Introduction to Outbreak Investigation

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Introduction to Outbreak Investigation

• Epidemiology

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What is Epidemiology?

• Epidemiology is the study of the distribution
  and determinants of health-related states or
  events in specified populations, and the
  application of this study to the control of
  health problems. -CDC

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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

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What does Epidemiology study?

• Just about anything
• Health-related
• Infectious disease
• Chronic disease

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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

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Epidemiologic Information

• Case definition
• Person
• Place
• Time

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Case Definition

• Standard set of criteria
• Clinical and lab
• Allows for comparison
• Sensitive vs. Specific

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Case Definition

• Smallpox
• Clinical Description
• An illness with acute onset of fever gt101 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)

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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.

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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.

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Confirmed Case of Smallpox

• A case of smallpox that is laboratory confirmed.

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Case Definition Gradient
Low Specificity
High Specificity
Suspected
Probable
Confirmed
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Working Case Definition

• Smallpox Outbreak
• - Anyone who meets original case definition
• - Anyone with fever (gt101 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.

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Person

• Age
• Sex
• Race/Ethnicity
• Socio-Economic Status
• Behaviors

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Place

• Geographic Distribution
• Natural
• Clustering vs. uniform
• Home
• Work
• School
• Hospital room

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Geographic Distribution of LaCrosse
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Place in LaCrosse Case Investigation

• Neighborhood
• Home visits
• Mosquito Breeding Sites
• Tires
• Pots
• Standing Water

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Clustered
Scattered
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Time

• Onset of symptoms
• Incubation Period
• Infectious Period
• Seasonality
• Baseline vs. epidemic
• Interval
• Long-term trends
• Shorter for environmental exposure

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Measles Outbreak

• Baseline
• Normal occurrence
• Onset
• Incubation period
• Infectious period
• Mode of transmission

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Time in Outbreak investigation
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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

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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

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Information for any outbreak

• 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

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Exposure

• Possible Cause of illness
• Know your agent/disease
• Clinical picture
• Pathogenesis
• Mode of transmission
• Natural Reservoir
• Common Vehicle or Vector

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Chain of Infection
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Transmission

• Direct
• Contact
• Droplet
• Portal of Exit
• Portal of Entry

• Indirect
• Airborne
• Vehicle
• Vector
• Mechanical vs. biologic

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Types of Outbreaks

• Propagated
• Indicative of person to person transmission
• Point-source
• Indicative of a common exposure to a contaminated
  vehicle or reservoir

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Analytic Epi Studies

• Associations between exposure and disease
• Experimental
• Observational
• Cohort
• Case Control
• Cross-sectional

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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

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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

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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

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Use of Two-By-Two Tables

• Calculate association between disease and exposure

b
a
d
c
ad/bc Odd Ratio ? Relative Risk
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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

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People who ate from menu A were about 20 times
more likely to have diarrhea than those that did
not eat from menu A.
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More OR interpretation

• OR odds of exposure among ill
• odds of exposure among well
• If OR(RR) gt 1, then the exposure is a risk factor
  for being ill
• If OR lt 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!

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P-value

• Used in LOTS of statistical tests
• A guide to tell us that a result is significant
• Generally at the 95 level
• p lt 0.05 Significant at the 95 level there
  is a 95 probability that the result is accurate
  (not by chance)
• Example OR4, p-value0.01
• Example OR15, p-value0.36

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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 95CI (5.2, 8.8)
• Example OR 7 95CI (0.4, 18.7)

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Strength of Association

• Magnitude of Odd Ratio
• 95 Confidence Interval or P-value
• Be careful
• Plausibility
• Confounding

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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

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Secondary Attack Rate

• Measure of frequency of new cases among contacts
  of known cases
• cases among contacts of primary cases SAR
  during the period
• total of contacts

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The calculations

• of children in day care center 70
• of ill children at day care center 7
• of contacts (of those 7) at home 25
• of ill contacts 5
• AR 7/70 0.1 x 100 10 AR
• SAR 5/25 0.2 x100 20 SAR

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Summary

• Case definition, person, time, place
• Know disease/agent
• Recognize Point-Source vs. Propagated
• Set up analytic epi study
• Measures of association