What would you like to understand?

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.

$R_0$ should be used early in an outbreak when the whole population is susceptible and when a growth rate and serial interval are available. Once some immunity builds up in the population, one should switch to using the effective reproduction number instead.

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.

$R_0$ should be used early in an outbreak when the whole population is susceptible and when a growth rate and serial interval are available. Once some immunity builds up in the population, one should switch to using the effective reproduction number instead.

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.