Making an epidemic curve (epicurve)

Preben Aavitsland(with a lot of help from Katarina Alpers, RKI)

The epidemic curve

• Time components of an outbreak• Shows

– Start – End – Duration – Peak– Outliers

• Helps to form hypotheses on– Route of transmission– Probable exposure period– Incubation time

The epidemic curve

Figure 1. Reported cases of campylobacteriosis (n=45) in Svolvær, Norway, by

date of onset J anuary and February 1997.

10 = 1 primary case

= 1 secondary household case

5

22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10

J anuary February

The epidemic curve

• Histogram– Area proportional to number

– No space between columns

– One population

• X-axis = time– Start before epidemic, continue after

– Interval < 1/4 of incubation period

• Y-axis = number of cases– Usually one square = one case

• Easy to make in Excel

Point source

0

5

10

15

20

No of cases

Time

Duration ~ Incubation time

Continous source

Mean IP

0

5

10

15

20

1 4 7 10 13 16 19 22 25 28 31 34 37

No of cases

Weeks

Intermittent sourceNo of cases

Time (days)

0

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Human to human transmission

0

5

10

Zeit

No of cases

Generation

time

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Probable exposure time

0

5

10

15

Time

1 Median onset time

23

50% 50%Probable exposure time

Median incubation time (Intervall between first and last disease onset)

No of cases

1 2 3 4 5 6 7 8 9 1011 12 1314 1516 171819 20 21 22

Probable exposure period

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 TimeProbable exposure period

minimum incubation time

maximum incubation time

No of cases

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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)