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The herd immunity threshold (HIT) is computed using the formula $\mathrm{HIT} = 1 - \frac{1}{R_0}$. This represents the immune fraction of the population needed so that each case infects fewer than one other on average, stopping sustained spread.

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.

The herd immunity threshold (HIT) is computed using the formula $\mathrm{HIT} = 1 - \frac{1}{R_0}$. This represents the immune fraction of the population needed so that each case infects fewer than one other on average, stopping sustained spread.

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.

$R_0=1$ is the threshold separating outbreak decline from growth. If $R_0 < 1$, each case infects fewer than one other person on average, causing transmission chains to break and the outbreak to fade.

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

$R_0=1$ is the threshold separating outbreak decline from growth. If $R_0 < 1$, each case infects fewer than one other person on average, causing transmission chains to break and the outbreak to fade.

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

When $R_0 > 1$, each generation of infections is larger than the previous one because every case produces more than one new infection on average. This repeated multiplication by a factor greater than one results in exponential growth of the outbreak.

Conditions: Assumes the same average transmission per case across generations.; The worked example uses $R_0=2.5$.

When $R_0 > 1$, each generation of infections is larger than the previous one because every case produces more than one new infection on average. This repeated multiplication by a factor greater than one results in exponential growth of the outbreak.

Conditions: Assumes the same average transmission per case across generations.; The worked example uses $R_0=2.5$.