Why is R-naught equal to one the dividing line for an outbreak?

Conditions

• The video states the growth side only until immunity builds up.
• The example value shown is $R_0=2.5$.

Reasoning, step by step

1. Compare the value of $R_0$ to 1.
2. If $R_0 < 1$, conclude that transmission chains break and the outbreak dies out.
3. If $R_0 > 1$, conclude that the epidemic grows until immunity accumulates.
4. Identify 1 as the critical dividing line between these two behaviors.

Example

The video displays a number line with "< 1 -> outbreak dies out" and "> 1 -> epidemic grows", placing the example value 2.5 on the growth side.

Common misconceptions

• Believing that $R_0 < 1$ means the outbreak stops immediately rather than fading out.
• Assuming that $R_0 > 1$ guarantees infinite growth without considering the buildup of immunity.

Watch the explanation

Explore next

• How do you compute the herd immunity threshold from R-naught?
• When should you use R-naught instead of the effective reproduction number?