Transcript Document

I is for Investigation
Outbreak Investigation Methods from
Mystery to Mastery
Session II
Study Design
Session Overview
• Developing and testing hypotheses
• Study designs
– Cohort studies
– Case-control studies
• Sampling
Learning Objectives
• Understand how to develop and test a
hypothesis about an exposure and an
outcome
• Describe the design of cohort studies and
case control studies
• Learn how to assess which study design
to apply during an outbreak investigation
• Explain the rationale and options for
sampling in an outbreak investigation
Basic Steps of an
Outbreak Investigation
1.
2.
3.
4.
5.
6.
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Verify the diagnosis and confirm the outbreak
Define a case and conduct case finding
Tabulate and orient data: time, place, person
Take immediate control measures
Formulate and test hypotheses
Plan and execute studies
Implement and evaluate control measures
Communicate findings
Developing and Testing
Hypotheses
Analytic Epidemiology
Studies Assess Exposures and
Outcomes
• Exposures refer to factors that might influence
one’s risk of disease
– Examples
Smoking
Eating at a particular restaurant
• Outcomes refer to case definitions
– Individuals who have the specified disease/condition
of interest
– Examples
Small cell carcinoma
Laboratory diagnosis of Campylobacter
What is an Epidemiologic
Hypothesis?
An educated guess about an association
between an exposure and an outcome that
is testable in a scientific investigation
• Data from descriptive epidemiology
provide information to develop hypotheses
• Hypotheses tend to be broad initially and
are then refined to have a narrower focus
Example
• Broad hypothesis: People who ate at the church
picnic were more likely to become ill
– Exposure is eating at the church picnic
– Outcome is illness - diarrhea and fever, where diarrhea
is defined as at least 3 soft stools in a 24 hour period
• Narrower hypothesis: People who ate the egg
salad at the church picnic were more likely to
have laboratory-confirmed Salmonella
– Exposure is eating egg salad at the church picnic
– Outcome is laboratory confirmation of Salmonella
Analytic Studies
• Designed to test an epidemiologic
hypothesis:
– Is there an association between exposure and
disease?
– How strong is the association?
• Two types used in outbreak investigations:
– Cohort
– Case-control
Study Designs
Cohort studies and
Case-control studies
Definition of a Cohort
In epidemiology,
“Any designated group of individuals who
are followed or traced over a period of time.”
- Last, JM. A Dictionary of Epidemiology, 3rd ed. New York: Oxford University Press, 1995
Cohort Studies
Study
Population
Time:
Present
Exposure is
self selected
Exposed
Non-exposed
Follow through time
Future
Disease
No Disease
Disease
No Disease
Cohort Study Types
A cohort study analyzes an exposure / disease
relationship within the entire cohort
• Prospective
– Identified population is followed forward through time
– The Framingham Study
• Retrospective
– Exposure is documented and cohort is “followed” for
outcomes that have already occurred
– Usually used in outbreak investigations
Identifying a Cohort
• Must be
straightforward to
define
• Must be at risk for
disease
• Should not be selected so that everyone
is exposed, or everyone is diseased
– Study group: exposed
– Comparison group: non-exposed
When to Use a Cohort Study
Design
• Members of cohort are easily identifiable
• Members of a cohort are easily
accessible
• Exposure is rare
• There may be multiple diseases involved
Cohort Study Example
• Recent norovirus
outbreaks on cruise
ships
• Attempt to interview
all passengers
• Collect food history
information
CDC Vessel Sanitation Program, 2012. http://www.cdc.gov/nceh/vsp/surv/GIlist.htm
Cohort Study Examples
• Skin disorders among Hurricanes Katrina and
Rita reconstruction workers
– Enrolled civilian construction workers living and
working at a New Orleans military base
Noe R, Cohen AL, Lederman E, et al. Skin Disorders Among Construction Workers
Following Hurricane Katrina and Hurricane Rita: An Outbreak Investigation in New Orleans,
Louisiana. Arch Dermatol. 2007;143(11):1393-1398.
