Transcript What is Epidemiology

Introduction to
Outbreak Investigation
Epidemiology
What is Epidemiology?
• “Epidemiology is the study of the
distribution and determinants of healthrelated states or events in specified
populations, and the application of this
study to the control of health
problems.” -CDC
What is Epidemiology?
• Science of Public Health that studies:
– Distribution of disease
– Determinants of health/disease
– Specific populations
• Look for patterns of disease
– Time, place, personal characteristics
• Interventions
– Prevention is key
What does Epidemiology study?
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Just about anything
Health-related
Infectious disease
Chronic disease
How is Epidemiology Used?
• Population/community health
assessment
• Personal decision-making
• Complete clinical picture
• Evaluate interventions
• Search for cause
– Exposure and relationship to disease
– Outbreak investigation
Epidemiologic Information
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Case definition
Person
Place
Time
Case Definition
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Standard set of criteria
Clinical and lab
Allows for comparison
Sensitive vs. Specific
Case Definition
• Smallpox
– Clinical Description
• An illness with acute onset of fever >101 °F followed by a rash
characterized by vesicles or firm pustules in the same stage of
development without other apparent cause.
– Laboratory Criteria for Confirmation
• Isolation of smallpox (variola) virus from a clinical specimen, or
• Polymerase chain reaction (PCR) identification of variola DNA in
a clinical specimen, or
• Negative stain electron microscopy (EM) identification of variola
virus in a clinical specimen (Level D laboratory or approved
Level C laboratory)
Suspected Case of Smallpox
• A case that meets the clinical case definition
but is not laboratory confirmed and does not
have an epidemiological link to a confirmed or
probable case of smallpox, OR a case that
has an atypical presentation that is not
laboratory confirmed but has an
epidemiological link to a confirmed or
probable case of smallpox. Atypical
presentations of smallpox include a)
hemorrhagic lesions OR b) flat, velvety
lesions not appearing as typical vesicles nor
progressing to pustules.
Probable Case of Smallpox
• A case that meets the clinical case
definition that is not laboratory
confirmed but has an epidemiological
link to another confirmed or probable
case.
Confirmed Case of Smallpox
• A case of smallpox that is laboratory
confirmed.
Case Definition Gradient
Low Specificity
High Specificity
Suspected
Probable
Confirmed
Working Case Definition
• Smallpox Outbreak
- Anyone who meets original case definition
- Anyone with fever (>101 °F ) or rash who
was in a confirmed exposed area during the
BT event or came in contact with a
confirmed or probable case should be
considered a case.
Person
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Age
Sex
Race/Ethnicity
Socio-Economic Status
Behaviors
Place
• Geographic Distribution
– Natural
– Clustering vs. uniform
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Home
Work
School
Hospital room
Geographic Distribution of LaCrosse
“Place” in LaCrosse Case Investigation
• Neighborhood
• Home visits
• Mosquito Breeding Sites
– Tires
– Pots
– Standing Water
Clustered
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Scattered
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Time
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Onset of symptoms
Incubation Period
Infectious Period
Seasonality
Baseline vs. epidemic
Interval
• Long-term trends
• Shorter for environmental exposure
Measles Outbreak
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Baseline
Normal occurrence
Onset
Incubation period
Infectious period
Mode of transmission
Time in Outbreak investigation
Visited health dept.
Conference in Nairobi
Home
Church activities
Rash onset
ER Visit
Health dept. notified
IgM results
Interventions
28 19
July
26
August
2
9
September
16
Measles
Southwestern VA, 1999
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Index case
First generation case
Second generation case
Third generation case
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0
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81
11 0
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14 3
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17 6
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20 9
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23 2
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26 5
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57
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14 3
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17 6
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9
Cases
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September
October
Date of Onset
Incidence
•Number of new events occurring in a defined
population during a specified period of time
•Incidence =
New cases/population at risk/time
•Used to measure current disease activity
•Allows comparison between areas with
different populations
Contrasts with Prevalence
• Prevalence is the number of new and existing
cases divided by the total population (can be
during a period of time or at a given point)
• Prevalence =
(New cases + existing cases)/Total population
- Can be expressed as a percent
- Can give a picture of disease burden within a
population
Information for any outbreak
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Symptom onset date
Symptoms present and agent if known
Suspected exposure date if known
Residence
Age
Gender
Laboratory testing
Outbreak Case Definition
Organized case information in a line list
Exposure
• Possible Cause of illness
• Know your agent/disease
– Clinical picture
– Pathogenesis
– Mode of transmission
– Natural Reservoir
– Common Vehicle or Vector
Chain of Infection
Transmission
• Direct
– Contact
– Droplet
• Portal of Exit
• Portal of Entry
• Indirect
– Airborne
– Vehicle
– Vector
• Mechanical vs.
