EPIDEMIOLOGYEPIDEMIOLOGYEPIDEMIOLOGYEPIDEMIOLOGYCrisbert I. Cualteros, M.D.Crisbert I. Cualteros, M.D.
http://crisbertcualteros.page.tlhttp://crisbertcualteros.page.tl
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
COMPONENTS OF EPIDEMIOLOGY
I. Descriptive Epidemiology study of the distribution of disease variables commonly examined are
descriptive of person, place and time
II. Analytic Epidemiology use of epidemiologic methods to explain
disease occurrence or elucidate causal mechanisms
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.
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
DESCRIPTIVE EPIDEMIOLOGY
III. Community Reaction to Disease1. Absence of disease
»No cases on current record»Disease absent from the beginning or has been eradicated
2. Sporadic »Occurrence of few and unrelated cases
3. Endemic»Constant occurrence of disease
4. Epidemic»Occurrence of a number of cases of disease in excess of the normal expectancy
DESCRIPTIVE EPIDEMIOLOGY
IV. Descriptive VariablesA. 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
VARIABLES OF PERSON2. 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 treatmentBetter compliance with treatmentMore cases recorded – artifactual reason
VARIABLES OF PERSON3. 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
DESCRIPTIVE VARIABLES
B. Place1. Variables Of Place
–geographic divisions
–physical environmentclimate, altitude, soil, vegetation, faunabiological environment: infectious disease
agents, animal reservoirs, vectors
–socio-cultural-political-economic environment
related to level of development
VARIABLES OF PLACEpopulation characteristics
density urban vs. rural mobility herd immunity
PLACE2. Etiology of Disease Variations
Real Causes - reflect true increase in risk deteriorating or improving environmentquality, availability and distribution of medical care
Artifactual Causesreliability of diagnosiscompleteness of reporting and recording of
diseases, births and deaths
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
TEMPORAL VARIATIONSSecular 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
TIME2. Reasons for Changes in Mortality
Rates– Artifactual or non-etiologic
Error in the numeratoroguess diagnosis / misdiagnosisoinaccurate countingochange in the International Classification of Diseases
Error in the denominatoroover or underestimate of the population
TIMEChange in Case Fatality Rate, w/o
change in Incidence Ratechange in availability or utilization
of health care serviceschange in treatment modalitieschange in risk to superimposed
infections
TIMEChange in Incidence Rate, w/o
change in Case Fatality RateArtifactual or non-etiologicReal or etiologic
ochange in disease agentochange in herd resistanceochange in the environment
TIME3. Types of Cyclic Fluctuations
Cyclic Intrinsic Fluctuationchange/s in the hostchange in herd resistanceaccumulation of susceptibles
Cyclic Extrinsic Fluctuation (seasonal variation)
change in the environmentchange in the disease agent
Analytic Epidemiology I. Definition
study the determinants of disease or reasons for low and high frequency in specific groups
employs Epidemiologic Methods:Definition of the problemAppraisal of existing factsFormulation of hypothesisTesting of hypothesisConclusion and practical application
ANALYTIC EPIDEMIOLOGY
II. Principal UsesCommunity diagnosisInvestigation of epidemicsDetermination of disease etiologyEvaluation of community
intervention and programs
COMMUNITY DIAGNOSIS
I. 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
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
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
EPIDEMICSI. Definition
the occurrence of any number of cases of a disease clearly in excess of the normal expectancy or what usually prevails
EPIDEMICSII. Causes of Epidemics
flare up of an old or existing diseaseincreased virulence of existing strainintroduction of a new strain of the
existing agentincreased capacity to multiplydecreased resistance of the population dilution of herd resistance with a
susceptible populationchanges in the environment favoring
disease transmission, e.g. calamities destroying health facilities, factors favoring survival and multiplication of vectors, changes in climate, temperature, etc.
