Herd Immunity Threshold Explorer
Drop an infection into a grid of people and watch it spread neighbour to neighbour. Vaccinate a slider-controlled fraction first and the outbreak shrinks; cross the herd-immunity threshold (1 − 1/R₀) and it fizzles out even though plenty of unvaccinated people remain — they are protected because the chains of transmission break. Adjust how contagious the disease is and the vaccine coverage to find the tipping point yourself. Runs entirely in your browser.
Runs 100% in your browser — simulations are computed locally on your device.
Read the full guide to this tool
Notes
- The herd-immunity threshold is 1 − 1/R₀: a disease with R₀ = 4 needs about 75% immune to stop spreading. More contagious diseases (higher R₀) demand higher coverage.
- Above the threshold the unvaccinated are still protected because most of their contacts are immune, so outbreaks cannot chain — that indirect protection is what "herd immunity" means.
- Runs 100% in your browser — simulations are computed locally on your device.
- This is an educational estimate, not medical advice; consult a professional for decisions about your health.
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