How Herd Immunity Works: Why Measles Needs ~95%

What this infographic shows

A disease spreads only while each case infects more than one other person. If enough people are immune, the virus keeps hitting dead ends and the outbreak fades. That tipping point is the herd immunity threshold.

Each dot is one person in a group of 100:

• Teal: immune because they were vaccinated and the vaccine worked.
• Light coral: can still catch the disease.
• Amber ring: the people who need to be immune for herd immunity.

When every ringed dot is teal, the outbreak shrinks.

How to use it

1. Set how contagious the disease is. The slider starts at 15, in the middle of the range for measles.
2. Set the vaccine’s effectiveness.
3. Move share vaccinated until “each case infects” drops below one person.
4. Try a much less contagious disease, such as an R0 of 2. Far fewer people need to be immune.

Why the number is so high for measles

Measles spreads through the air and can linger in a room after an infected person has left. Because one case can infect 12 to 18 people, even a small drop in vaccination can let outbreaks start again.

This is a simplified model. It assumes people mix evenly; real communities have clusters where vaccination is lower, and outbreaks start there.