Epidemiology Review Notes

Epidemiology

1. EPIDEMIOLOGY
2. Crisbert I. Cualteros, M.D.
   
3. http://crisbertcualteros.page.tl
4. DEFINITION
   • Study of the distribution and determinants of health and disease among populations and the application of such study to the prevention and control of health problems.
   • Determination of the nature, extent and determinants of disease or health problems
   • among populations
5. COMPONENTS OF EPIDEMIOLOGY
   • Descriptive Epidemiology
   • study of the distribution of disease
   • variables commonly examined are descriptive of person, place and time
   • Analytic Epidemiology
   • use of epidemiologic methods to explain disease occurrence or elucidate causal mechanisms
6. CHARACTERISTICS / FEATURES OF EPIDEMIOLOGY
   • It is a quantitative science.
   • It is an applied science.
   • Its methods are generally observational.
   • Its focus is the group or community of persons.
   • Its methods are systematic and orderly.
7. DESCRIPTIVE EPIDEMIOLOGY
   • I. Definition
   • study the amount and distribution of disease within a population
   • II . Uses
   • evaluate trends in health and make comparisons
   • health planning
   • identify problems to be studied by analytical methods = hypothesis
8. DESCRIPTIVE EPIDEMIOLOGY
   • III. Community Reaction to Disease
   • Absence of disease
   • No cases on current record
   • Disease absent from the beginning or has been eradicated
   • Sporadic
   • Occurrence of few and unrelated cases
   • Endemic
   • Constant occurrence of disease
   • Epidemic
   • Occurrence of a number of cases of disease in excess of the normal expectancy
9. DESCRIPTIVE EPIDEMIOLOGY
   • IV. Descriptive Variables
   • A. Person
   • 1. Age
   • Different diseases show different age patterns
   • Disease Characteristic Pattern of Magnitude
   • Confers long lasting immunity decreasing with age
   • Degenerative diseases or increasing with age
   • w/ long latency
   • Reflects low resistance high at extreme ages
   • of young and old
   • Reflects high exposure high in middle age groups
   • during middle age
10. VARIABLES OF PERSON
    • 2. Sex
    • Difference in sexual constitutional, e.g. hormonal balance
    • Greater exposure of males due to habits,
    • recreation, occupation, lifestyle
    • Greater health consciousness of females:
    • Early consultation, diagnosis and treatment
    • Better compliance with treatment
    • More cases recorded – artifactual reason
11. VARIABLES OF PERSON
    • 3. Civil Status
    • Differences in lifestyle that are causally related to particular diseases
    • Self-selection
    • Concordance between marital partners
    • Greater family support among the married
    • 4. Socio-economic class – affects state of nutrition,
    • level of health awareness or knowledge, etc
    • 5. Genetics
12. DESCRIPTIVE VARIABLES
    • B. Place
    • 1. Variables Of Place
    • geographic divisions
    • physical environment
    • climate, altitude, soil, vegetation, fauna
    • biological environment: infectious disease agents, animal reservoirs, vectors
    • socio-cultural-political-economic environment
    • related to level of development
13. VARIABLES OF PLACE
    • population characteristics
    • density
    • urban vs. rural
    • mobility
    • herd immunity
14. PLACE
    • 2. Etiology of Disease Variations
    • Real Causes - reflect true increase in risk
    • deteriorating or improving environment
    • quality, availability and distribution of medical care
    • Artifactual Causes
    • reliability of diagnosis
    • completeness of reporting and recording of diseases, births and deaths
15. DESCRIPTIVE VARIABLES
    • C. Time
    • Temporal variations – changes / fluctuations in disease frequency with the passage of time
    • 1. Types of Temporal Variations:
    • Secular Trends
    • Cyclic Fluctuations
    • Short-term Irregular – e.g. Epidemics / outbreaks
16. TEMPORAL VARIATIONS
    • Secular Trend Cyclic Fluctuations
    • Measure of frequency Mortality rates Incidence Rates
    • Nature of change Increase or Almost regular rises
    • decrease
    • Period of observation 10 years or hours, days, weeks,
    • longer months, years (≤5)
    • Type of Disease Chronic Acute
17. TIME
    • 2. Reasons for Changes in Mortality Rates
    • Artifactual or non-etiologic
    • Error in the numerator
    • guess diagnosis / misdiagnosis
    • inaccurate counting
    • change in the International Classification of Diseases
    • Error in the denominator
    • over or underestimate of the population
18. TIME
    • Change in Case Fatality Rate, w/o change in Incidence Rate
    • change in availability or utilization of health care services
    • change in treatment modalities
    • change in risk to superimposed infections
19. TIME
    • Change in Incidence Rate, w/o change in Case Fatality Rate
    • Artifactual or non-etiologic
    • Real or etiologic
