How do you compute an attack rate?

• A. Attack rate = (number of new cases during the period / population at risk during the period) × 100.
• B. Attack rate = (total cases / population) × 1000.
• C. Attack rate = (deaths during period / population at risk) × 100.
• D. Attack rate = (recoveries / total cases) × 100.

Answer: (A)

An outbreak-focused metric that tells you the risk of getting ill over a defined period is the attack rate. It measures how many people who were at risk actually developed the disease during that time, expressed as a percentage. The correct approach uses the number of new cases during the period divided by the population at risk in that period, then multiplied by 100 to convert to a percent. This captures the rising cases among those susceptible, rather than counting everyone or including people who were already ill.

If you used total cases, you’d be counting people who were already sick, not the new risk during the period. Using deaths or recoveries doesn’t reflect new illness either. And while you could express the rate per 1,000 people, the fundamental idea remains the same, but the standard practice in many contexts is to use new cases over the population at risk times 100 to yield a percent. For example, 40 new cases in a population at risk of 800 yields 40/800 × 100 = 5%.