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Epidemiology: Steps in Investigation of an Epidemic

Definition of an ‘Epidemic’
The occurrence in a community, of cases of a particular disease in numbers which are clearly ‘in excess’ of that ‘expected’ for that population and for that time of the year

What is the expected number of cases?
• The definition differs for different diseases but mostly an arbitrary limit of mean±2SE of the cases reported in last 3 years.
• In some cases it may be desirable to take the mean± 2SE of the cases reported in last 5 or 7 years.
• For some diseases like poliomyelitis, even a single case is considered to be an ‘epidemic’

Types of epidemics:

Mostly of two types:

1. Common – source epidemic
Common source – single exposure (point source epidemic)
Common source – multiple exposure (or continuous exposure)

2. Propagated epidemic

Epidemic curve:
It is a graphical representation of occurrence of cases of a disease with passage of time during an epidemic.
Epidemic curve provides information about the following:

  • The behaviour of the epidemic, where and how it started
  • The speed with which it spread; when did it reach its peak?
  • When did it start declining or is it still increasing
  • It can tell about the type of the epidemic; i.e. common – source or propagated

Steps in the investigation of an epidemic:
1. Confirmation of the diagnosis
2. Confirmation of the existence of an epidemic; counting the number of cases, reported and searched and then comparison with previous years’ records
3. Defining the population which is exposed; using census data, voters list etc.
4. Rapid search for cases
5. Analysis of initial data: Description of the cases in terms of time, place and person
6. Study of ecological factors, i.e. the environment
7. Formation of a few tentative hypotheses; as to why did the epidemic occur?
8. Further and more detailed investigation of the disease and the associated risk factors in the given population
9. Testing of different hypotheses and agreeing on one of them
10. Writing the report:
• Findings and interpretation
• Control measures undertaken
• Lessons learnt and
• Recommendation for preventing recurrence of the epidemic

Mnemonic:Diagnose the Epidemic in the Population by Rapid Analysis of Environment and HIT it with the Report
(HIT= Hypothesis formation +Investigation in detail + Testing the hypothesis)

Tiwari P. Epidemiology Made Easy. New Delhi: Jaypee Brothers; 2003
Gordis, L. (2014). Epidemiology (Fifth edition.). Philadelphia, PA: Elsevier Saunders.
Bonita, R., Beaglehole, R., Kjellström, T., & World Health Organization. (2006). Basic epidemiology. Geneva: World Health Organization.
Schneider, Dona, Lilienfeld, David E (Eds.), 4th ed. Lilienfeld’s Foundations of Epidemiology. New York: Oxford University Press; 2015
K. Park. Principles of Epidemiology and Epidemiologic Methods. In Park’s Textbook of Preventive and Social Medicine. 24th Ed. Jabalpur: Banarasidas Bhanot, 2017: pg 58 – 145

Steps in Investigation of an Epidemic:
Tests of Significance:
Monitoring and Evaluation:
Advantages and Disadvantages of Case Control Studies:
Advantages and disadvantages of cohort study:
Basic Concepts in Epidemiology:
Types of Epidemiological Studies:
Differences between Case – control and cohort study:
Uses of epidemiology:
Blinding in Experimental Studies:
Evaluation of a Screening Test: