epicurve-150417183744-conversion-gate02.ppt

Epidemic Curves: Definition, Types, Interpretation, and Applications
1. Introduction

Epidemic curves are among the most fundamental tools in the practice of epidemiology and outbreak investigation. They provide a visual representation of the temporal pattern of disease occurrence among a population. By plotting the number of cases of a disease against the time of onset, epidemiologists can quickly grasp essential characteristics of an outbreak: its magnitude, pattern, duration, and potential source.

The use of epidemic curves dates back to the foundational years of modern epidemiology, when pioneers like John Snow and William Farr used graphical tools to illustrate patterns of cholera outbreaks in 19th-century London. Today, epidemic curves (commonly referred to as epi curves) remain vital components of descriptive epidemiology. They are used in field investigations, research, and public-health response frameworks by the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and other global health agencies.

This paper explores the principles, construction, and interpretation of epidemic curves, discusses their different types, and examines their applications, advantages, and limitations.

2. Definition and Purpose

An epidemic curve (epi curve) is a histogram that displays the distribution of the number of new cases of a disease according to the time of onset. The x-axis represents the time (date or period of illness onset), and the y-axis represents the number of cases.

The fundamental purposes of an epidemic curve are:

To describe the time course of an outbreak.

To estimate the magnitude and duration of the outbreak.

To identify the pattern of spread — whether it is common source, propagated, or mixed.

To help estimate the incubation period and possible period of exposure.

To assess the effectiveness of control measures and predict the outbreak’s course.

3. Construction of an Epidemic Curve
3.1 Data Requirements

To construct an epidemic curve, the epidemiologist needs:

The date (or time) of onset of illness for each case.

The number of cases corresponding to each date or time interval.

Sometimes, the classification of cases (e.g., confirmed, probable, suspected).

3.2 Steps in Construction

Collect Data: Compile case data from line lists or surveillance records.

Choose Time Interval: The interval should be about one-fourth to one-third of the average incubation period of the disease. For instance, for cholera (incubation period 1–5 days), a 1-day interval is appropriate.

Plot the Data: On the x-axis, plot the time of onset; on the y-axis, plot the number of new cases.

Label the Graph Clearly: Include a descriptive title, axes labels, and units of time.

Differentiate Case Types (optional): Use different colors or patterns for confirmed vs. probable cases.

The resulting histogram provides a visual summary of how the outbreak evolved over time.

4. Types of Epidemic Curves

Epidemic curves vary a

1. Epidemic CurvesEpidemic Curves
Jen-Hsiang Chuang, M.D., Ph.D.Jen-Hsiang Chuang, M.D., Ph.D.
Taiwan Centers for Disease ControlTaiwan Centers for Disease Control
[email protected]@cdc.gov.tw

2. 7/18
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通報病例： 31
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3. What is an Epidemic Curve What is an Epidemic Curve
(Epi Curve) ?(Epi Curve) ?

An epidemic curveAn epidemic curve（（a histograma histogram）） is a is a
graphical depiction of the number of cases of graphical depiction of the number of cases of
illness by the date of illness onsetillness by the date of illness onset

There should not be any space between the x-There should not be any space between the x-
axis categoriesaxis categories

4. How Can it Help in an Outbreak?How Can it Help in an Outbreak?

An epi curve can provide information on the An epi curve can provide information on the
following characteristics of an outbreak:following characteristics of an outbreak:

Pattern of spreadPattern of spread

MagnitudeMagnitude

OutliersOutliers

Time trendTime trend

Exposure and/or disease incubation periodExposure and/or disease incubation period

5. Outbreak Pattern of SpreadOutbreak Pattern of Spread

The overall shape of the epi curve can reveal the The overall shape of the epi curve can reveal the
type of outbreaktype of outbreak

Common sourceCommon source

Point sourcePoint source

PropagatedPropagated

6. Common Source OutbreakCommon Source Outbreak

An outbreak due to transmission from a single An outbreak due to transmission from a single
environmental or human sourceenvironmental or human source

Period of exposure may be brief or longPeriod of exposure may be brief or long

Intermittent exposure often results in an epi Intermittent exposure often results in an epi
curve with irregular peaks that reflect the timing curve with irregular peaks that reflect the timing
and the extent of exposure and the extent of exposure

Continuous exposure will often cause cases to Continuous exposure will often cause cases to
rise gradually (and possibly to plateau, rather rise gradually (and possibly to plateau, rather
than to peak)than to peak)

7. An Epi Curve for a Common Source An Epi Curve for a Common Source
Outbreak with Intermittent ExposureOutbreak with Intermittent Exposure

8. An Epi Curve for a Common Source An Epi Curve for a Common Source
Outbreak with Continuous ExposureOutbreak with Continuous Exposure

9. Point Source Outbreak Point Source Outbreak

Typically shows a sharp upward slope and a Typically shows a sharp upward slope and a
gradual downward slopegradual downward slope

Is a common source outbreak in which the Is a common source outbreak in which the
period of exposure is brief, and all cases occur period of exposure is brief, and all cases occur
within one incubation periodwithin one incubation period

