Which measure describes how many people at risk develop disease within a short period during an outbreak?

• A. Incidence rate
• B. Prevalence
• C. Attack rate
• D. Mortality rate

Answer: (C)

In an outbreak, you want a simple measure of risk over a defined, short time window for those who are at risk. The attack rate does just that: it is the proportion of people who develop the disease among those who were at risk during a specific, limited period. It’s essentially cumulative incidence calculated for the outbreak window, with the at-risk population as the denominator and the new cases during the window as the numerator.

This makes it especially useful for outbreak investigations where you’re tracking new illness over a short timeframe and a defined group (for example, people who ate a particular food).

Prevalence looks at existing cases at a point or period and doesn’t reflect new disease arising in a short outbreak. Mortality rate counts deaths and not illness incidence. Incidence rate is a true rate per person-time and requires tracking time-at-risk continuously; it’s more complex and less directly interpreted as the short-term risk within a fixed group during an outbreak.