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$R_0$ 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.