What is the basic reproduction number, R0?

• A. The average number of secondary cases produced by one infectious case in a completely susceptible population.
• B. The number of new cases per day.
• C. The number of individuals immune in a population.
• D. The threshold for achieving herd immunity.

Answer: (A)

R0 is the average number of secondary infections produced by one infectious person in a completely susceptible population. It reflects how easily a pathogen spreads when no one has immunity and no control measures are in place. If R0 is above one, the outbreak can grow because each case causes more than one new case on average; if it’s below one, transmission tends to die out.

This isn’t the daily number of new cases—that describes incidence. It isn’t the number of people who are already immune—that describes immune proportion. And while herd immunity threshold is related to R0, it’s a derived concept (the portion of the population that must be immune to stop transmission), not the reproduction number itself.