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Introduction of Epidemiology

Weiqing Chen PhD Prof.Department of Biostatistics and EpidemiologyTel: 87332199 E-mail:

wqchen@gzsums.edu.cn

Contents

Brief historyDefinitionPrinciple and utilizationResearch methodTraits of EpidemiologyRelationship with other

disciplines and prospect

Part One: Brief history

• Originating form long term accumulated experience of humankinds’ fighting against disease

• Originating from development and improvement of research methods– From description to analysis– From qualitative to quantitative

Early History: before form of epidemiology

• From civilization of humankinds to 18th century

Earlier notions of etiological reasoning by Hippocrates (5th Century B.C.):

Observed seasons of the year and their effect

Change in the environment followed by change in water used (hard, soft)

Peoples’ behavior and health (drinking, eating, exercise and labor)

Diseases had natural rather than supernatural causes

《 Suwen▪on acupuncture 》：“ When five yi (epidemic) come ， all person are easy infected ， symptoms and signs are similar in both youth and aged” 。

Han Dynast 《 On balance▪about life 》“ When Yi occurs, thousands families are destroyed”

“疫 -Yi” refers to epidemic Gold, Wood, Water, Fire, Earth

Negative: Positive

Middle of 15th century ， Italy establishing quarantine

Shui Dynasty setting up “ 疠人坊” for quarantining lepra patients

1546 - ‘Living contagion’ proposed by Girolamo Fracastoro, De Contagione (1546)

1700 - ‘Miasma theory’, air quality is responsible for disease

• John Graunt (Observations on the Bills of Mortality London,1662):– Weekly report on birth and mortality– Patterns of disease occurrence : noted h

igher mortality for men Vs. women– Seasonal variation – Recognized value of routinely collected

data on human illness

Early History: form of epidemiology

• From end of 18th century to early stage of 20th century

• 1747 - Lind performed first ‘experiment’ to establish cause and treatment of scurvy

• 1768 - Jenner first vaccinated human with cowpox

• Cholera epidemic and John Snow (London, 1855)

• Formulated and tested hypothesis of increased risk associated with water supply - induction and deduction

Snow’s Original Map of Cholera cases in Proximity to the Broad Street water pump

His nomenclature and classification of disease,

accepted by the International Statistical Congress in Paris in 1864, remains the bedrock for the International Classification of Diseases, now in its 10th revision

Farr’s Law of Epidemics

William Farr (London, 1807-1883) establishes the death registryAnnual reports and application of vital statistics Recognized that mortality data could be used to learn about disease

• Farr also initiated comparison studies– Married Vs. singles– Metal miners Vs. earth ware industry workers

• Epidemiological issues:– Population at risk, comparison group,effect of

age, duration of exposure, health status

• Establishing the first committee of epidemiology in London, which implicating the form of epidemiologic discipline

• Loius Pasteur (1822~1895) ：Proving disease is caused by microbiology and vaccination is the best way to preventing disease 。

• Robert Koch

(1843~1910) ： establishing the criteria of bacteria to causing disease.

Modern Perspective

• Modern Epidemiology - – Deductive / Inductive process– Logic and reasoning– Causation– Causation in Epidemiology– Estimation, statistical testing, and bias

• 1945 - Fundamental shift in paradigm after World War II to study chronic diseases

• 1948 - Framingham cohort study begun to study coronary artery disease

• 1956 - Discovery of DNA by Watson and Crick• 1958 - Doll and Hill apply case-control study

design to establish smoking as cause of lung cancer

• 1980 - DNA fingerprinting, Micro-computing industry begins

• 1982 - AIDS epidemic recognized in US, Cox proportional hazards regression analysis

Development of Epidemiology in China

Before Liberation

Wu Liande （ 1879 ～ 1960 ） : famous Chinese epidemiologist.

He made serious surveys on rat plague from 1910 to 1920 in Northern of China, and firstly proved that rat plague was spread through air in the world.

After Liberation

Epidemic station, Research institute for vaccine, establishment of health system

Law for management of communicable diseases

Expanded Program for Immunization

Law for coping with emergency public health events

Su Delong （ 1906 ～ 1985 ） : modern epidemiologist in China.

He conducted a lots of studies on schistosome and cholera.

He suggested that liver cancer was associated with quality of drink water.

Part Two: Definition of Epidemiology

Epidemiology• epi:

– Indo-European root, opi, near, at, or against

– Greek root, epi, over, on

• - dem: – Indo-European root, da, meaning ‘to divide’

– Greek root deme, meaning a township, or division, of ancient Attica; whence demos, for people

• -logy:– Indo-European root, leg-, to collect, with derivatives m

eaning ‘to speak’

– Greek root logos, speak, word, or reason

– Contemporary English, a branch of learning

Epidemiology• Epidemic - ‘all over the people’• Epidemiology

– A branch of learning that deals with events or conditions that are ‘all over the people’

– A branch of medicine that deals with the cause, distribution, and control of disease in populations (American Heritage Dictionary of the English Language, third edition)

Epidemiology: Definitions• The study of the distribution of a disease or a physiological

condition in human populations and of the factors that influence this distribution. Lilienfeld

• The study of disease occurrence in human populations. Friedman

• The study of the occurrence and distribution of disease and other health-related conditions in [human] populations. Kelsey

• Patterns of disease occurrence in human populations according to person, place, and time

Definition of epidemiology

“Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the applications of this study to the control of health problems and evaluating their effects.”

