1. Epidemic curves Madrid, 3 February 2015
Marie Roseline Darnycka Bélizaire
Spanish National Institute of Health Carlos III (ISCIII)

2. Learning Objectives
Discuss the importance of epidemic curves as a method to graphically depict outbreaks
Define and review:
Outbreak patterns of spread
Outbreak magnitude
Outbreak outliers
Period of exposure
Incubation period
Describe methods to create epidemic curves
This presentation will discuss the importance epidemic curves as a method to graphically depict outbreaks. It will also review concepts such as outbreak patterns of spread, magnitude of the outbreak, outbreak time trend, outbreak outliers, period of exposure and incubation period. Finally, methods to make epidemic curves will be described.

3. Introduction
A histogram is a graphical version of a table that shows what proportion of cases fall into each category
An epidemic curve (epi curve) is a type of a histogram with the number of infected cases (y axis) by a selected time unit (x axis)
Epidemic curves are used to describe the course of an epidemic over time. The shape of the curve can give clues to the transmission route and source of the epidemic.
Week

4. Epidemic curve message
An epidemic curve reports on the time course of an epidemic in a defined population
Epidemic curves:
the type of epidemic and mode of transmission
(source, patterns of spread, magnitude)
the difference between maximum and minimum incubation period
the probable time and duration of exposure, time trend
outliers might provide important clues

5. Histogram for epidemic curve
Frequency distribution of quantitative variable (y)
Area of each column represents number of cases
No spaces between columns
Equal class intervals Y X
Time unit of x-axis based on:
Incubation period of disease
Length of time over which cases are distributed

6. Frequency distribution of the incubation period
infection
median
cases
time
minimum
maximum
incubation period

7. Cases of salmonellosis (n=65) by date and time of onset of illness
Cases of salmonellosis (n=65) by date and time of onset of illness. Hospital A, Dublin, August 1996
IP=8-48h 16

8. Outbreak Pattern of Spread
The overall shape of the epi curve can reveal the type of outbreak
Point source
Common source
Continuous
Intermittent
Propagated or progressive
The overall shape of the curve can reveal the type of outbreak (common source, point source or propagated).

9. Types of epidemic curves
Point source
Common continuous source
cases
cases
days
days
Common intermittent source
Propagated source
cases
cases
days
weeks

10. Outbreak Pattern of Spread-Point Source
Typically shows a sharp upward slope and a gradual downward slope
Is a common source outbreak in which the period of exposure is brief and all cases occur within one incubation period
An epi curve with a sharp upward slope and a gradual downward slope typically describes a point source outbreak. A point source outbreak is a common source outbreak in which the exposure period is relatively brief, and all cases occur within one incubation period. The next graph is an example of a point source epidemic curve.

11. Point source number of cases 20 15 10 5 time
salmonella
time
width ~ (max.-min.) incubation period

12. Outbreak Pattern of Spread-Common Source
People are exposed continuously or intermittently to a harmful source
Period of exposure may be brief or long
Intermittent exposure often results in an epi curve with irregular peaks that reflect the timing and the extent of exposure
A common source outbreak is one in which people are exposed continuously or intermittently to a common harmful source. The period of exposure may be brief or long. An intermittent exposure in a common source outbreak often results in an epi curve with irregular peaks that reflect the timing and extent of the exposure (2). The following figure represents an example of a common source outbreak with intermittent exposure.

13. Example of an Epi Curve for a Common Source Outbreak with Intermittent Exposure
This graph shows an example of an epi curve for a common source outbreak with an intermittent exposure. The irregular peaks represent the timing and extent of the exposure.

14. Outbreak Pattern of Spread-Common Source
Continuous exposure will often cause cases to rise gradually (and possibly to plateau, rather than to peak)
A continuous exposure will often cause cases to rise gradually (and possibly to plateau, rather than peak) (2). The following figure is an example of a continuous exposure.

15. Continuous common source
Mean IP
Gastroenteritis with contaminated tap water
width ~ (max.-min.) incubation period + duration of exposure

16. Outbreak Pattern of Spread-Propagated
Is spread from person to person
Can last longer than common source outbreaks
May have multiple waves
The classic epi curve for a propagated outbreak has progressively taller peaks, an incubation period apart
A propagated outbreak is one that is spread from person to person, as seen in the next figure. Because of this, propagated epidemics can last longer than common source epidemics, and may lead to multiple waves of infection if secondary and tertiary cases occur. The classic propagated epi curve has a series of progressively taller peaks, each an incubation period apart, but in reality the epi curve may look somewhat different (2).

17. Outbreak Pattern of Spread-Propagated
In a propagated epidemic due to person-person transmission you may see different waves of the epidemic as different groups of people are exposed. A curve like the one below might be seen as an infection moves through classes in a school or from village to village. The time interval between the peaks corresponds to the average serial interval. The serial interval is the time between the same stage of disease (e.g. fever onset or rash) in successive cases in a chain of transmission.

18. Outbreak Pattern of Spread-Propagated

19. Person-to-person transmission5 10
time
number of cases
Generation
period
Hepatitis A, measles;
the basic reproductive number R= pXcXd probability of transmission per contact, contacts per unit time, duration of infectiousness
Imunity influeces probability of transmission per contact!

20. Ebola transmission (Dec 2014)

21. Ebola transmission (Dec 2014)

22. Outbreak Magnitude Can provide a sense of the magnitude of an outbreak
Additional information can be obtained by stratifying the epi curve
Separating the sample into several subsamples according to specific criteria, e.g. age and gender
An epidemic curve can provide a sense of the magnitude of the outbreak as well. For example, there were 73 cases reported in the point source outbreak shown in a prior slide (slide # 11), this is a fairly large outbreak for certain diseases in a small geographical area! Additional information about the magnitude of the outbreak within subpopulations can be obtained by stratifying (separate sample into several subsamples) the epi curve by characteristics such as gender, age, clinical symptoms or geographic location.

