What is the formula for attack rate as described?

• A. ill / (ill + well) × 100
• B. ill / population × 100
• C. deaths / cases × 100
• D. new cases / total population × 100

Answer: (A)

Attack rate shows the proportion of people who become ill during an outbreak among those who were at risk. The proper way to express this is the number who become ill divided by the number who were at risk (ill plus well) at the start, multiplied by 100. This focuses on the group that could have fallen ill, not including those who aren’t at risk. In many outbreak settings, ill plus well equals the total population, but framing it as ill over ill plus well clarifies that we’re accounting for everyone who could potentially become ill. This is different from a case fatality rate (deaths divided by cases) or from simply ill divided by the total population, which can misrepresent risk if some individuals were not at risk initially.