Key points
- Developing a clear case definition is critical to effective investigation of an outbreak.
- The below content explains the difference between an outbreak and cluster and how to develop a case definition.
Defining an outbreak of unexplained respiratory illness
Cluster: An aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known.
Outbreak: When there are more disease cases than what is usually expected:
- For a given time (e.g., within 2 weeks)
- Within a specific location (e.g., linked by institution, affiliation, exposure, small geographic area)
- For a target population (e.g., students, long-term care residents)
Purpose of a case definition
Development of a clear case definition is critical to effective investigation of an outbreak.
Use of a common case definition allows for standardization of the cases of interest. This is applicable both within an ongoing outbreak investigation and possibly between outbreak investigations that differ over time or geographic location.
Developing a case definition
A case definition includes criteria for person, place, time, and clinical features. These should be specific to the outbreak under investigation.
Person: Describes key characteristics the patients share in common, such as:
- Age
- Sex
- Race
- Occupation
- Exclusion criteria (e.g., persons with no history of X disease)
Place: Typically describes a specific geographic location (state, county) or facility associated with the outbreak (X nursing home, Y high school).
Time: Used to delineate a period of time associated with illness onset for the cases under investigation. Limiting the time period enables exclusion of similar illnesses which are unrelated to the outbreak of interest.
Clinical features: Initially, should be simple and objective (e.g., sudden onset of fever and cough). The clinical criteria may later be characterized by the presence of specific laboratory findings.
Table: Common components and examples of an outbreak case definition
| Element* | Descriptive features | Examples |
|---|---|---|
| Person | Age group | Children under the age of 5 years |
| Sex | Males | |
| Occupation | Healthcare workers at hospital X | |
| Exclusion criteria | Persons with no previous history of chronic cough or asthma | |
| Race | ||
| Place | Geographic location | Resident of Y county or state |
| Facility | Living in X nursing home; student at A high school | |
| Time | Illness onset | Onset of illness between May 4 and August 31, 2007 |
| Clinical features | Pneumonia | Clinical or radiographically confirmed pneumonia; shortness of breath and fever |
| Laboratory criteria | Cultures; serology | Pneumococcus isolated from blood; rapid influenza test positive |
*Please note that components of an outbreak case definition vary for each outbreak.
Further characterization of a case definition
Case definitions are often further categorized by the degree of certainty regarding the diagnosis as:
- Suspected
- Probable
- Confirmed
Adjusting a case definition
As additional information about a particular outbreak becomes available, it may make sense to adjust the outbreak case definition.
Consider incorporating:
- Predominant clinical presentations (e.g., pneumonia, pharyngitis)
- Severity of illness (e.g., hospitalizations, deaths)
- Demographic profiles
- Illness onset dates of the cases (see line list template)
- A comparison to the historical or baseline rates for similar disease in the community
Examples of a case definition
Example 1
"Student attending X High School who has onset of fever and cough between January 4 and 24, 2023."
Example 2
"A resident of, or visitor to, Rapid City, South Dakota who was diagnosed by a physician, either clinically or radiographically, with community-acquired pneumonia (CAP) with symptom onset after May 1, 2022, and who had laboratory confirmation of Legionnaires' disease by culture of Legionella, by urinary antigen test for Legionella pneumophila serogroup 1 (Lp1), by a four-fold or greater rise in serum antibody titer to Lp1, or detection of specific Legionella antigen by direct fluorescent antibody staining."
Example 3
"A clinical case was defined as an acute cough illness lasting >2 weeks with onset during September 2004–February 2005 and without other apparent cause in a person living in the Amish community in Kent County. A confirmed case was defined as a clinical case of pertussis that:
All other clinical cases were considered probable."
- Gregg, M.B. Field Epidemiology. New York: Oxford University Press, 2002.