Vaccination & herd immunity
About this app
A population grid with a vaccination-rate slider: an outbreak spreads, the vaccinated block the chains. Above a threshold vaccination rate the epidemic dies out (herd immunity).
How herd immunity works
Vaccination makes a person immune: they do not get infected and do not pass the pathogen on. Once a large enough part of the population is vaccinated, infected people rarely meet susceptible contacts and chains of infection break. That also protects people who cannot be vaccinated themselves. The vaccination level needed depends on how contagious the pathogen is. In simple models the threshold is 1 − 1/R₀, where R₀ is the number of people one infected person infects in a fully susceptible population.
What the simulation shows
The app shows the population as a grid in which each person can only infect their neighbours. Colours mark healthy, vaccinated, infected and recovered people. A chart tracks the share of infected people over time, next to the total number of cases and the outcome: epidemic or outbreak fizzled out.
Things to try
Start an outbreak at a low vaccination rate and watch it spread across the whole grid. Raise the vaccination rate step by step and look for the threshold above which the outbreak almost always dies out early. Then change the infection rate: with a more contagious pathogen the required vaccination rate moves up.
Frequently asked questions
- What does herd immunity mean?
- Herd immunity means that so many people are immune that a pathogen can no longer spread persistently in the population. Chains of infection break because infected people have few susceptible contacts left. People who are not vaccinated benefit as well.
- How is the herd immunity threshold calculated?
- In simple models the threshold is 1 − 1/R₀. With R₀ = 4, about 75 % would need to be immune; with R₀ = 10, about 90 %. Real values also depend on contact patterns, vaccine effectiveness and waning immunity.
- Why does high vaccination coverage protect unvaccinated people?
- A pathogen needs a chain of susceptible people to spread. If most contacts are vaccinated, the chain ends before it reaches the unvaccinated. This protects, for example, infants or people who cannot be vaccinated for medical reasons.
- What can I set in the Vaccination & Herd Immunity app?
- You choose the vaccination rate and infection rate and start an outbreak on the population grid. You see live who is infected, recovered or protected, and a chart shows the share of infected people. At the end the app reports whether an epidemic occurred and how many people fell ill in total.
Subject: Biology (Gymnasium)
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