Curves in epidemiology: An overview

Life is full of curves and thus the epidemiology. However, some curves are important as Epidemic Curves and Survival Curves. This presentation is an attempt to know about epidemic curves.

1. Curves in epidemiology
Life path is full of Curves thus the Epidemiology too.
Dr. Bhoj R Singh
Act. Head of Division of Epidemiology
Indian Veterinary Research Institute, Izatnagar-243122, India
Email: [email protected]

2. Curves
•Depiction of frequency distribution in
graphical format is a curve.
Characteristics of frequency distribution:
•Central value (most common: the arithmetic
mean, the median, and the mode.
Rare: the midrange and the geometric mean)
•Variance (the range, variance, and the
standard deviation)
•Shape (Symmetric, Skewed)

6. B=Symmetrical, normal; A= Positively skewed, skewed to right; C= Skewed to left,
negatively skewed

7. Types of Curves
•Epidemic curves
•Survival curves
•Cumulative frequency (incidence curves)
•Distribution curves

8. Epidemic curve
•An epidemic curve isn’t a curve at all, but a
histogram that shows cases of disease during
a disease outbreak or epidemic by their date
of onset.
•the number of cases on the vertical axis and
time on the horizontal axis (The duration of
the epidemic is shown along the x-axis in
equal time periods).

9. Epidemic (outbreak) and Hypodemic
(sudden banishment of a disease from a
population)

13. The X axis
•Time line
•Intervals are decided on the basis of incubation period
and length of illness.
•Hours for an outbreak of C. perfringens gastroenteritis,
or 3-5 days for an outbreak of hepatitis A. As a general
rule, we make the intervals less than one-fourth of the
incubation period of the disease shown. The x axis
begins before the first case of the outbreak, and show
any cases of the same disease which occurred during
the pre-epidemic period. These cases may represent
background or unrelated cases.

15. Cumulative frequency/ survival curves

16. Interpreting the Epidemic curve
•An epidemic curve which has a steep upslope and a more gradual
down slope (a log-normal curve) indicates a point source epidemic
in which susceptible are exposed to the same source over a relative
brief period. In fact, any sudden rise in the number of cases
suggests sudden exposure to a common source. In a point source
epidemic, all the cases occur within one incubation period.
•If the duration of exposure is prolonged, the epidemic is called a
continuous common source epidemic, and the epidemic curve will
have a plateau instead of a peak.
•Intermittent common source epidemics produce irregularly jagged
epidemic curves which reflect the intermittency and duration of
exposure, and the number of individuals exposed.
•Animal to animal spread – a propagated epidemic – should have a
series of progressively taller peaks one incubation period apart, but
in reality few produce this classic pattern.

17. Point source
0
5
10
15
20
No of cases
Time
Duration ~ Incubation time

18. Continous source
Mean IP
0
5
10
15
20
14710131619222528313437
No of cases
Weeks

19. Is it Continuous source Outbreak?

20. Intermittent source
No of cases
Time (days)
0
1
2
3
4
5
6
7
8
9
10
12345678910111213141516171819202122

21. Propagated epidemic
Animal-to-Animal transmission
0
5
10
Zeit
No of cases
Generation
time
12345678910111213141516171819202122

22. Is it propagated outbreak?

23. Middle East respiratory syndrome corona virus

24. Probable exposure time
0
5
10
15
Time
1
Median onset time
2
3
50% 50%Probable exposure time
Median incubation time: Intervall between first and last disease onset
No of cases
12345678910111213141516171819202122

25. Probable exposure period
1 3 5 7 9 1113151719212325272931
Time
Probable
exposure
period
minimum incubation time
Maximum incubation time
No of cases
12345678910111213141516171819202122

26. Two ways of making epicurves in Excel
•The ”square method”
–Turn a work sheet in to squares
–Fill in each patient as a square
•The ”chart method”
–Make a table of onset times
–Use the chart wizard
–Make a histogram (by removing intervals between
bars in a bar chart)

27. Applications of epidemic curves
•Time components of an outbreak
•Shows
–Start
–End
–Duration
–Peak
–Outliers
•Help to frame hypothèses on
–Route of transmission
–Probable exposure period
–Iincubation time
•Refining the estimate of the point in time of occurrence of a focal point
source
•Identifying multiple possible sources of exposure
•Distinguishing primary and secondary cases
•Unmasking outbreak source through epidemic curve segmentation