    • change in disease agent
    • change in herd resistance
    • change in the environment
20. TIME
    • 3. Types of Cyclic Fluctuations
    • Cyclic Intrinsic Fluctuation
    • change/s in the host
    • change in herd resistance
    • accumulation of susceptibles
    • Cyclic Extrinsic Fluctuation (seasonal variation)
    • change in the environment
    • change in the disease agent
21. Analytic Epidemiology
    • Definition
    • study the determinants of disease or reasons for low and high frequency in specific groups
    • employs Epidemiologic Methods :
    • Definition of the problem
    • Appraisal of existing facts
    • Formulation of hypothesis
    • Testing of hypothesis
    • Conclusion and practical application
22. ANALYTIC EPIDEMIOLOGY
    • II. Principal Uses
    • Community diagnosis
    • Investigation of epidemics
    • Determination of disease etiology
    • Evaluation of community intervention and programs
23. COMMUNITY DIAGNOSIS
    • Definition of the Problem
    • Determining the extent and magnitude of the problem using statistical indices
    • Comparison of the statistical indices with those of other places and other diseases
    • computation of economic burden of disease: cost of losses due to disability, death, treatment and prevention
24. COMMUNITY DIAGNOSIS
    • II. Appraisal of Existing Facts
    • Determining the:
    • state of knowledge of disease or health problem etiology
    • distribution of the disease/ problem in terms of person, place and time
    • factors associated with the disease/ problem
25. COMMUNITY DIAGNOSIS
    • III. Formulation of Hypothesis
    • Explanations for the existence and magnitude of the disease / problem
    • IV. Testing of Hypothesis
    • V. Conclusion and Practical Solutions to the
    • Problem
26. EPIDEMICS
    • Definition
    • the occurrence of any number of cases of a disease clearly in excess of the normal expectancy or what usually prevails
27. EPIDEMICS
    • II. Causes of Epidemics
    • flare up of an old or existing disease
    • increased virulence of existing strain
    • introduction of a new strain of the existing agent
    • increased capacity to multiply
    • decreased resistance of the population
    • dilution of herd resistance with a susceptible population
    • changes in the environment favoring disease transmission, e.g. calamities destroying health facilities, factors favoring survival and multiplication of vectors, changes in climate, temperature, etc.
28. Causes of Epidemics
    • new disease
    • introduction of a disease not previously present in the community
    • disease previously affecting lower animals affecting man for the first time
    • recognition for the first time of previously occurring disease known by another name
29. EPIDEMICS
    • III. Classification of Epidemics according to:
    • 1. Onset (of epidemic)
    • explosive , abrupt, sudden – majority of cases occurring within one incubation period
    • staggering , insidious, gradual
    • 2. Exposure (of cases)
    • mass or simultaneous – exposure occurred about the same time
    • progressive – cases were exposed one after the other from a primary case
30. CLASSIFICATION OF EPIDEMICS
    • 3. Transmission
    • common vehicle – single or multiple exposure
    • propagated
    • contact-transmitted: person to person
    • vector-transmitted
31. CLASSIFICATION OF EPIDEMICS
    • 4. Epidemic Curve
    • classical – short ascending, long descending limbs; water-borne
    • inverted – long ascending, short descending limbs; vector-borne
32. Epidemic Curve
    • bell-shaped – ascending and descending limbs about equal, peak is rounded; contact-transmitted
    • point - ascending and descending limbs about equal, peak is pointed; food poisoning
33. EPIDEMICS
    • IV. Termination of Epidemics
    • eradication / killing of disease agents at the source or reservoirs
    • interruption or closure of transmission
    • exhaustion of susceptibles
34. EPIDEMICS
    • V. Steps in the Investigation of Epidemics
    • 1. Definition of the problem
    • verify the diagnosis
    • establish existence of an epidemic
    • 2. Appraisal of existing facts
    • characterize the distribution of cases by person, place and time
35. INVESTIGATION OF EPIDEMICS
    • 3. Formulation of hypothesis
    • as to source of infection, mode of transmission, factors that may have given rise to the epidemic
    • 4. Testing of hypothesis
    • conduct an epidemiological investigation (case control)
    • 5. Conclusion and recommendations for control
    • and prevention
36. DETERMINATION OF DISEASE ETIOLOGY
    • I. Types of Epidemiologic Studies
    • A. Descriptive Studies
    • 1. Uses
    • determination of distribution of disease according to person, place and time
    • delineation of syndrome as a disease entity
    • establishment of the natural history of disease
    • classification of disease
    • manifestational: pathologic and symptomatic