• Gastroenteritis on a Greek island during a
religious festival
– All island residents were enrolled
Karagiannis I, Detsis M, Gkolfinopoulou K, Pervanidou D, Panagiotopoulos T, Bonovas
S. An outbreak of gastroenteritis linked to seafood consumption in a remote Northern
Aegean island, February-March 2010. Rural and Remote Health 10: 1507. (Online) 2010.
Available: http://www.rrh.org.au.
Cohort Study Examples
• Shigellosis among swimmers in a Georgia park
– Used park registry to identify park visitors
Iwamoto M, Hlady G, Jeter M et al. Shigellosis among Swimmers in a Freshwater Lake
Georgia, 2003. Presented at the 53rd Annual Epidemic Intelligence Service Conference.
Atlanta, GA. April, 2004.
• Whirlpools and Methicillin-Resistant
Staphylococcus aureus
– Occurred on a college football team
Begier EM, Barrett FK, Mshar PA et al. Body Shaving, Whirlpools, and Football: An Out
break of Methicillin-Resistant Staphylococcus aureus Cutaneous Infections in a College
Football Team-Connecticut, 2003. Presented at the 53rd Annual Epidemic Intelligence
Service Conference. Atlanta, GA. April, 2004.
Case-Control Studies
Time:
Past
Exposed
Not Exposed
Exposed
Cases
Controls
Study
Population
Present
Not Exposed
When to Use a Case-Control
Study
• When the study population is hard to
define
• When the study population is difficult to
contact
• When the study population is very large
• When the outcome is rare
A case-control study is the alternative to a
cohort study.
Initial Steps in a
Case-Control Study
1. Identify the source population
– The population that the cases came from
– Similar to the cohort in a cohort study
2. Establish a case definition and select
cases
– A standard set of criteria for determining
disease status that includes clinical criteria,
time, place, and person
Initial Steps in a
Case-Control Study
3. Select controls
– Controls represent exposure distribution of
the source population
– Sources of controls
• Random sample
• Friends of case-patients
Cohort versus Case-Control
Best when…
Cohort Study
Case-Control Study
Members are easily
identifiable
Source population is not
easily defined
Members are easily
accessible
Accessing entire cohort
would be too costly or
time consuming
Exposure is rare
Illness is rare
There are multiple
diseases of interest
Study Group
Exposed persons
Comparison Group Unexposed persons
Persons with illness
(cases)
Persons without illness
(controls)
Study
Design
Cohort
CaseControl
Advantages
Disadvantages
1. Least prone to selection bias 1. Prospective can be
expensive, time-consuming
2. Can reasonably conclude that
cause preceded disease
2. Prospective can lead to
loss to follow up
3. Can study several diseases
at once
3. Exposed may be followed
more closely than
4. Can examine rare exposures
unexposed, yielding invalid
5. Retrospective can be lowconclusions about causality
cost
1. Less expensive and quicker
than cohort
2. Can examine the effect of
multiple exposures
3. Requires a smaller sample
population
1. Inefficient for studying rare
exposures
2. Susceptible to selection
bias
3. Cannot directly estimate
the risk of disease
4. Cannot study several
diseases at once
Matching in Case-Control
Studies
• Matching attributes are “third factors”
– Age, gender, residence
• Can clarify relationship between exposure
and outcome
• Unmatched study design is usually
preferred
Matching: Points to Consider
• Special analytic techniques required
• Cannot assess role of matching factor on
disease status
– Do not match on a potential exposure
• Over-matching may result in not
representing source population
Sampling
What is sampling?
The systematic selection of a portion of
the larger source population. A sample
should be representative of the larger
source population.
Why sample?
Because it is more efficient – saves time
and money!
Sample Size
• Is the purpose of the study to determine
the source of the outbreak?
– A small number of cases and controls can
reveal risk factors for infection
• Is the purpose of the study to determine
the number of persons who become sick
over a specific period of time?
– A cohort study would require a larger sample
Types of Sampling
• Simple random sample (SRS)
– Randomly select persons to participate in
study. There are many variations of SRS.
• Convenience sample
– Choose those individuals who are easily
accessible.
