Principles of Epidemiology

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Principles of Epidemiology
For Epidemiology Support Staff
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Section Overview

• Definition of Epidemiology its role in disaster
• Epidemiologic data collection basics
• Counting cases
• Collecting information on cases
• Data collection challenges solutions
• Surveillance

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Epidemiology

• The study of the occurrence, distribution, and
  control of diseases in populations
• Typically characterized by
• Person
• Place
• Time

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Crucial Role of Epidemiology in Disasters
Response

• To collect, analyze and disseminate
  health-related data on the affected community so
  that decision-makers can use the information to
  guide a rational response

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Types of questions addressed

• What is the cause of an epidemic?
• How much disease or injury has occurred?
• How effective was a public health intervention?

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Type of information collected

• Number of cases
• Information on the cases ( non-cases)

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Counting cases
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What is a case

• ????

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Case
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Case definition

• Establishing the definition of a case is the
  foundation of epidemiology
• Why?

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Why a case definition is key
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Elements of a case definition

• Some or all of the following
• Clinical characteristics
• Laboratory results
• Epidemiologic characteristics (description of
  person, place, time)
• May have a case classification

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Case classification

• Degree of certainty regarding diagnosis
• Example
• Confirmed - Laboratory confirmed
• Probable - Typical clinical features without lab
  confirmation
• Possible - Fewer of the typical clinical features
• Why??

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Rational for case classification

• Allows you to determine a possible range for the
  number of cases

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Rational for case classification

• Estimate based on confirmed only
• Vast majority of identified cases would be real
  cases
• Some real cases probably missed
• Referred to as specific

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Rational for case classification

• Estimate based on cases under investigation,
  probable, possible confirmed
• Would include vast majority of cases
• Probably would include some none cases
• Referred to as sensitive

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Norovirus Outbreak in an Elementary School -
District of Columbia, February 2007

• A case of gastrointestinal illness illness
  in a student or staff member who reported nausea,
  vomiting, or diarrhea and who was present at the
  school any time during February 2-18

Morbidity and Mortality Weekly Report 2007
56(51) 1340-3
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Chickenpox case definition

• Clinical case definition - An illness with acute
  onset of diffuse (generalized) maculo-papulovesicu
  lar rash without other apparent cause
• Laboratory criteria for diagnosis
• Isolation of varicella virus from a clinical
  specimen, or
• Direct fluorescent antibody (DFA), or
• Polymerase chain reaction (PCR), or
• Significant rise in serum varicella
  immunoglobulin G (IgG) antibody level by any
  standard serologic assay

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Chickenpox

• Case classification
• Probable a case that meets the clinical case
  definition, is not laboratory confirmed, and is
  not epidemiologically linked to another probable
  or confirmed case
• Confirmed a case that is laboratory confirmed or
  that meets the clinical case definition and is
  epidemiologically linked to a confirmed or
  probable case

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CO poisoning after 2 major hurricanes

• Case Definition illness among persons of any
  age residing in Alabama during 8/28-11/1 or Texas
  9/20-11/1 with a diagnostic code consistent with
  CO
• Case classification
• Confirmed case Affected person had an elevated
  blood carboxyhemoglobin (COHb) level
• Probable case Did not have an elevated COHb
  level or did not have a COHb level documented.

MMWR 2006 55(9) 236-9
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2 types of cases

• Prevalent cases
• Incident cases

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Prevalent cases

• People with the condition at a given point in
  time
• Onset may be recently or in the past
• Examples
• Ever received a diagnosis of diabetes
• Infected with HIV

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Prevalence rate (ratio)

• Number of prevalent cases / number of people in
  population
• during a given period
• Interpretation
• Probability that someone in the population has
  the condition

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Incident cases

• People who newly meet case definition in a given
  period
• Examples
• Recent onset of gastroenteritis
• Recent injury

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Morbidity and Mortality Weekly Report 2007
56(51) 1340-3
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Use of information on the number of incident cases

• Drawing an epidemic curve
• Calculating incidence rate

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Epidemic curve

• Epidemic curve - figure that shows the number of
  new cases of disease caused by an infection over
  time

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Use of epidemic curve

• To help understand the cause of an epidemic
• Agent
• Time of exposure to the agent
• To examine the effectiveness of an intervention
  measure

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Effects of MMR Intervention on Mumps cases
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Incidence of Acute Hepatitis B United States,
1980-2003
HBsAg screening of pregnant women recommended
Source National Notifiable Diseases Surveillance
System (NNDSS)
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Incident rate

• Number of incident cases / number of people at
  risk of becoming an incident case
• during a given period
• Interpretation
• Risk of developing a condition at a given time

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Use of incidence rate

• Quantifying the effect of a disaster
• Evaluate the effectiveness of a control measure

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Collecting information on cases

• (and non-cases)

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Types of information collected on cases

• Clinical information
• Demographics (age, sex, race)
• Location
• Risk factors for disease / injury

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Uses of this information

• To describe how severe illness or injury is
• To describe the groups of people most affected by
  the disaster
• To identify people who have been exposed to
  infection
• To identify causes and risk factors

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Norovirus Outbreak in an Elementary School -
District of Columbia, February 2007

• Collected information from school children on
• Clinical symptoms onset
• Grade and classroom
• Contact with a sick person
• Use of library computers
• Attendance at art an other special classes

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Norovirus Outbreak in an Elementary School -
District of Columbia, February 2007

• Risk factors
• First grade classroom J only classroom with
  shared computers
• Contact with a sick person

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CO poisoning cases by generator placement
Alabama and Texas, Aug-Oct 2005
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Need for information on non-cases

• You are investigating an outbreak of E. coli
  O157H7. You suspect that the ground beef at
  market A may be contaminated.
• You interview cases and find that 50 of cases
  ate ground beef from market A.
• How do you know if this is more than you would
  expect?

