CPH Focus: Evidence-Based Approaches to Public Health: Epidemiology – Measures of Disease Frequency: Attack Rates and Case Fatality Rates
In this tutorial, we’ll explore two more important epidemiological measures: attack rates and case fatality rates. These metrics are particularly useful in outbreak investigations and for understanding the severity of diseases. They are also essential for the Certified in Public Health (CPH) exam.
By the end of this tutorial, you’ll understand what attack rates and case fatality rates are, how they are calculated, and some potential use cases. As usual, we’ll provide practice questions with answers to solidify your understanding.
Table of Contents:
- Introduction to Attack Rates and Case Fatality Rates
- Understanding Attack Rates
- Primary Attack Rate
- Secondary Attack Rate
- Understanding Case Fatality Rates
- Differences Between Attack Rates and Case Fatality Rates
- The Importance of Attack Rates and Case Fatality Rates in Public Health
- Practice Questions
- Conclusion
1. Introduction to Attack Rates and Case Fatality Rates
In epidemiology, attack rates and case fatality rates (CFR) are vital for assessing the impact of disease outbreaks and determining the severity of specific diseases:
- Attack Rate refers to the proportion of people who become ill after exposure to a disease during an outbreak.
- Case Fatality Rate (CFR) indicates the percentage of individuals diagnosed with a disease who die from it.
Both rates are widely used in public health, particularly during the investigation and management of outbreaks or epidemics, with the latter often being a key determinant of how the public reacts to a disease outbreak.
2. Understanding Attack Rates
An attack rate is a measure of how many people in a population develop a disease over a short period, often during an outbreak. It reflects the proportion of people exposed to an infectious agent who become ill.
Formula for Attack Rate:
[math] \text{Attack Rate} = \frac{\text{Number of New Cases}}{\text{Number of People at Risk}} \times 100 [/math]
The attack rate is expressed as a percentage, allowing public health officials to quickly gauge the spread of an illness within a community or population.
2.1 Primary Attack Rate
The primary attack rate measures the proportion of people who become ill after the first exposure to a pathogen.
Formula for Primary Attack Rate:
[math] \text{Primary Attack Rate} = \frac{\text{Number of Primary Cases}}{\text{Total Population at Risk}} \times 100 [/math]
Example:
If 50 people out of 200 exposed to a contaminated food source develop food poisoning, the primary attack rate would be:
[math] \text{Primary Attack Rate} = \frac{50}{200} \times 100 = 25\% [/math]
2.2 Secondary Attack Rate
The secondary attack rate measures the spread of disease from the initial group of infected individuals (the primary cases) to others who come into contact with them, such as family members or close contacts.
Formula for Secondary Attack Rate:
[math] \text{Secondary Attack Rate} = \frac{\text{Number of Secondary Cases}}{\text{Number of Susceptible Contacts}} \times 100 [/math]
Example:
If 10 out of 30 family members of the primary cases develop the same illness, the secondary attack rate would be:
[math] \text{Secondary Attack Rate} = \frac{10}{30} \times 100 = 33.33\% [/math]
3. Understanding Case Fatality Rates
The case fatality rate (CFR) is a measure of the severity of a disease. It represents the proportion of individuals diagnosed with a disease who die from it within a specified time period.
Formula for Case Fatality Rate:
[math] \text{Case Fatality Rate} = \frac{\text{Number of Deaths from Disease}}{\text{Number of Diagnosed Cases}} \times 100 [/math]
A higher CFR indicates a more lethal disease, while a lower CFR suggests a higher survival rate among those diagnosed.
Example:
If 100 people are diagnosed with disease X, and 10 of them die, the CFR would be:
[math] \text{Case Fatality Rate} = \frac{10}{100} \times 100 = 10\% [/math]
This means that 10% of the people diagnosed with disease X died from it.
4. Differences Between Attack Rates and Case Fatality Rates
While both attack rates and case fatality rates are useful in public health, they measure different aspects of disease dynamics:
- Attack Rate focuses on the risk of becoming ill after exposure to a disease during an outbreak.
- Case Fatality Rate focuses on the risk of death once someone has been diagnosed with a disease.
| Measure | Focus | Calculation Basis |
|---|---|---|
| Attack Rate | Risk of becoming ill after exposure | Number of people exposed and those who became ill |
| Case Fatality Rate | Risk of death after diagnosis | Number of diagnosed cases and those who died |
5. The Importance of Attack Rates and Case Fatality Rates in Public Health
Both attack rates and case fatality rates provide essential information during disease outbreaks and epidemics:
- Attack Rates help determine the spread of a disease and identify high-risk populations.
- Case Fatality Rates provide insight into the severity of a disease and its impact on those affected.
Public health workers use these measures to guide responses to outbreaks, allocate resources, and develop intervention strategies. In some cases, particularly with highly infectious or deadly diseases, both rates may be used to communicate the seriousness of a public health threat to the public and policymakers.
6. Practice Questions
Let’s reinforce your understanding with some practice questions. Try answering these before checking the solutions.
Question 1:
In an outbreak of foodborne illness, 120 people ate contaminated food, and 30 of them became ill. What is the attack rate?
Answer 1:
Answer, click to reveal
[math] \text{Attack Rate} = \frac{30}{120} \times 100 = 25\% [/math]
Question 2:
Out of 50 people diagnosed with a new virus, 5 died. What is the case fatality rate?
Answer 2:
Answer, click to reveal
[math] \text{Case Fatality Rate} = \frac{5}{50} \times 100 = 10\% [/math]
Question 3:
In a household of 20 people, 2 family members become infected after one person brings home a highly contagious disease. What is the secondary attack rate?
Answer 3:
Answer, click to reveal
[math] \text{Secondary Attack Rate} = \frac{2}{19} \times 100 = 10.53\% [/math]
7. Conclusion
In public health, understanding attack rates and case fatality rates is crucial for evaluating disease outbreaks, developing interventions, and communicating the risks to the public. By mastering these measures, you will be better equipped to address public health emergencies and to succeed on the Certified in Public Health (CPH) exam.
Remember:
- Attack Rates measure the risk of becoming ill after exposure to a disease.
- Case Fatality Rates measure the risk of death once diagnosed with a disease.
Final Tip for the CPH Exam:
Practice calculating attack rates and case fatality rates using real-world scenarios, or making your own questions using both high and low CFR scenarios and varied attack rates. Understand the differences between primary and secondary attack rates and how CFR is used to assess disease severity. With regular practice, you’ll be well-prepared for questions on these topics in the exam.
Humanities Moment
The featured image for this article is Attack in Pleistocene England by Benjamin Waterhouse Hawkins (1807-1894). Waterhouse was an English sculptor and natural history artist known for creating the first life-sized models of dinosaurs, notably the Crystal Palace Dinosaurs. His work blended science and art, including designing the world’s first mounted dinosaur skeleton. He faced a major setback when his models for New York’s Central Park were destroyed in a wider political scandal of corrupt patronage and politics in New York.