When should you use R-naught instead of the effective reproduction number?

Conditions

• Use $R_0$ early in an outbreak with a fully susceptible population.
• Use it when a growth rate and serial interval are available.
• Switch to effective $R$ once immunity builds.

Reasoning, step by step

1. Check if the outbreak is in its early stages.
2. Verify if the population is fully susceptible.
3. Ensure that a growth rate and serial interval are available for calculation.
4. If these conditions are met, use $R_0$.
5. If some immunity already exists, use the effective reproduction number.

Example

The closing slide lists "Early outbreak, fully susceptible population" and "You have a growth rate and serial interval" as conditions for using $R_0$, with a gray arrow noting "Already some immunity? Use the effective R".

Common misconceptions

• Treating $R_0$ as the right measure throughout the whole outbreak, even after some people are immune.
• Assuming that $R_0$ can be used without knowing the growth rate and serial interval.

Watch the explanation

Explore next

• What metric should be used if some immunity is already present?
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