What is the denominator in the attack rate definition?

• A. Number initially exposed
• B. Number at risk at end
• C. Total population
• D. Number of cases

Answer: (A)

Attack rate measures the risk of illness among those exposed during an outbreak. The denominator should be the number at risk at the start of the outbreak—the people who were exposed to the source. The numerator is the number who actually became ill. For example, if 60 people were exposed and 15 develop illness, the attack rate is 15/60 = 0.25 (25%).

Using the number at risk at the end would misrepresent risk because people who are no longer at risk (already ill or removed) would distort the denominator. Using the total population includes individuals who were not exposed, diluting the risk calculated for this outbreak. Using the number of cases as the denominator would cancel the numerator and yield a meaningless value for risk.