group, skipping the infectious group.

A substance, typically injected into the body, that helps an individual develop immunity against a pathogen. Vaccines work by triggering the body’s response against the pathogen without causing disease. The body is then better prepared to recognize and destroy the same type of pathogen in the future.

In the “Epidemic Simulator,” vaccination can be modeled by increasing the “Initial Removed Individuals” setting. This action can represent the movement of susceptible individuals directly to the removed group before the start of the simulation.

An illustration showing the two paths which result in individual immunity (removed).
Individuals can gain immunity through two different pathways: recovery from an infection or vaccination.

Herd Immunity Protects the Population

When enough individuals in a population have immunity, the pathogen can’t spread easily. This protects still-susceptible individuals from being exposed to and infected by the pathogen. This effect is called herd immunity.

The herd immunity threshold (HIT) is the proportion of immune individuals needed in a population for that population to have herd immunity. HIT depends on the basic reproduction number (R0) as follows:

HIT = 1 - ( 1 R 0 )

The higher the R0 of a given pathogen, the more quickly it can spread, which means a higher HIT may be required to prevent an epidemic.

A graph with the x-axis measuring basic reproduction number and the y-axis measuring herd immunity threshold percent.
The relationship between R0 and HIT plotted on a graph. As R0 increases, HIT (the percentage of the population that must be immune in order to achieve herd immunity) also increases.
An illustration showing how susceptible and close contact individuals can be protected from an infectious individual when surrounded by removed (immune) individuals. Icons for Susceptible and Close Contact, Infectious, and Removed or Immune individuals.
This population has achieved herd immunity. Although several susceptible individuals remain in the population, herd immunity protects them from becoming infected.
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