Causes of Epidemicsnew 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
EPIDEMICSIII. 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
CLASSIFICATION OF EPIDEMICS
3. Transmissioncommon vehicle – single or
multiple exposurepropagated
contact-transmitted: person to person
vector-transmitted
CLASSIFICATION OF EPIDEMICS
4. Epidemic Curveclassical – short ascending, long
descending limbs; water-borne
inverted – long ascending, short descending limbs; vector-borne
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
EPIDEMICSIV. Termination of Epidemics
eradication / killing of disease agents at the source or reservoirs
interruption or closure of transmission
exhaustion of susceptibles
EPIDEMICSV. 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
INVESTIGATION OF EPIDEMICS
3. Formulation of hypothesisas to source of infection, mode of
transmission, factors that may have given rise to the epidemic
4. Testing of hypothesisconduct an epidemiological investigation
(case control)
5. Conclusion and recommendations for control
and prevention
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
Descriptive Studies2. Types
Case reportunit of study: single person with a
diseaselimitation: based on experience of a
single personprovides first clues in the
identification of a disease or adverse effects of exposure
TYPES OF DESCRIPTIVE STUDIES
Case seriesunit of study: group of persons with a similar
diseaseUses:
o formulation of criteria for diagnosiso formulation of indications for
treatmento identification of prognostic factorso determination of survival rates
CASE SERIES Limitation
o limited generalizability because of unrepresentativeness of subjects and absence of comparison group
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
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
PREVALENCE/CROSS-SECTIONAL/ SURVEYS
Advantagesquick and easy to perform
Disadvantages temporality cannot be ascertained selects for longer-lasting and
indolent cases
TYPES OF DESCRIPTIVE STUDIES
Ecological Studiescrude 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
ECOLOGICAL STUDIES
Advantage simple to conduct
Disdavantageindividual link between exposure
and effect cannot be made (ecologic fallacy)
Types of Epidemiologic Studies
B. Analytic Studies1. Use
to determine whether a factor is causally associated with disease
to test epidemiologic hypotheses 2. Categories
observational/ non-experimentalobserves natural course of eventscase control study, cohort study, cross-
sectional study
CATEGORIES OF ANALYTIC STUDIES
Experimental/ Interventionalexposure to the factor or
treatment under study controlled by investigator
randomized clinical trial (rct), community trial, laboratory trial
ANALYTIC STUDIES3. Types
Case-control Studiescases (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
CASE-CONTROL STUDIES
1. Usesto test risk factors preferred if disease is rarepreferred if several factors are
associated with disease of interest
CASE-CONTROL STUDIES
2. Requirements for valid resultsCases 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.
CASE-CONTROL STUDIES
3. AnalysisOdd’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
ANALYSIS OF CASE CONTROL STUDIES
Outcome (Disease) + -
+ a bExposure (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
CASE-CONTROL STUDIES
4. Advantagesmore economical smaller sample size requiredsuitable for rare diseasessuitable for diseases associated
with multiple exposures
CASE-CONTROL STUDIES
5. Disadvantagesmore susceptible to bias of recallestimate of risk is indirectcontrols more difficult to assembletemporal relationship between
factor and outcome cannot be ascertained
TYPES OF ANALYTIC STUDIES
Cohort Studiesgroups 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
COHORT STUDIES
1. Usesto test prognostic factorsto directly measure risk of development of disease or outcome
provide more definitive information about disease etiology
preferred for study of rare exposures
COHORT STUDIES2. Requirement for valid results
Similarity of comparison groups
3. Typesconcurrent
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.
TYPES OF COHORT STUDIES
. Non-concurrentSubjects 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.
COHORT STUDIES4. 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
ANALYSIS OF COHORT STUDIES
Outcome (Disease)
+ -
+ a bExposure (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
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)
COHORT STUDIES5. Advantages
provides direct estimate of risktemporality can be ascertained (for concurrent
studies)less biases of recall and observationallows for determination of population-based ratescontrols easier to assemblevariations in exposure can be followed-upunsuspected effects of the exposure may be
observed
COHORT STUDIES6. Disadvantages
more expensivefollow-up period may be longhigh attrition ratelarge sample size requiredchange in exposure rates over long
periods of time
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
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
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 trialsused if strong evidence for association
already exists
TYPES OF EXPERIMENTAL STUDIES
Field or community trialssubjects 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
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
EXPERIMENTAL STUDIES
2. Analysiscomparison of disease or outcome
rate in experimental (P1) = (a/a+c) and
control groups (P2) = (b/b+d)
ANALYSIS OF EXPERIMENTAL
STUDIESTherapeutic / Preventive Measure + -
+ a b
Disease/ Outcome - c d
a+c b+d
ANALYSIS OF EXPERIMENTAL
STUDIES
Protective Value = P2 – P1
P2
EXPERIMENTAL STUDIES
3. AdvantageProvide the strongest evidence for
testing hypothesis
4. Limitationethical issues, especially for clinical
trials
Determination of Disease Etiology
II. Assessment of Results1. determine if statistical association
between factor and outcome occurs2. if association exists, determine if due to:
chance perform significance testing
extraneous or confounding variablesmatchingspecification or restrictionstandardization of rates stratified analysis
Assessment of Results
if association exists, determine if due to:
causal relationshipo criteria:
1. measures of strength of association – OR, RR, Protective Value2. temporality – exposure occurred prior
to outcome3. dose-response relationship
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