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Data collection problems and how to avoid them
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Inaccuracy in data collection

• Usually occurs unintentionally
• Can result from
• Random mistakes
• Systematic errors bias

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Types of Bias

• Selection Bias Bias caused by systematically
  including or excluding some people
• Information Bias Bias caused by introducing
  systematic inaccuracies or inconsistencies to
  data collection

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Importance of minimizing inaccuracies

• Inaccuracy can lead to incorrect conclusions
  regarding -
• The size of the problem
• The cause of the problem
• The effectiveness of a control measure

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Tips for avoiding inaccuracies

• Use a consistent method to identify cases
• Use a consistent method to collect data on cases
• Record information carefully
• Dont excluded people

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Tip 1 Use a consistent method to count cases

• Use the case definition to establish case status
• Do it carefully
• Use it each time
• If not, you may under or over count

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Tip 2Use a consistent method to collected
information

• Use the data collection instrument faithfully
  each time
• Data collection instrument
• Interview questionnaire
• Record review form
• Inconsistency / inaccuracy is particularly
  problematic if it is systematic (information
  bias)

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Example of information bias

• You are investigating an outbreak of
  gastroenteritis at a wedding. It is suspected
  that the potato salad was contaminated.
• You are interviewing cases and controls.
• You send extra time helping cases to remember
  if they ate potato salad.
• You dont do this for controls

Potential Effect?
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Example of information bias
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Tip 3Record information carefully

• How to do this
• Write clearly
• Dont work more quickly then you need to
• Review work periodically
• Make sure you understand the form you are using
  to record the information

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Tip 3 Dont exclude people

• How to do it
• To the extent possible
• Assess all possible cases assigned to you
• Collect data on all cases assigned to you
  (interview or record review)
• Excluding cases is particularly problematic if it
  is systematic (selection bias)

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How selection bias might occur

• You dont review charts that are difficult to
  read
• You dont interview people who
• are difficult
• Live farthest way

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Surveillance
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Sources of epidemiologic data

• Routine Surveillance
• Interview
• Face-to-face
• Telephone
• Review of records
• Computer or paper
• Clinical charts and other records

Discussed now
Discussed in later modules
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Public Health Surveillance

• Ongoing, systematic collection, analysis, and
  interpretation of health-related data essential
  to the planning, implementation, and evaluation
  of public health practice
• Closely integrated with the timely dissemination
  of these data to those responsible for prevention
  and control.

Principles of Epidemiology Introduction to
Public Health Surveillance
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Basic elements

• Systematic, ongoing
• Collection
• Analysis
• Interpretation
• Dissemination of health-related data
• Link to action (public health practice)

CDC. Principles of Epidemiology Introduction to
Public Health Surveillance
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Routine Surveillance Methods

• Reportable Diseases
• Reportable Laboratory Significant Findings

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Reportable Disease

• Commissioner of the Department of Public Health
  declares an annual list of reportable disease
• Health care provider who treats or examines any
  person who has or is suspected of having a
  reportable disease shall report the case to
• the local director of health for the jurisdiction
  in which the person resides
• CT Department of Public Health

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Reportable Disease

• Report should include
• Name and address of reporting physician
• Attending physician
• Disease being reported
• Name, address, dob, race/ethnicity, sex and
  occupation of person affected
• Standard forms are available

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Categories of Reportable Disease

• Category 1 Report immediately by telephone on
  the day of recognition or strong suspicion and
  mail report within 12 hours
• Category 2 Report by mail within 12 hours of
  recognition or strong suspicion.

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Laboratory Reportable Significant Findings

• Directors of clinical laboratories must report
  laboratory evidence suggestive of reportable
  disease
• Reported on a standard form
• Not a substitute for reportable disease

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Partial list of laboratory Significant findings
Diseases that are possible indictors of
bioterrorism
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Partial list of laboratory Significant findings
Others that may result in a public health
emergency
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Authority to conduct surveillance

• Connecticut General Statutes Section 19a-2 and
  Regulation, Section 19a-36-A2 gives the
  Commissioner the authority to
• Prepare annual lists of Reportable Disease and
  Reportable Laboratory Significant Findings
• Convene an Advisory Committee annually to review
  proposed changes to the annual lists of
  Reportable Disease and Reportable Laboratory
  Significant Findings

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Surveillance data

• Especially important source of data for
• Identifying cases of disease
• Establishing an expected baseline level of
  disease
• Identifying possible deviations from baseline
  level which many indicate an outbreak