biologic
Types of Outbreaks
• Propagated
– Indicative of person to person transmission
• Point-source
– Indicative of a common exposure to a
contaminated vehicle or reservoir
Measles
Southwestern VA, 1999
5
Index case
First generation case
Second generation case
Third generation case
3
2
1
0
24
57
81
11 0
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14 3
-1
17 6
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20 9
-2
23 2
-2
26 5
-2
8
29
-1
24
57
810
11
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14 3
-1
17 6
-1
9
Cases
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September
October
Date of Onset
Figure 3. Onset of Illness in Wilsonburg Grade School
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Number of Cases
14
12
10
Sick
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6
4
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12pm
5/1
6pm
12am
5/2
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12pm
6pm
12am
5/3
Date and Time of Onset in 6 Hour increments.
6am
12pm
6pm
Analytic Epi Studies
• Associations between exposure and
disease
• Experimental
• Observational
– Cohort
– Case Control
– Cross-sectional
Cohort Study
• Objective: In a population of
individuals initially free of disease,
determine the risk factors associated
with development of disease.
– Select Population
– Categorize individuals based on presence
or absence of select risk factors
– Follow individuals to determine which
develop disease
• Good for rare exposures
Case-Control Study
• Objective: Compare risk factors in diseased
and non-diseased individuals to determine
possible associations.
– Select population
– Select cases that meet the disease case definition
– Select non-diseased individuals from the
population to act as controls
– Gather previous exposure/risk factor histories from
both groups
• Many outbreak investigations use this study
design
• Good for rare diseases
John Snow and Cholera
• “Broad Street Pump”
– 1854 cholera epidemic in London
– Looks at cholera deaths
– Geographical mapping
• Where do cases live?
• Where do cases work?
– Water source
Use of Two-By-Two Tables
• Calculate association between disease
and exposure
Ill
Well
Exposed
a
b
Unexposed
c
d
Total
Total
N
ad/bc = Odd Ratio  Relative Risk
Interpreting Measures of Association
• Odds ratios and risk ratios measure the
degree of relatedness of an exposure
and a health event (an outcome)
• The farther away the OR/RR is from 1,
the more we would say the exposure
and outcome are associated
• Confidence intervals and p-values help
to determine if association is due to
chance
OR = 12 x 17/2 x 5
= 204/10
OR = 20.4
People who ate from menu A were about 20 times more
likely to have diarrhea than those that did not eat from
menu A.
More OR interpretation
• OR = odds of exposure among ill
odds of exposure among well
• If OR(RR) > 1, then the exposure is a risk
factor for being ill
• If OR < 1, then the exposure is protective
of illness
• If OR = 1, then there is no association
between exposure and illness
• OR can NEVER be negative!
P-value
• Used in LOTS of statistical tests
• A guide to tell us that a result is
“significant”
• Generally at the 95% level
• p < 0.05 = Significant at the 95% level =
there is a 95% probability that the result
is accurate (not by chance)
• Example: OR=4, p-value=0.01
• Example: OR=15, p-value=0.36
95% Confidence Intervals
• Used with risk ratios and odds ratios
• Tell us about both precision and accuracy
• With an OR or RR we have estimated the
magnitude of the association – 95%
confidence intervals tell us that we can be
95% sure that the true association is
somewhere in that interval
• Example: OR = 7 95%CI= (5.2, 8.8)
• Example: OR = 7 95%CI= (0.4, 18.7)
Strength of Association
• Magnitude of Odd Ratio
• 95% Confidence Interval or P-value
• Be careful
– Plausibility
– Confounding
Attack Rate
AR = # of people who became ill x10n
# of people at risk
– People at risk could be those at a party, in
a class, on a cruise ship…
– Usually express AR as a %, so n = 2
Secondary Attack Rate
• Measure of frequency of new cases
among contacts of known cases
SAR =
# cases among contacts of primary cases
during the period
total # of contacts
The calculations
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#
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of
of
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children in day care center = 70
ill children at day care center = 7
contacts (of those 7) at home = 25
ill contacts = 5
• AR = 7/70 = 0.1 x 100 = 10% AR
• SAR = 5/25 = 0.2 x100 = 20% SAR
Summary
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Case definition, person, time, place
Know disease/agent
Recognize Point-Source vs. Propagated
Set up analytic epi study
Measures of association