    • experiential: based on similarity of experience
37. Descriptive Studies
    • 2. Types
    • Case report
    • unit of study: single person with a disease
    • limitation: based on experience of a single person
    • provides first clues in the identification of a disease or adverse effects of exposure
38. TYPES OF DESCRIPTIVE STUDIES
    • Case series
    • unit of study: group of persons with a similar disease
    • Uses:
    • formulation of criteria for diagnosis
    • formulation of indications for treatment
    • identification of prognostic factors
    • determination of survival rates
39. CASE SERIES
    • Limitation
    • limited generalizability because of unrepresentativeness of subjects and absence of comparison group
40. TYPES OF DESCRIPTIVE STUDIES
    • Prevalence/Cross-sectional/ Surveys
    • measures prevalence of disease or an event
    • information about exposure and outcome are obtained simultaneously in a well-defined population
41. PREVALENCE/CROSS-SECTIONAL/ SURVEYS
    • Uses
    • determination of prevalence of risk factors
    • determination of frequency of prevalent
    • cases
    • determination of health status and health
    • needs
    • formulation of hypothesis
42. PREVALENCE/CROSS-SECTIONAL/ SURVEYS
    • Advantages
    • quick and easy to perform
    • Disadvantages
    • temporality cannot be ascertained
    • selects for longer-lasting and indolent cases
43. TYPES OF DESCRIPTIVE STUDIES
    • Ecological Studies
    • crude way of exploring relationship between environment or occupation and disease
    • unit of study: populations or groups of people rather than individuals
    • hypothesis generating rather than hypothesis testing
44. ECOLOGICAL STUDIES
    • Advantage
    • simple to conduct
    • Disdavantage
    • individual link between exposure and effect cannot be made (ecologic fallacy)
45. Types of Epidemiologic Studies
    • B. Analytic Studies
    • Use
    • to determine whether a factor is causally associated with disease
    • to test epidemiologic hypotheses
    • 2. Categories
    • observational/ non-experimental
    • observes natural course of events
    • case control study, cohort study, cross-sectional study
46. CATEGORIES OF ANALYTIC STUDIES
    • Experimental/ Interventional
    • exposure to the factor or treatment under study controlled by investigator
    • randomized clinical trial (rct), community trial, laboratory trial
47. ANALYTIC STUDIES
    • 3. Types
    • Case-control Studies
    • cases (with disease) and controls (no disease) are selected from a chosen population
    • both are questioned or records are reviewed about presence or absence of a suspected cause/risk factor in the past
48. CASE-CONTROL STUDIES
    • 1. Uses
    • to test risk factors
    • preferred if disease is rare
    • preferred if several factors are associated with disease of interest
49. CASE-CONTROL STUDIES
    • 2. Requirements for valid results
    • Cases must be representative of all those with disease and clearly defined.
    • Controls must be representative of all those without the disease and come from same community or source as the cases .
50. CASE-CONTROL STUDIES
    • 3. Analysis
    • Odd’s Ratio (OR)
    • proportion of those with history of exposure to the factor among the cases (a/a+c) is compared to those with history of exposure (b/b+d) to the factor among the controls
    • OR = ad/bc
51. ANALYSIS OF CASE CONTROL STUDIES
    • Outcome (Disease)
    • + -
    • + a b
    • Exposure
    • (Factor)
    • - c d
    • a+c b+d
    • * statistical association between factor and outcome
    • exists if (a/a+c) ≠ (b/b+d)
    • * association is probably causal , if OR > 1
52. CASE-CONTROL STUDIES
    • 4. Advantages
    • more economical
    • smaller sample size required
    • suitable for rare diseases
    • suitable for diseases associated with multiple exposures
53. CASE-CONTROL STUDIES
    • 5. Disadvantages
    • more susceptible to bias of recall
    • estimate of risk is indirect
    • controls more difficult to assemble
    • temporal relationship between factor and outcome cannot be ascertained
54. TYPES OF ANALYTIC STUDIES
    • Cohort Studies
    • groups of subjects are chosen on the basis of having been exposed to a factor or not
    • groups are followed up to identify those who develop the disease or outcome
55. COHORT STUDIES
    • Uses
    • to test prognostic factors
    • to directly measure risk of development of disease or outcome
    • provide more definitive information about disease etiology
    • preferred for study of rare exposures
56. COHORT STUDIES
    • 2. Requirement for valid results
    • Similarity of comparison groups
    • 3. Types
    • concurrent
    • Subjects are free of disease or outcome of interest at the time of initiation of the study.
    • Investigator follows-up the groups or cohorts from exposure to appearance of disease or outcome.
