Is the attack rate really a rate?

No — despite the name, the attack rate is a proportion (incidence proportion or risk): new cases divided by the population at risk at the start of the outbreak, usually shown as a percentage.

What is a food-specific attack rate?

The number of people who ate a specific food and became ill, divided by everyone who ate that food. Comparing the attack rate among those who ate each food with the rate among those who did not — the relative risk — points to the likely vehicle.

Why subtract the primary cases for the secondary attack rate?

The primary cases cannot become secondary cases, so CDC often calculates the denominator as the total population in the households of the primary cases minus the number of primary cases.

Free tool · CIC® exam topic

Attack rate calculator

The proportion of an exposed group that becomes ill — and the secondary attack rate among contacts of the first cases.

Attack rate

Result

Attack rate: 30.0%

Secondary attack rate

Result

Secondary attack rate: 25.0%
Calculation: 17 ÷ (86 − 18)

Attack rate formula

Attack rate = new cases during the outbreak ÷ population at risk at the start × 100

CDC also calls this the incidence proportion or risk. It is the core measure for outbreaks in a defined group.

Secondary attack rate formula

Secondary attack rate = cases among contacts of primary cases ÷ total number of contacts × 100

The number of contacts is often the total population in the primary cases’ households minus the primary cases. CDC’s example: 17 ÷ (86 − 18) = 17 ÷ 68 = 25.0%.

Using attack rates to find the source

In a foodborne outbreak, calculate a food-specific attack rate for those who ate and those who did not eat each item. The likely vehicle has a high attack rate among those who ate it, a low rate among those who did not, and accounts for most cases. The ratio of the two attack rates is the relative risk.

Test yourself: a CIC-style question

During an outbreak investigation, 40 of 200 exposed patients developed illness. What is the attack rate?

Show answer

B. 20%

Attack rate = new cases ÷ population at risk × 100 = 40 ÷ 200 × 100 = 20%. Comparing attack rates between exposure groups helps identify the source.

Questions like this come up in the Surveillance and Epidemiologic Investigation domain of the CIC® exam. The 654 questions in the app each come with a written explanation.

FAQ

Is the attack rate really a rate?: No — despite the name, the attack rate is a proportion (incidence proportion or risk): new cases divided by the population at risk at the start of the outbreak, usually shown as a percentage.
What is a food-specific attack rate?: The number of people who ate a specific food and became ill, divided by everyone who ate that food. Comparing the attack rate among those who ate each food with the rate among those who did not — the relative risk — points to the likely vehicle.
Why subtract the primary cases for the secondary attack rate?: The primary cases cannot become secondary cases, so CDC often calculates the denominator as the total population in the households of the primary cases minus the number of primary cases.

Sources

CDC. Principles of Epidemiology in Public Health Practice (SS1978), Lesson 3: Measures of Risk

Checked against these sources on October 21, 2026. For education and CIC® exam preparation only — not a substitute for your facility’s policies, NHSN’s official reports, or clinical judgment. Calculations run in your browser; nothing you enter is sent anywhere.