When enough people in a community are protected against a disease — through vaccination or prior infection — the disease struggles to spread and cannot reach those who are most vulnerable. This is called community immunity, sometimes called herd immunity. Explore it here through real-world scenarios.
Community immunity protects everyone — including people who cannot get vaccinated — by limiting how easily a disease can spread through a population.
When you get vaccinated, your immune system learns to fight the disease without you getting sick. Most people who are vaccinated become immune — the vaccine teaches the body to recognize and fight the germ.
Babies, elderly people, and those with weakened immune systems often cannot receive certain vaccines. When those around them are protected, the disease has nowhere to go and cannot reach these vulnerable individuals.
Every disease has a community immunity threshold — the minimum share of people who need to be immune before the disease stops spreading. Fall below that level and outbreaks become more likely.
R₀ (said "R-naught") is the average number of people one sick person infects in a fully susceptible population. A higher R₀ means a disease spreads more easily and requires a higher vaccination rate to achieve community immunity.
No vaccine is 100% effective for every person. Vaccine effectiveness varies by disease, age, and individual biology. This means some vaccinated people may still get sick, though usually with milder illness.
If vaccination rates fall — due to exemptions, access barriers, or vaccine hesitancy — the community immunity threshold may no longer be met. Exemptions in U.S. schools have hit record highs, putting entire communities at risk.
Choose a real-world scenario below, or adjust the sliders yourself to explore how vaccination rates and vaccine effectiveness shape the outcome of an outbreak.
Adjust the sliders below to explore any combination of parameters. Click Start Simulation to run it.
Based on your R₀ and vaccine settings, are you above or below the protection threshold?
93.3% |
Effective coverage = vaccinated% × effectiveness% = 92.2%
Counts at the start and end of the simulation
| Group | Start | End |
|---|---|---|
| Vaccinated & Immune | — | — |
| Vaccinated, Not Immune | — | — |
| Unvaccinated | — | — |
| Total Infected | — | — |
| Recovered (gained immunity) | 0 | — |
The "attack rate" is the share of each group that got sick. A high rate ratio means being unvaccinated carried much more risk.
| Measure | Value |
|---|---|
| Attack Rate — Vaccinated | — |
| Attack Rate — Unvaccinated | — |
| Rate Ratio (Unvax ÷ Vax) | — |
This chart shows how many new infections happened each simulated day. Epidemiologists call this an "epi curve" — the shape tells you a lot about how fast an outbreak moved.
The R₀ number describes how contagious a disease is in a population with no immunity. Diseases with a higher R₀ require more of the community to be vaccinated before outbreaks are prevented.
All models simplify reality. Here's what this simulator assumes — and where real life is more complex.
All vaccine effectiveness values, R₀ estimates, and epidemiological data used in this simulator are drawn from the sources below.