Tipping Points

R = 1 and herd immunity

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• $R > 1$: growth. $R < 1$: decline.
• $R_{\text{eff}} = R_0 \times (\text{susceptible fraction})$.
• Herd immunity threshold: $1 - 1/R_0$.

Steps

1. Above or below 1
2. Herd immunity
3. Measles
4. Imperfect vaccines
5. Protecting the vulnerable
6. Not enough