Problems with Convenience
Sampling
• Based on subjective judgment
• Cases may or may not be representative
of the total population
• May lead to biased results
Session II Summary
• An analytic study is used to test scientific
hypotheses
• A case definition with specific criteria is used
to identify cases from the study population,
and should not include the study hypothesis
• Cohort studies provide a direct estimate of
the risk of disease, whereas case-control
studies do not
Session II Summary
• Cohort studies may be preferable when you
work with easily identifiable and accessible study
populations
• Case-control studies, when conducted properly,
can be an efficient alternative to cohort studies
• Controls should represent the source population,
and not be matched on the exposure factor if
matching is used.
References and Resources
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Begier EM, Barrett FK, Mshar PA et al. Body Shaving, Whirlpools, and
Football: An Outbreak of Methicillin-Resistant Staphylococcus aureus
Cutaneous Infections in a College Football Team-Connecticut, 2003.
Presented at the 53rd Annual Epidemic Intelligence Service Conference.
Atlanta, GA. April, 2004.
Centers for Disease Control and Prevention (1992). Principles of
Epidemiology: 2nd Edition. Public Health Practice Program Office: Atlanta,
GA.
Centers for Disease Control and Prevention "Gastroenteritis at a University
in Texas" http://www.phppo.cdc.gov/phtn/casestudies/classroom/gastro.htm
Gordis, L. (2000). Epidemiology: 2nd Edition. W.B. Saunders Company:
Philadelphia, PA.
Gregg, M.B. (2002). Field Epidemiology: 2nd Edition. Oxford University
Press: New York.
Hennekens, C.H. and Buring, J.E. (1987). Epidemiology in Medicine. Little,
Brown and Company: Boston/Toronto.
References and Resources
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Iwamoto M, Hlady G, Jeter M et al. Shigellosis among Swimmers in a
Freshwater Lake-Georgia, 2003. Presented at the 53rd Annual Epidemic
Intelligence Service Conference. Atlanta, GA. April, 2004.
Kleinbaum, D., Sullivan, K., and Barker, N. (2003). ActivEpi Companion
Textbook. Springer-Verlag: New York.
Last, J.M. (2001). A Dictionary of Epidemiology: 4th Edition. Oxford
University Press: New York.
McNeill, A. (January 2002). Measuring the Occurrence of Disease:
Prevalence and Incidence. Epid 160 lecture series, UNC Chapel Hill School
of Public Health, Department of Epidemiology.
Morton, R.F, Hebel, J.R., McCarter, R.J. (2001). A Study Guide to
Epidemiology and Biostatistics: 5th Edition. Aspen Publishers, Inc.:
Gaithersburg, MD.
University of North Carolina at Chapel Hill Gillings School of Global Public
Health, UNC Center for Public Health Preparedness, and the NC Institute of
Public Health. E is for Epidemiology. Session 3: Descriptive and Analytic
Epidemiology; 2012. Available from:
http://cphp.sph.unc.edu/trainingpackages/E/index.htm
References and Resources
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University of North Carolina at Chapel Hill School of Public Health,
Department of Epidemiology, and the Epidemiologic Research &
Information Center (June 1999). ERIC Notebook. Issue 2.
http://cphp.sph.unc.edu/trainingpackages/ERIC/issue2.htm
University of North Carolina at Chapel Hill School of Public Health,
Department of Epidemiology, and the Epidemiologic Research &
Information Center (July 1999). ERIC Notebook. Issue 3.
http://cphp.sph.unc.edu/trainingpackages/ERIC/issue3.htm
University of North Carolina at Chapel Hill School of Public Health,
Department of Epidemiology, and the Epidemiologic Research &
Information Center (September 1999). ERIC Notebook. Issue 5.
http://cphp.sph.unc.edu/trainingpackages/ERIC/issue5.htm
University of North Carolina at Chapel Hill School of Public Health,
Department of Epidemiology (August 2000). Laboratory Instructor’s Guide:
Analytic Study Designs. Epid 168 lecture series.
http://www.epidemiolog.net/epid168/labs/AnalyticStudExerInstGuid2000.pdf