Community immunity, aka herd immunity, occurs when enough people are immune to a certain infectious disease that even those who aren’t immune are protected because the virus can’t spread through the population. The easier a virus spreads from person to person, the more people need to be vaccinated to reach herd immunity. People can become immune in two ways: through getting the infection (and developing immunity naturally) or through getting a vaccine (and developing immunity without getting infected). Achieving herd immunity through vaccination with a safe and effective vaccine is the safest way to protect people from the disease.

You can use this tool to simulate the spread of an imaginary disease in a community. Test out the animation below to watch a disease spread through a population and see how vaccination can protect the entire community.

In this simulation, people can be:

  • Healthy (not immune)
  • Sick (infected)
  • Recovered (immune after being sick)
  • Dead
  • Vaccinated (healthy and immune)

In this simplified scenario, we assume that if a person who is not immune comes into contact with someone who is sick, they will get sick 100% of the time. Here, if a person gets sick, they will either recover and become immune, or die. With this imaginary disease, 15 out of 100 people who get sick will died from the disease (15% case fatality).

To see the impact of vaccination, choose the percentage of the population to vaccinate and watch what happens in the community.

  • How does the shape of the red curve (people who are sick) change as the percent vaccinated increases?
  • What happens to the number of total deaths as the percent vaccinated increases?

You will be able to track the number of people in this simulated community who are healthy (yellow), sick (red), recovered (blue), and vaccinated (green) in real-time.

Interpreting the Plots

These plots summarize what happens in our simulated community when 0%, 30%, 50% 70%, 90%, and 100% of the population is vaccinated. Since the movement is random in our community, the resulting curves will look different each time you run the simulation. Here we captured the typical results under these scenarios.

The higher the percentage of people vaccinated, the flatter the red curve (number of sick people).

What does this mean? As the percentage of people vaccinated increases, individuals will get sick at a slower rate (rather than all at the same time), and there will be fewer people overall who fall sick or die from the disease.

Why is this important? If hospitals are less likely to be flooded all at once, sick people will receive better care and hospitals will not run out of supplies to control the outbreak.

Even better, when a high percentage of people are vaccinated, they will indirectly protect those who are unable to get vaccinated (e.g. elderly, babies, cancer or transplant patients, pregnant women, etc.).

It will be more difficult for the virus to spread through the population when most people in the population are vaccinated and have immunity.