What is an attack rate?

• A. Incidence rate for a defined population during a single outbreak, expressed as a percentage
• B. Overall incidence rate for a year
• C. Prevalence rate in the population
• D. Mortality rate during an outbreak

Answer: (A)

Attack rate is the risk of becoming ill during a defined outbreak in a population at risk. It’s the proportion of people who develop the illness over the outbreak period, usually expressed as a percentage. Because it looks at who among a defined at‑risk group becomes a case during a finite time, it’s essentially an incidence proportion for that outbreak rather than a long-term rate.

You calculate it as: attack rate = (number of new cases during the outbreak) / (number of people at risk in the population) × 100.

For example, if 12 students become ill during a two‑week outbreak in a school of 200 at‑risk students, the attack rate is 12/200 × 100 = 6%. This measure helps compare risk between exposed and unexposed groups and identify which subgroups are more affected.

It’s not the overall incidence rate for a year, not the prevalence (how many people have the disease at a point in time), and not the mortality rate (deaths during the outbreak).