10. An Epi Curve for a Point Source An Epi Curve for a Point Source
OutbreakOutbreak

11. Propagated OutbreakPropagated Outbreak

Is spread from person to personIs spread from person to person

Can last longer than common source outbreaksCan last longer than common source outbreaks

May have multiple wavesMay have multiple waves

The classic epi curve for a propagated outbreak has The classic epi curve for a propagated outbreak has
progressively taller peaks, an incubation period apartprogressively taller peaks, an incubation period apart

Outbreaks may begin as a common-source one Outbreaks may begin as a common-source one
followed by person-to-person spread (e.g., viral followed by person-to-person spread (e.g., viral
gastroenteritis caused by ingestion of contaminated gastroenteritis caused by ingestion of contaminated
shrimp)shrimp)

12. An Epi Curve for a Propagated An Epi Curve for a Propagated
OutbreakOutbreak

13. Outbreak MagnitudeOutbreak Magnitude

Can provide a sense of the magnitude of an Can provide a sense of the magnitude of an
outbreakoutbreak

Additional information can be obtained by Additional information can be obtained by
stratifying the epi curvestratifying the epi curve

Separating the sample into several subsamples Separating the sample into several subsamples
according to specific criteria, e.g. age, gender, & according to specific criteria, e.g. age, gender, &
geographic informationgeographic information

15. Outbreak Time TrendOutbreak Time Trend

Allow information about the time trend of the Allow information about the time trend of the
outbreak to be gleanedoutbreak to be gleaned

Consider:Consider:

Date of illness onset for the first caseDate of illness onset for the first case

Date when the outbreak peaked Date when the outbreak peaked

Date of illness onset for the last caseDate of illness onset for the last case

16. Outbreak OutliersOutbreak Outliers

Outliers are cases at the very beginning and end Outliers are cases at the very beginning and end
that may not appear to be relatedthat may not appear to be related

First check to make certain they are not due to a First check to make certain they are not due to a
coding or data entry errorcoding or data entry error

17. Outbreak OutliersOutbreak Outliers

If they are not an error, they may representIf they are not an error, they may represent

Baseline level of illnessBaseline level of illness

Outbreak sourceOutbreak source

A case exposed earlier than the othersA case exposed earlier than the others

An unrelated caseAn unrelated case

A case exposed later than the othersA case exposed later than the others

A case with a long incubation periodA case with a long incubation period

18. 0
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19. How do I Make an Epi Curve?How do I Make an Epi Curve?

Plot the number of cases of disease reported Plot the number of cases of disease reported
during an outbreak on the y-axisduring an outbreak on the y-axis

Plot the time or date of illness onset on the x-Plot the time or date of illness onset on the x-
axisaxis

20. How do I Make an Epi Curve?How do I Make an Epi Curve?

Technical tipsTechnical tips

Choice of time unit for x-axis depends upon the Choice of time unit for x-axis depends upon the
incubation periodincubation period

Begin with a unit approximately one third or less of Begin with a unit approximately one third or less of
the incubation periodthe incubation period

If the incubation period is not known, graph several If the incubation period is not known, graph several
epi curves with different time units epi curves with different time units

21. How do I Make an Epi Curve?How do I Make an Epi Curve?

Usually the day of illness onset is the best unit Usually the day of illness onset is the best unit
for the x-axis for the x-axis

If the incubation period is very short, hour of onset If the incubation period is very short, hour of onset
may be more appropriate (e.g., may be more appropriate (e.g., Staphylococcus aureusStaphylococcus aureus))

If the incubation period is very long, week or month If the incubation period is very long, week or month
may be more appropriate (e.g., hepatitis B or TB)may be more appropriate (e.g., hepatitis B or TB)

Effects for changing time intervalsEffects for changing time intervals
http://www.cdc.gov/cogh/descd/modules/MiniModules/E
pidemic_Curve/page05.htm

22. How do I Make an Epi Curve?How do I Make an Epi Curve?

Epi curves are histogramsEpi curves are histograms

Label each axis Label each axis

Provide a descriptive titleProvide a descriptive title

Include the pre-epidemic period to show the Include the pre-epidemic period to show the
baseline number of casesbaseline number of cases

23. ConclusionConclusion

Epi curves are useful in outbreaks toEpi curves are useful in outbreaks to

Identify the pattern of spreadIdentify the pattern of spread

Assess the magnitude Assess the magnitude

Evaluate time trendsEvaluate time trends

Examine OutliersExamine Outliers

Estimate the exposure periodEstimate the exposure period

24. ReferencesReferences
1.1.CDC. Epidemic CurveCDC. Epidemic Curve
http://www.cdc.gov/cogh/descd/modules/MiniModules/
Epidemic_Curve/page01.htm
2.2.North Carolina Center for Public Health North Carolina Center for Public Health
Preparedness. Preparedness. Epidemic Curves AheadEpidemic Curves Ahead
http://www.sph.unc.edu/nccphp/focus/vol1/issue5/index
.htm
3.3.CDC. Principles of Epidemiology, 2nd Edition. CDC. Principles of Epidemiology, 2nd Edition.
Atlanta, GA: Public Health Practice Program Office, Atlanta, GA: Public Health Practice Program Office,
1992.1992.
http://www.phppo.cdc.gov/PHTN/catalog/pdf-file/Epi_
Course.pdf