General Epidemiological Definitions
Technical Manual / 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 application of this study to inform public health action.
Disease distribution refers to the frequency and pattern of health events in a population.
Frequency
Frequency refers to the number of health events (counts) in a population (e.g., the number of people living with HIV in a population at a particular time).
It also refers to the occurrence of a health event relative to the size of the population (rates). This measure allows comparison of disease frequency across different populations.
Pattern
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 diseases patterns tend to be both seasonal and hourly, where risk is impacted by the season as well as the vector biting times (e.g., dusk). 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, behavioural factors or other environmental factors.
Refers to the causes or factors that bring about a change in 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.
Health event
In this booklet, ‘health event’ includes anything that could be considered to affect the health of a population. Often in field epidemiology this refers to communicable 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 communicable disease outbreaks. A primary goal of field epidemiology is to rapidly guide the selection and implementation of public health interventions to prevent negative health impacts, including death.
Case
A countable instance of the health condition under investigation (e.g., one person being diagnosed with a certain condition is a ‘case’). These counts can be used for broade public health surveillance or during an outbreak response.
In the context of public health surveillance, a case is a single event meeting the identified case definition for a surveillance system. In a surveillance system, one person may contract a disease more than once and be counted as a case each time they contract that disease.
In an outbreak, a case is a single person meeting the outbreak case definition.
Case Definition
A set of standard criteria for deciding whether a person has a particular disease or health- related condition (to be classified as a case), by specifying clinical criteria and limitations on time, place, and person.
Surveillance system case definition
Include person, place, and time criteria, as well as clinical, laboratory (when available), and epidemiologic factors. Surveillance system case definitions remain consistent over time to enable quality comparison of data over time.
Case definitions can change when there is a system change, such as the introduction of a new laboratory diagnostic test.
Example
Case definition for COVID-19 for an indicator- based surveillance system at a hospital may be ‘Laboratory confirmation of SARS-CoV-2’
Example
Case definition for influenza-like-illness for a syndromic surveillance system may be ‘Fever ≥38oC AND cough AND onset within past 10 days’
Outbreak case definition
Specific to each outbreak; they must specify person, place, time, and clinical criteria (signs and symptoms; laboratory diagnosis if available). Outbreak case definitions can change over time as more information is gathered throughout the outbreak investigation.
Example
Following 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 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 intelligence: Any person who attended the wedding party in Village X presenting with diarrhoea from June 10, 2021.
