What is the attack rate?

• A. Attack rate = number of new cases in a defined group during an outbreak divided by the population at risk in that group.
• B. Attack rate equals total cases in a city over a year.
• C. Attack rate measures the number of deaths.
• D. Attack rate is the same as prevalence.

Answer: (A)

Attack rate is a measure of risk during an outbreak, expressing the proportion of people in a defined group who develop the disease over the outbreak period. It is calculated by dividing the number of new cases by the number of people at risk in that group, often expressed as a percentage. The denominator should include only those who could become cases (not already immune or ill), which makes this a form of cumulative incidence for that specific population and time frame. This is different from simply counting total cases in a city over a year, which doesn’t focus on a defined at‑risk population during a defined outbreak, and it differs from death counts or from prevalence, which counts existing cases at a point in time. For example, if 20 people become ill in a village of 1,000 during the outbreak, the attack rate is 20/1,000 = 0.02 or 2%.