1. Outbreak Investigation
Tran Minh Nhu Nguyen
Technical Adviser
WHO Vietnam

2. What is an outbreak ? Occurrence of more cases of disease
than expected
in a given area
over a particular period of time
among a specific group of people

3. “Endemic” vs “Epidemic”
Number of cases
“Endemic”
“Epidemic”
Time

4. Objectives for this session
Highlight a systematic approach to outbreak investigation
Define steps of an outbreak investigation
Provide an overview of the course content

5. Scenario
It’s a late Friday afternoon at the local health department. You were about to leave when the phone rings.
On the line, a panicked voice explains that there have been 2 patients from a local garment factory admitted to the hospital with severe acute diarrhea, and you need to do something about it.
You hang up the phone. “Right, right,” you say to yourself, “I need to do something about this problem! PROBLEM?”
But what should you do?

6. Outbreak with delayed detection and response
DAY
Delayed
response
CASES
Late
detection
First case
Potentially
preventable cases
Outbreak with delayed detection and response
DAY
CASES
Early
detection
Potentially
preventable cases
First case
Prompt
response
Outbreak with timely detection and response

7. Objetives of your investigation
Investigation must meet these OBJETIVES:
To implement these INTERVENTIONS:
Determine the etiologic agent involved
Determine the potential source
Determine the mode of transmission
Optimize the case management
Interrupt the chain of transmission
Prevent future outbreaks

8. Clinicians Epidemiology Co-ordination Food safety Laboratory
Dead
Sick
Investigation
Surveillance
Prediction
Exposed
Clinicians
Epidemiology
Co-ordination
Vector
Reservoir
Food safety
Laboratory
Clinical
Supply channels
Specimen
transfer
Trace
back
Investigation
Diagnostic
Media
Authorities
Decisions
Infrastructure
Regulations
Vaccinations etc

9. Specific demands when investigating outbreaks
Unexpected event
Act quickly
Rapid control
Interdisciplinary coordination
Work carried out in the field
Systematic approach

10. Steps of an outbreak investigation
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Implement control measures
Communicate results

11. Confirm outbreak Is this an outbreak? More cases than expected?
Surveillance data
Surveys: hospitals, labs, physicians
Caution!
Seasonal variations
Notification artefacts
Diagnostic bias (new technique)
Diagnostic errors (pseudo-outbreaks)

12. Confirm diagnosis Review the clinical findings Examine some cases
Meet attending physicians
Review the lab findings
Contact the laboratories
Obtain additional lab specimens
Not always necessary to confirm all the cases
but confirm a proportion
throughout the outbreak

13. Immediate control measures? Further investigation?
Outbreak confirmed 
Immediate control
measures?
Further
investigation?
- prophylaxis
- exclusion / isolation
- public warning
- hygienic measures
- others
- causative agent
- mode of transmission
- vehicle of transmission
- source of contamination
- population at risk
- exposure causing illness

14. Outbreak confirmed, further investigations warranted
Form Outbreak Control Team
Epidemiologist
Microbiologist
Clinician
Environmentalist
Engineers
Veterinarians
Others
Team coordinates field investigation
Prepare for field work:
Gather material and equipment
Arrange personal and administrative issues
Determine your role in the investigation
Determine local contacts

15. Descriptive epidemiology
Who are the cases? (person)
- Where do they live? (place)
- When did they become ill? (time)

16. Epidemiologic triangle
Case definition
Standard set of criteria for
deciding if a person should be classified as suffering from the disease under investigation
Epidemiologic triangle
Person
Place
Time 16

17. Case definition Simple, practical, objective Sensitive? Specific?
Multiple case definitions
confirmed
probable
possible

18. Identify & count cases notifications hospitals, GPs laboratories
schools
workplace, etc
Identify & count cases

19. Identify & count cases Obtain information
Identifying information Demographic information
Clinical details
Exposures and known risk factors

20. Organize information: Line list
Names
Date of birth
Address
Onset of symptoms
Treating physician
Hospital stay
Laboratory results

21. Line List Case No. Name Date of birth Address Date of onset Lab
results 1 2 3 4 5 6 XY AB CD …

22. Analysis of descriptive data
Identify & count cases
Obtain information
Describe in
- time
- place
- person
Analysis of descriptive data

23. Time: Epi curve Describe Helps to develop hypotheses atypical cases
start, end, duration
peak
importance
atypical cases
Helps to develop hypotheses
incubation period
etiological agent
type of source
type of transmission
time of exposure
Days
Histogram
Distribution of cases by time of onset of symptoms, diagnosis or identification
Time interval depends on incubation period

24. Common intermittent source
Examples of Epicurves
Common persistent source
Common point source
cases
cases
hours
days
Propagated source
cases
Common intermittent source
cases
weeks
days

25. Outbreak of typhoid fever, Dong Thap 2002

26. Place Place of residence Place of possible exposure work meals
travel routes,
day-care
leisure activities
Maps
identify an area at risk

27. Using maps

28. Person Distribution of cases age sex occupation, etc
Distribution of these variables in population
Attack rates

29. Develop hypotheses - What is the disease?
- Who is at risk of becoming ill?
- What is the disease?
- What is the source and the vehicle?
- What is the mode of transmission?

30. Test specific hypotheses
Compare hypotheses with facts
Test specific hypotheses
Analytical studies
- cohort studies
- case-control studies

31. Testing hypothesis Cohort attack rate exposed group
attack rate unexposed group
Case control
proportion of cases exposed
proportion of controls exposed

32. Additional investigations
Verify hypothesis
Additional investigations
Microbiological investigation of food samples
Environmental investigation
Veterinarian investigation
Molecular Typing
Trace back investigations (origin of foods)
Meteorological data
Entomological investigations

33. Implement control measures
1) Control the source of pathogen
2) Interrupt transmission
3) Modify host response
At first, general measures
According to findings, more specific measures
May (must) occur at any time during the outbreak!!
Boiling water, closure of establishment, recall contaminated product
2) Isolation, quarantine, PPE
3) Chemoprophylaxis, immunization

34. Outbreak report Regular updates during the investigation
Detailed report at the end
communicate public health messages
influence public health policy
evaluate performance
training tool
legal proceedings
Outbreak investigation is not finished before submission of a report
Oral briefing for local health authorities
Written report for epidemiologic network
Scientific publication in public health journals
Deal carefully with the press

35. Steps of an outbreak investigation
Confirm outbreak and diagnosis
Form Outbreak Control Team
Define a case
Identify cases and obtain information
Describe data by time, place, person
Develop hypothesis
Test hypothesis: analytical studies
Additional studies
Implement control measures
Communicate results

36. The reality…. time Site visit Case definition Recommendations Report
Publication
Confirmation
Organize Data
Descripitve
Epidemiology
Confirm Diagnosis
Info:
Outbreak
suspected
Line list
Analytic
Epidemiology
Form Outbreak Control Team
Control measures