Which epidemiologic measure is sometimes used during outbreaks to describe the average number of secondary cases generated by one primary case?

• A. Attack rate.
• B. Basic reproduction number R0.
• C. Mortality rate.
• D. Case fatality rate.

Answer: (B)

Basic reproduction number, R0, captures how transmissible a pathogen is by describing the average number of secondary cases generated by one infectious person in a completely susceptible population. This is exactly what the phrase “average number of secondary cases generated by one primary case” refers to, making R0 the standard measure of transmission potential. It differs from other epidemiologic measures because those focus on disease occurrence or outcomes rather than how widely a pathogen spreads. Attack rate reflects the proportion of people who become ill in a defined group over a period, not how many new infections a single case creates. Mortality rate counts deaths in a population, and case fatality rate is the proportion of diagnosed cases that die; neither describes transmission between people. In outbreaks, R0 helps gauge potential growth: if R0 is above 1, transmission can expand; below 1, it tends to wane. In practice, the effective reproduction number Rt is used as real-time conditions change, but R0 remains the foundational descriptor of inherent contagiousness.