How do you compute the herd immunity threshold from R-naught?

Conditions

• The narration adds assumptions of random mixing and a fully effective vaccine.
• The goal stated is that each case infects fewer than one other on average.

Reasoning, step by step

1. Determine the value of $R_0$.
2. Calculate the reciprocal of $R_0$ ($\frac{1}{R_0}$).
3. Subtract this reciprocal from 1 to find the HIT.
4. Express the result as a proportion or percentage of the population.

Example

For $R_0=2.5$, the video computes $\mathrm{HIT} = 1 - \frac{1}{2.5} = 60\%$, visualized as 60 green squares out of 100.

Common misconceptions

• Reading $\mathrm{HIT}=1-\frac{1}{R_0}$ as universally sufficient without considering the assumptions of random mixing and vaccine efficacy.
• Confusing the herd immunity threshold with the basic reproduction number itself.

Watch the explanation

Explore next

• When should you use R-naught instead of the effective reproduction number?
• What metric should be used if some immunity is already present?