Which statement correctly differentiates Rt from R0?

• A. Rt accounts for immunity and interventions
• B. R0 is the time-varying reproduction number
• C. Rt is the basic reproduction number in a fully susceptible population
• D. R0 accounts for current immunity

Answer: (A)

The key idea is how the reproduction number changes with time and with population conditions. The statement that Rt accounts for immunity and interventions is the best because Rt is the number of secondary cases generated at time t under the current reality: how many people are still susceptible, who is immune, and what control measures or behavior are in place. It reflects what’s happening now, so Rt can rise or fall as immunity builds (for example through recovery or vaccination) or as interventions like social distancing or mask use are implemented or relaxed.

R0, by contrast, is the baseline number of secondary cases in a completely susceptible population with no interventions. It does not change with time in response to immunity or behavior, and it does not reflect current conditions. So the other statements mix up these definitions: R0 is not the time-varying number, and Rt is not the basic reproduction number in a fully susceptible population, while R0 does not account for current immunity. When immunity increases or interventions are strong, Rt tends to drop, potentially below 1, signaling a shrinking outbreak.