Attack rate is best described as what?

• A. The proportion of at-risk individuals who develop disease during a defined period
• B. The total number of cases in the population
• C. The rate of new cases per unit time per person
• D. The proportion of the population diseased at a point in time

Answer: (B)

Attack rate reflects the risk among those at risk during a defined time period. It is the proportion of people who develop the disease within that time window among those who could have become cases. For example, at a picnic outbreak, if 20 people are at risk and 6 become ill, the attack rate is 6 divided by 20, or 30%. This is essentially the incidence proportion for a short outbreak. It differs from the total number of cases, which is just a count, and from prevalence, which is the proportion diseased at a single point in time. It also differs from an incidence rate expressed per person-time, which incorporates time into the denominator.