Unit 2 – Public HealthEpidemiology

Chapter 4 – Epidemiology: The Basic Science of Public Health

EpidemiologyThe diagnostic discipline of public healthA major part of public health’s assessment

functionInvestigates causes of diseasesIdentifies trends in disease occurrenceEvaluates effectiveness of medical and public

health interventionsAn observational science

Patterns of Disease OccurrenceWho is getting the disease?When did they get the disease?Where is the disease occurring?From this information, epidemiologists can

infer why the disease is occurring.

Epidemic SurveillanceEndemic vs. EpidemicNotifiable diseasesRecognition of new diseaseIncreased importance with threat of

bioterrorism

Outbreak InvestigationVerify the diagnosisConstruct a working case definitionFind cases systematically – active

surveillanceAsk the who, where, and when questions to

describe the epidemic by person, place, and time. Consider the incubation period

Look for common source of exposure

Question #1What has epidemiology contributed to

peoples understanding of Heart Disease?Lung Cancer?

Epidemiology and Chronic DiseasesIdentify risk factorsObserve long-term trendsHeart disease – leading cause of death in U.S.

Framingham Study – started in 1948Lung cancer and smoking – early 1950s

British Physicians’ StudyHammond-Horn study in U.S.

Unit 2 – Public HealthEpidemiology

Chapter 5 – Epidemiologic Principles and Methods

Definition of EpidemiologyEpidemiology is the study of the distribution

and determinants of disease frequency in human populations

Step 1: Define the DiseaseDeath is easy to determine – death

certificates have cause of deathSome diseases need blood tests or stool

cultures to verify diagnosisSome diseases are hard to define – e.g.

EMS, SARSSometimes definition changes as more is

learned – e.g. AIDSOther health outcomes – injuries, risk

factors

Question #2 Explain the interaction between incidence,

prevalence and prognosis. Give examples

Disease FrequencyCount number of people with disease and

relate to the population at risk (PAR)PAR (denominator) may be total population

or exposed population, or one gender or age group; often comes from census

Two ways to measure frequency:Incidence – number of new casesPrevalence – number of existing cases

Incidence is used for studying causes of disease

Disease Frequency, ctd.If causes or risk factors increase, incidence

and prevalence increaseIf ability to diagnose increases, incidence and

prevalence increasePrevalence depends on incidence and

prognosis

Question #3Why are the who, when, and where questions

useful in determining the causes of disease?  Give examples.

Distribution of DiseaseWho – sex, age, occupation, raceWhen – season, year (long-term trends),

elapsed time since an exposure (epidemic curve)

Where – neighborhood (e.g. clusters), latitude (climate), urban vs. rural, national variations

Determinants of DiseaseWhy is distribution as it is?Can make inferences from distributionRisk factors

Human PopulationEpidemiology observes humans, in contrast

to biomedical sciences, which can do experiments on laboratory animals

Question #4Explain the three major types of

epidemiologic studies. 

Kinds of Epidemiologic StudiesGoal is to determine an association between

an exposure and a disease or other health outcome

May be prospective or retrospectiveIntervention studyCohort studyCase-control study

Question #5Which is most likely to yield a valid result? 

Why?

Intervention StudyClosest thing to an experimentStart with two groups: experimental group

(gets the intervention or exposure) and control group

Watch them over time and compare outcomesExperimenter chooses who is in which groupTwo groups should be as similar as possible

so that intervention is the only difference

Intervention Study, ctd.Randomized, double-blind, placebo control

is the idealPharmaceutical companies conduct many

clinical trials for new drugsPhysicians’ Health Study – prevention study

Aspirin to prevent heart diseaseBeta carotene to prevent cancer

Field trial of polio vaccine – 1954Kingston-Newburgh study of fluoridation to

prevent tooth decay

Cohort StudyFor situations when doing an intervention

study would be unethical or too difficultConsidered the next most accurateChoose a large number of healthy people,

collect data on their exposures, and track outcomes over time

The only difference from intervention is that people choose their own exposures

Cohort Study -- examplesFramingham Heart StudyNurses’ Health StudyBritish physicians study on smoking and lung

cancerHammond-Horn study on smoking and lung

cancer in U.S.

Question #6Which is likely to yield an answer in the

shortest period of time? Why?

Case-Control StudyChoose people who already have diseaseChoose a healthy control group of individuals

as similar as possible to casesInterview them all and ask for their previous

exposuresAdvantage: faster and cheaperLeast accurate approach

Unit 2 – Public HealthEpidemiology

Chapter 6 – Problems and Limits of Epidemiology

Problems with Studying HumansIntervention study: subjects may not follow

prescribed behavior throughout study periodCohort study: sometimes hard to isolate

which of many factors are responsible for health differences

Case-control study: control group may not be truly comparable; also errors in reporting or recall

For all studies, must worry about differential drop-outs

Sources of ErrorRandom variationConfounding variablesBias

Selection biasReporting bias or recall bias

Factors that Lend Validity to ResultsStrong associationDose-response relationshipKnown biological explanationLarge study populationConsistent results from several studies

Hormone Replacement TherapyConflicting results from two major studiesClinical trial is the gold standardResults of cohort study were confounded by

associated factors that made women taking HRT healthier, even without the therapy

Ethical IssuesNazi experiments on humansTuskegee syphilis studyNew rules – informed consentNew rules -- institutional review boardsImportance of clinical trialsPossibility of conflict of interest by medical

providers who stand to profit

Question #7Look up the Tuskegee Syphilis Study?  Why

was it unethical?  What influence has it had on the conduct of clinical trials?