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