Attack rate is defined as the proportion of a defined population affected during an outbreak, with the denominator equal to which of the following?

• A. The total population at risk at the start of the outbreak
• B. The incidence proportion in the population at risk
• C. The number of cases divided by the total population
• D. Proportion of exposed individuals who become cases, measured over time

Answer: (A)

Attack rate measures the risk that members of a defined group who are susceptible at the start of an outbreak will become ill during the outbreak. Therefore the denominator must be the population at risk at the beginning—those who could potentially become cases. Excluding people who are already ill, immune, or not exposed keeps the rate true to the chance of developing disease in that at-risk group. The calculation is new cases during the outbreak divided by that at-risk population. If you used the total population without restricting to those at risk, the rate would be biased downward. Restricting to an exposed subgroup yields a different measure (an exposure-specific attack rate), not the overall attack rate.