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Insert name of presentation on Master Slide
Epidemiology Toolkit for
Outbreak Investigation
Meirion Evans
Communicable Disease Surveillance Centre
What is an outbreak?
Occurrence of more cases of disease
than expected
• Over a particular period of time
• In a given area
• Among a specific group of people
(incidents, clusters)
Key questions
What is going on? ASSESS
Who is affected? DESCRIBE
What is the cause? ANALYSE
What should be done? ACT
1. Confirm existence of an outbreak
2. Corroborate diagnosis
3. Define and identify cases
4. Collect and collate data
5. Characterise cases (person - place - time)
6. Develop hypotheses
7. Test hypotheses
8. Verify biological coherence
9. Communicate results and write report
10. Implement control measures
ASSESS
ANAL
YSE
ACT
10 steps in outbreak investigationDESCRIBE
Confirm existence of an outbreak
Corroborate diagnosis
3. Define and identify cases
4. Collect and collate data
5. Characterise cases (person - place - time)
6. Develop hypotheses
7. Test hypotheses
8. Verify biological coherence
Communicate results and write report
Implement control measures
ANAL
YSE
10 steps in outbreak investigationDESCRIBE
Descriptive epidemiology
Analytical epidemiology
Descriptive epidemiology
Define & identify cases
Develop hypotheses
Characterise cases
Collect & collate data
To refine the case definition To develop a demographic profile To identify people at risk To develop hypotheses about
• Potential sources of exposure
• Potential routes of transmission
Descriptive epidemiology OBJECTIVES
Case definition
Set of criteria… • for deciding if a person should be classified as having the disease• for the purposes of that stage of the investigation
Clinical and/or laboratory criteria Time Place Person
• Tiered definitions: confirmed, probable, possible
Case definitionoutbreak of salmonellosis in Swansea, 2011
Confirmed case
diarrhoea • (> 2 liquid stools per day)
and/or fever > 38°C • (for at least one day)
and
an isolate of S. Typhimurium
in a resident of Swansea after May 2011
Probable case
diarrhoea • (> 2 liquid stools per day)
and contact (same household) with a
confirmed case in a resident of Swansea after May 2011
Case definitionsensitivity vs. specificity
Possible Probable Confirmed
Low Specificity
High Specificity
High Sensitivity
Low Sensitivity
Identify & count cases
Collect data
demographics
clinical details (outcome)
risk factors (exposure)
reports from stafflaboratory dataoccupational healthhospitals recordsGPs, etc
Collect data
Detailed interviews• symptoms and date of onset• case characteristics • all relevant exposures in relevant period
Visit (examine) some cases
Speak with clinicians
Obtain lab confirmation
Example line list
Case
No. Name
Date
of birthDate of onset
Date of report
Lab
results
1
2
3
4
5
6
XY
AB
CD
…
…
…
Line listing - principles
Constitutes a unique MASTER LIST• avoids confusion with multiple versions• suitable for sharing
Contains unique identifier for each record Ensures confidentiality Contains essential information on each case
• time, place, person, clinical, lab, etc. Can be updated as the investigation develops Prepares data for simple descriptive analysis
Characterise cases
Who are the cases? Where do they live, work, etc.? When did they become ill?
Classify cases by:• Person
• Place
• Time
Person
Place
Time
Characterise cases
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 100
200400600800
10001200
0-4 '5-14 '15-44 '45-64 '64+
Age Group
Develop hypotheses
Pathogen? Source? Transmission?
PersonWHO is getting the disease?
Sex and age group Ethnicity Pre-existing conditions Medication Invasive procedures Surgical treatment
PlaceWHERE is the disease occurring?
In the community• Place of residence• Place of work
In hospital• Floor• Ward or unit• Operating theatre• Outpatient departments
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
Figure. Cases of gastroenteritis (n=45) in Hospital X, Wales by date of onset, January and February
2012
1 patient case
1 staff case
TimeWHEN does the disease occur?
To describe the outbreak
• Start date, end date, duration
• Peak, shape, magnitude
• Outliers and atypical cases
To develop hypotheses
• Incubation period
• Aetiological agent
• Type of source and transmission
• Time of exposure
Time - use of the epidemic curve
What is the disease? Who is at risk of becoming ill? What is the source and the vehicle? What is the mode of transmission?
Develop hypotheses
To test hypotheses• Is there an association between exposure
and disease?• How strong is the association?• What proportion of cases are explained by
the exposure?• Is there an increased risk of disease with
increased exposure (dose-response)?
Analytical epidemiology OBJECTIVES
Test hypotheses
Analytical studies
• Cohort study
• Case-control study
These must test specific hypotheses Compare the predictions of your hypotheses
with further investigations
Testing hypothesis - comparing groups
Cohort study
- attack rate exposed group
- attack rate unexposed group
= risk ratio
Case control study
- proportion of cases exposed
- proportion of controls exposed
= odds ratio
Cohort Study
Identify a cohort• Categorise individuals based on whether or
not they were exposed• Compare attack rates
exposed vs unexposed
Suitable when a cohort is easily identifiable e.g. specific ward(s), operating theatre list(s)
Case-Control Study
Identify cases • that meet the case definition Select non-diseased individuals from the
same population to act as controls Compare proportions exposed• cases vs. controls
Suitable when a distinct group is not easily identifiable e.g. long-term outbreak, OPD
Cohort study two-by-two table
Calculate association between exposure & disease
dc
ba
NTotal
c + dUnexposed
a + bExposed
TotalWellIll
Risk ratio [RR] = a/(a+b) / c/(c+d)
CC study two-by-two table
Calculate association between disease & exposure
dc
ba
NTotal
Unexposed
Exposed
TotalControlCase
Odds ratio [OR] = ad/bc
Table from a case control studyRisk factors for MRSA bacteraemia
ExposureCases n=42
Controls n=90
Odds Ratio
On admission
Indwelling catheter on admission 5 3 3.9
Prior admission 35 66 1.8
On or during admission
Bed sore 5 1 12.0
Skin ulcer 5 5 2.3
During admission
Central line during admission 17 1 60.5
Urinary catheter during admission 22 2 48.4
Blood transfusion 15 7 6.6
Verify biological coherence
Corroborative studies• Microbiological investigation
suspected sources or vehicles of transmission typing and molecular diagnostics
• Environmental investigation
• Traceback investigations (origin of supplies)
• Air circulation data
The reality….
Outbreak suspected
time
Confirmation
Form Outbreak Control Team
Confirm Diagnosis
Site visit
Case definition
Line list
Organise data
Descriptive Epidemiology
Control measures
AnalyticalEpidemiology
Recommendations
Outbreakreport