23. Outbreak Outliers
Outliers are cases at the very beginning and end that may not appear to be related
First check to make certain they are not due to a coding or data entry error
Cases at the very beginning or end that may not appear to be related to the outbreak are referred to as “outliers.” The first thing that should be done when considering outliers is to make sure they are not mistakes due to miscoding or data entry error.

24. Example of an Epi Curve for a Point Source Outbreak
This graph is an example of an epi curve for a point source outbreak. A point source outbreak is a type of common source outbreak in which all of the cases are exposed within one incubation period. Note how the graph shows a steep upslope and a comparatively gradual downslope. in a point source epidemic everyone is exposed at about the same time. The epidemic curve therefore depends on the incubation period.

25. Point source infection

26. Outbreak Outliers If they are not an error, they may represent
Baseline level of illness
Outbreak source
A case exposed earlier than the others
An unrelated case
A case exposed later than the others
A case with a long incubation period
Assuming they are not errors, important information can be deducted from outliers. For example, an early case may not be part of the outbreak; it may just represent the baseline level of illness. However, it may also represent the source of the outbreak, such as an infected food handler, or a case that was exposed earlier than the others. Also, a late case may not be part of the outbreak. Alternatively, a late case may represent an individual who had a long incubation period, who was exposed later than the other cases or who was a secondary case (2).

27. Period of Exposure/Incubation Period for the Outbreak
If the timing of the exposure is known, epi curves can be used to estimate the incubation period of the disease
The time between the exposure and the peak of the epi curve represents the median incubation period
If the timing of the presumed exposure is known, epi curves can be used to estimate the incubation period of the disease, and this may facilitate the identification of the causative agent. This is because the period between the known or hypothesized exposure time and the peak of the epi curve represents the hypothesized median incubation period (3).

28. Period of Exposure/Incubation Period for the Outbreak
In common source outbreaks with known incubation periods, epi curves can help determine the average period of exposure
Find the average incubation period for the organism and count backwards from the peak case on the epi curve
In common source outbreaks involving diseases with known incubation periods, epi curves can help determine the probable period of exposure (5). This can be done by looking up the average incubation period for the organism and counting back from the peak (median) case the amount of time of the average incubation period.

29. Period of Exposure/Incubation Period for the Outbreak
This can also be done to find the minimum incubation period and count backwards from the earliest case on the epi curve
Average and minimum incubation periods should be close and should represent the probable period of exposure
Likewise, to estimate the minimum incubation period count back from the earliest case on the epi curve.

30. Determining exposure period1 3 5 7 9 15 19 21 23 25 27 29
time
median time of the illness onset 2
50% %
probable time of
exposure
median incubation period 5
*Incubation period is known or at least suspected !!! (clinically or by microorganism isolation)
Median incubation period is time from the onset of the first to the last case in point source outbreaks.

31. Determining exposure period2 4 6 8 10 12 14 16
minimum incubation period
for the first case
Time
probable period
of exposure
maximum incubation period
for the last case
N of cases

32. Hepatitis A by date of onset Ogemaw county, Michigan, April - May 1968
Number of cases
50 days 15
one case
30 days 10 5
15 days 2 8 14 20 26 2 8 14 20 26 1 7
Exposure
Days
*Incubation period of Hepatitis A days

33. How do I Make an Epi Curve?
Plot the number of cases of disease reported during an outbreak on the y-axis
Plot the time or date of illness onset on the x-axis
Begin with a unit approximately one quarter the length of the incubation period
The structure of an epi curve is straight forward. Simply plot the number of cases of disease reported during an outbreak on the y-axis (the vertical line) and the time/date of illness onset on the x-axis (the horizontal line).

34. How do I Make an Epi Curve?
Usually the day of illness onset is the best unit for the x-axis
If the incubation period is very short, hour of onset may be more appropriate
If the incubation period is very long, week or month may be more appropriate
For most diseases, date of onset is appropriate for the x-axis, but for illnesses with very short incubation periods (for example, Staphylococcus aureus food poisoning) hours of onset may be preferable. Likewise, for diseases with long incubation periods, such as tuberculosis, the best time interval may be days, weeks, or months (6).

35. How do I Make an Epi Curve?
Epi curves are histograms
There should not be any space between the x-axis categories
Label each axis
Provide a descriptive title
Include the pre-epidemic period to show the baseline number of cases
Epi curves are a type of histogram, so there should not be any space between the x-axis categories.
A simple but important point to remember is to label the axes correctly and to include a descriptive title with each epi curve. The epi curve, with its title and axes should provide enough information to be completely self explanatory.The pre-epidemic period should always be included on the graph to illustrate the baseline number of cases.

36. Epidemic curves in Epi Info V3
Command Graph  EpiGraph
Histogram, bar, line, area, xy, point, spline, …
« Square method » showing single cases
Possibility of modifying graph after it has been created
Export of graphs
as Metafile, BMP, JPG and data
to files, clipboard

37. Conclusion Epi curves are useful in outbreaks to
Identify the pattern of spread
Assess the magnitude
Evaluate time trends
Examine Outliers
Estimate the exposure period
Epi curves are useful for helping identify the pattern of spread, magnitude, time trends, and exposure period for an outbreak. Because they are so useful, be sure to make an epi curve (or more than one) to help put pieces of the puzzle together during your next outbreak investigation or during routine review of surveillance data.

38. Thank you