Which statement best describes the attack rate in epidemiology?

• A. The proportion of new cases among the population at risk during a specified period.
• B. An incidence rate calculated for a specific disease for a specific period of time and expressed as a percentage.
• C. The incidence rate for the disease over a lifetime.
• D. The case-fatality rate for the disease.

Answer: (B)

Attack rate is the probability that a person who is at risk will develop the disease during a defined, typically short, outbreak period. It’s calculated as the number of new cases during the period divided by the population at risk at the start, then expressed as a percentage. This makes it a measure of incidence proportion (a cumulative risk) for that specific time frame, which is why it’s described as an incidence figure expressed as a percentage. It’s not a lifetime measure, not the case-fatality rate, and it isn’t a true incidence rate that uses person-time.