Which statement best describes the effective reproduction number?

• A. It is always equal to R0
• B. It has no relation to susceptibility
• C. It is the reproduction number when the population is partially susceptible
• D. It is the number of secondary infections in a fully susceptible population

Answer: (C)

The effective reproduction number reflects how many people, on average, an infectious person will infect under current conditions, including how many people are susceptible, what control measures are in place, and how people behave. Because not everyone is susceptible at any given time, Re changes as immunity builds or interventions change.

This best description recognizes that the reproduction number is evaluated under partial susceptibility in the population. When some people are immune or protected, transmission is limited compared with a fully susceptible population, so Re is typically lower than R0. It isn’t always equal to R0, since immunity and interventions reduce transmission, and it is indeed related to susceptibility. The number of secondary infections in a fully susceptible population corresponds to R0, not Re.