Chatu Yapa, Masters in Applied Epidemiology trainee, Health Protection NSW

The flu season has hit states and territories throughout Australia exceptionally hard this year, with more than 42,300 laboratory confirmed cases recorded at the end of August. This compares to approximately 14,000 lab-confirmed cases at the same time last year. 

These kinds of numbers are able to be determined at a local and state level because of the application of a standardized case definition, which allows public health professionals to classify and count cases consistently across reporting jurisdictions.

Imagine that you are a public health officer investigating an influenza outbreak in an aged care facility (ACF X) in your local health district.  The experienced nurse in charge, in anticipation of your visit, has attempted to help in the investigation by developing some case definitions and creating a line list of cases as shown in the table below. 

Case definitions

Probable

Resident of ACF X with symptoms suggestive of influenza (measured fever of greater than 38.0oC AND at least 1 of the following 2 respiratory symptoms: cough or sore throat) where symptoms started after August 25th 2014.
 

Confirmed

Resident of ACF X with symptoms suggestive of influenza (as above) where symptoms started after August 25th 2014 AND with a throat swab positive for influenza. 
 
Case ID
Date symptom onset
Cough
Measured fever  >38.0oC
Sore throat
Throat swab taken
Throat swab positive for influenza​​
1
26/8/2014
Yes
Yes
Yes
Yes
Yes
2
28/8/2014
Yes​
Yes
No
Yes
Yes
3
29/8/2014
No
Yes
No
Yes
No
4
29/8/2014
Yes
Yes
Yes
No
Waiting for result
5
1/9/2014
Yes
Yes
Yes
No
Waiting for result
6
1/9/2014
Yes
Yes
No
Yes
Yes
7
2/9/2014
No
Yes
No
No
No
8
2/9/2014
No
Yes
Yes
Yes
Waiting for result

 

  1. According to the line list of cases, how many p​robable and confirmed cases are there at this time?
  2. What are some important elements to consider when developing a case definition for an outbreak investigation?
  3. How do the concepts of ‘sensitivity’ and ‘specificity’ apply to a case definition?

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Answer

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Page Updated: Tuesday 30 September 2014
Contact page owner: Health Protection NSW