What defines the secondary attack rate?

• A. The attack rate in susceptible people who have been exposed to a primary case.
• B. The attack rate in the entire population after exposure to a primary case.
• C. The proportion of all exposed people who become ill regardless of exposure status.
• D. The case fatality rate among exposed individuals.

Answer: (A)

Secondary attack rate measures how contagious a disease is within a close-contact group after exposure to a primary case. It focuses on those who are susceptible and have been exposed, and asks what proportion of them develop illness within one incubation period. The calculation uses the number of new cases among susceptible contacts as the numerator and the number of susceptible contacts exposed as the denominator, typically expressed as a percentage. This emphasis on susceptible contacts who have been exposed explains why this definition is the best fit. It’s distinct from the overall attack rate in the whole population or from fatality rates, and it doesn’t include non-susceptible individuals in the denominator.