57. TYPES OF COHORT STUDIES
    • . Non-concurrent
    • Subjects who are free of the disease or outcome of interest at some point in the past are identified in terms of their exposure level.
    • Disease or outcome status is determined through existing records.
    • At the time the study is conducted, the specified follow-up period has elapsed.
58. COHORT STUDIES
    • 4. Analysis
    • Relative Risk or Risk Ratio(RR)
    • proportion of subjects with the disease or outcome among the exposed (a/a+b) is compared to proportion of subjects with the disease or outcome among the unexposed (c/c+d)
    • RR = a/a+b ÷ c/c+d
59. ANALYSIS OF COHORT STUDIES
    • Outcome (Disease)
    • + -
    • + a b
    • Exposure
    • (Factor)
    • - c d
    • a+c b+d
    • * statistical association between factor and outcome
    • exists if (a/a+b) ≠ (c/c+d)
    • * association is probably causal , if RR > 1
60. ANALYSIS OF COHORT STUDIES
    • Attributable Risk (AR)
    • estimate of the amount of risk that is attributable to the risk factor
    • AR = a/(a+b) - c/(c+d)
61. COHORT STUDIES
    • 5. Advantages
    • provides direct estimate of risk
    • temporality can be ascertained (for concurrent
    • studies)
    • less biases of recall and observation
    • allows for determination of population-based rates
    • controls easier to assemble
    • variations in exposure can be followed-up
    • unsuspected effects of the exposure may be observed
62. COHORT STUDIES
    • 6. Disadvantages
    • more expensive
    • follow-up period may be long
    • high attrition rate
    • large sample size required
    • change in exposure rates over long periods of time
63. COMPARISON OF CHARACTERISTICS OF CASE CONTROL AND COHORT STUDIES
    • Case Control Cohort
    • Starting population diseased group exposed group
    • Control Group non-diseased unexposed
    • Information Sought frequency of disease rate
    • exposure to
    • risk factor
    • Principal bias knowledge of knowledge of
    • disease influences exposure influences
    • report of exposure diagnosis
    • 
      
64. COMPARISON OF CHARACTERISTICS OF CASE CONTROL AND COHORT STUDIES
    • Case Control Cohort
    • Time to Complete short usually long
    • Study
    • Measure of Odd’s Ratio Relative Risk
    • Association
65. TYPES OF ANALYTIC STUDIES
    • Experimental Studies
    • Requirement for validity: complete comparability of comparison groups
    • 1. Types
    • Clinical Trial - Randomized Controlled Trial (RCT)
    • investigator randomly places the subjects to one of the intervention groups
    • ex. drug or surgical trials
    • used if strong evidence for association already exists
66. TYPES OF EXPERIMENTAL STUDIES
    • Field or community trials
    • subjects are people in the general population
    • who are disease-free but are presumed to be
    • at risk
    • ex. trials of preventive measures, e.g.
    • immunization
67. FIELD OR COMMUNITY TRIALS
    • Requirements
    • high incidence of disease under study
    • availability of facilities for observation
    • accessibility of subjects
    • availability of resources for precise diagnosis
    • and follow-up
68. EXPERIMENTAL STUDIES
    • 2. Analysis
    • comparison of disease or outcome rate in
    • experimental (P 1 ) = (a/a+c) and control
    • groups (P 2 ) = (b/b+d)
69. ANALYSIS OF EXPERIMENTAL STUDIES
    • Therapeutic / Preventive
    • Measure + -
    • 
      
    • + a b
    • Disease/
    • Outcome - c d
    • a+c b+d
70. ANALYSIS OF EXPERIMENTAL STUDIES
    • Protective Value = P 2 – P 1
    • P 2
71. EXPERIMENTAL STUDIES
    • 3. Advantage
    • Provide the strongest evidence for testing hypothesis
    • 4. Limitation
    • ethical issues, especially for clinical trials
72. Determination of Disease Etiology
    • II. Assessment of Results
    • 1. determine if statistical association between factor and outcome occurs
    • 2. if association exists, determine if due to:
    • chance
    • perform significance testing
    • extraneous or confounding variables
    • matching
    • specification or restriction
    • standardization of rates
    • stratified analysis
73. Assessment of Results
    • if association exists, determine if due to:
    • causal relationship
    • criteria:
    • 1. measures of strength of association – OR, RR, Protective Value
    • 2. temporality – exposure occurred prior to outcome
    • 3. dose-response relationship
74. CRITERIA FOR CAUSAL ASSOCIATION
    • 4. specificity – factor associated with only 1 or
    • limited number of diseases
    • 5. consistency of association – distribution of factor and disease is similar in different
    • sub-groups
    • 6. biologic plausibility – consistency with existing knowledge