1 General Epidemiological Definitions
Epidemiology
The study of the distribution (who, where, when) and determinants (why, how) of health related events in human populations, and the use of this study to inform public health action.
Distribution
Disease distribution refers to the frequency and pattern of health events in a population.
Frequency
Frequency refers to the number of health events in a population (e.g., counts of the number of people living with HIV in a population at a particular time). It also refers to the number of health events relative to the size of the population (rates). This measure allows comparison of disease frequency across populations of different size.
Pattern
Patter refers to describing health events by time, place, and person. Time patterns may be yearly, monthly, weekly, daily, or hourly. They could also be seasonal, for example, vector-borne disease patterns tend to be seasonal, where risk increases during the rainy season. Place patterns can include differences between urban and rural geography, workplaces, altitude, or other geographic variations. Personal characteristics can include demographic factors (sex, age), socioeconomic status (education, income, job), personal habits or activities or environmental factors.
Determinants
Determinants are the factors that cause or contribute to a health event. Epidemiologists know that human health is influenced by a range of factors and that these factors can either increase or decrease the likelihood of experiencing a health event. These factors could be environmental (e.g., a weather event such as a tropical storm), social (e.g., socioeconomic status), behavioural (e.g., smoking or diet) as well as others.
Population
Groups of people who live in a defined place or share a common feature (e.g. age group or sex).
Health event
In this book, ‘health event’ includes anything that could be considered to affect the health of a population. In field epidemiology this mostly refers to infectious disease outbreaks, however, it can also refer to non-communicable disease, chronic disease, injury, occupational or environmental health, maternal and child health and any other health-related conditions.
Field epidemiology
The rapid, immediate investigation of urgent public health problems, such as infectious disease outbreaks. The main goal of field epidemiology is to rapidly choose and implement public health interventions to prevent bad health outcomes, including death.
Case
A person who has been diagnosed with a health condition of interest. Defining a case is very important in field epidemiology. Case definitions are always used for disease surveillance, outbreak response, and research.
Case definition
A set of standard criteria for deciding whether a person has a particular disease or health related condition and should be counted as a case.
Surveillance system case definition
Outlines the clinical signs and symptoms and laboratory test results (when available) that are used for a person to be counted as a case. Sometimes a surveillance case definition can include epidemiological information, such as contact with a confirmed case (e.g. measles). Surveillance system case definitions remain consistent over time to enable data to be compared. Typically, surveillance case definitions only change when there is a system change, such as a new laboratory test to diagnose the disease.
Example 1
Case definition for influenza-like-illness for a syndromic surveillance system may be ‘Fever ≥38o C AND cough AND onset within past 10 days’
Example 2
Case definition for COVID-19 for an indicator based surveillance system at a hospital may be ‘Detection of SARS-CoV-2 by nucleic acid amplification testing (NAAT)’
Outbreak case definition
Outbreak case definitions are specific to each outbreak; they outline the person, place, time, clinical (signs and symptoms) and laboratory (when available) factors that must be met for a person to be counted as being a case in the outbreak. Outbreak case definitions can change over time as more information is gathered throughout the outbreak investigation.
Example
Following the detection of a cluster of gastroenteritis cases from Village X on June 10, 2021, the initial case definition was: Any person living in Village X presenting with diarrhoea (e.g. three or more loose stools in 24 hours) from June 10, 2021. Following further investigation of the initial cases, they had all attended a wedding party together. As a result, the case definition was changed to reflect this new information: Any person who attended the wedding party in Village X presenting with diarrhoea (e.g. three or more loose stools in 24 hours) from June 10, 2021.
Surveillance case definitions |
Outbreak case definitions |
|
|---|---|---|
| Purpose | To count cases in the same way over long periods of time |
To count cases in an outbreak the same way |
| Information included in case definition | ||
|
Clinical signs and symptoms |
[latex]\checkmark[/latex] |
[latex]\checkmark[/latex] |
|
Laboratory result |
[latex]\checkmark[/latex] |
[latex]\checkmark[/latex] |
|
Epidemiological information |
Sometimes∗ |
Sometimes∗ |
|
Time# |
X |
[latex]\checkmark[/latex] |
|
Place |
X |
[latex]\checkmark[/latex] |
|
Person |
X |
[latex]\checkmark[/latex] |
| How often does case definition change? |
Not often (usually only when new laboratory diagnostic tests are used) |
Can change several times during the outbreak investigation as more information is gathered |
# During the investigation, only a start date is used. Once the outbreak is declared over, an end date is added to the final case definition.
∗ Epidemiological information (e.g. contact with another case) can be used sometimes both for surveillance and outbreak case definitions (e.g. measles).