Three levels of research contentDiseaseInjuryHealth

Three stages of taskExploring phenomenonFinding causeProviding measures

Explaining of Epidemiologic Definitions

Three categories of workDescriptive epidemiologic methodsAnalytical epidemiologic methodsExperimental epidemiologic methods

Three basic methodsObservationExperimentMathematics

Three essential factorsPrincipleMethodsUtilization

Part Three: Principle and Utilization

Epidemiology: Models

• Host, Agent, and Environment model

• Person, Place and Time model

• Exposure-Outcome model

• Cause-Effect model

• Observational - Experimental

• Mathematical and Statistical model

The epidemiologic triad of a disease

Host

Vector

Agent Environment

Host, Agent, Environment

Population

Environment

Host Agent

Host, Agent, EnvironmentHost Agent Environment

Age Sex Race/Ethnicity Religion SES Marital status Lifestyle Exercise Behavior Co-morbidity Genetic makeup

Biologic Microorganisms Chemical Toxins, tobacco, alcohol, drugs Physical Trauma, radiation, fire Nutrition Lack of, excess

Disease vectors Population density Substances in surroundings and workplace Air quality Weather Noise Food and water sources Special environments: Hospitals, day-care, institutions, bath houses, crack houses, refugee camps

Person, Time, and Place

• Individual risks and disease occurrence is examined in terms of geographic location and calendar time.

• Time and Place are used to link individuals – Chain of transmission, e.g., infectious diseases

– Clustered event, e.g., infectious disease, environmental exposures

Exposure - Outcome Model• Exposure and outcome are defined

• Conceptual model is proposed that connects the exposure to outcome

• Exposure can have two components:– Likelihood of exposure – Likelihood of outcome after exposure

• More complicated model accounts for confounding and interaction

Exposure - Outcome Model

Exposure Outcome

Exposure Outcome

Confounder(s)

Simple Model

Complicated Model

Spectrum of Disease

Healthy

Disease Onset

Symptoms

Seek Care

Diagnosis

TreatmentOutcomeCureControlDisabilityDeath

Infectious Disease EpidemiologyConceptual Model

Uninfected Infected Diseased Removed

Mediators of InfectionEnvironmentNon-immune defensesGenetic susceptibilityPrevious immunityPathogen virulence

Mediators of DiseaseHost immunityPathogen virulenceMedical intervention

Principle of disease and health distribution in population

“Iceberg” concept

Cause - Effect Model

• One goal of Epidemiology is to establish cause and effect between an exposure and an outcome

• Cause-effect models are multi-disciplinary and draw upon expertise in other fields to establish:– Biologic plausibility (biology, medicine)

– Dose-response relationship (pharmacology, biology)

– Specificity of exposure (clinical medicine)

– Strength of association, replication and consistency of findings (biostatistics)

Observational-Experimental Model• This model make a distinction between two broad

approaches in Epidemiology• Observational study

– The investigator does not determine the nature or extent of exposure

– e.g., cohort study after exposure to environmental toxin

• Experimental study– The investigator determines the nature and extent of

exposure– e.g., randomized clinical trial of new therapeutic agent

Mathematical and Statistical Models

• Mathematical Models– Simplified, mathematical representations of

biologic processes– Analysis focuses on the characteristics of this

representation

• Statistical Models– Mathematical approach to find and understand

patterns in data– Fundamental tool in the analysis of

epidemiologic data

Prevention

• Primary Prevention

• Secondary Prevention

• Tertiary

Part Four: Utilization

1. To identify the etiology or the cause of a disease and the risk factors - that is, factors that increase a person’s risk for a disease.

2. To determine the extent of disease found in the community.

3. To study the natural history and prognosis of disease.

4. To evaluate new preventive and therapeutic measures and new modes of health care delivery.

5. To provide the foundation for developing public policy and regulatory decisions relating to environmental problems.

6. Disease Surveillance

Part Five: Epidemiologic methods

Observational and Analytical Epidemiology

• Descriptive epidemiology– the occurrence of disease (cross-sectional)

• Etiologic epidemiology (quasi-experimental)– cause of disease (cohort, case-control)

• Experimental epidemiology– assessment of therapy (cohort, randomized

clinical trial)

Research Design Used in Research Design Used in EpidemiologyEpidemiology

• Case Reports / Case Series

• Case control

• Cohort: prospective or retrospective

• Cross sectional

• Qualitative (Randomized Clinical Trials)

• Secondary data analysis

Part Five: Traits of Epidemiology

Population

Comparison

Probability

Sociological medicine

Prevention

Multiple etiology

Part Six: Epidemiology: Future

• Molecular biology will revolutionize the way epidemiologists evaluate gene-environment interaction

• Micro-computing will become more sophisticated and more widely available

• Biomedical science will make significant advances that will require evaluation of findings in populations

• Evidenced medicine-Producing evidences

Part Seven: Relationship with other disciplines

• Basic medicines

• Clinical medicines

• Statistics and computer

• Other disciplines

Epidemiology

Clinical epidemiology

Nutritional epidemiology

Molecular epidemiology

Genetic epidemiology

Pharmacoepidemiology

Social epdemiology

New embranchment

Clinical epidemiology

• Clinical epidemiology is the application of epidemiologic principles and methods to problem encountered in clinical medicine.

• It is a science concerned with counting clinical events occurring in intact human beings, and it uses epidemiologic methods to carry out and analyze the count.

Part Eight: Why do Medical students study

Epidemiology?

• Many clinical decisions are based on information which is uncertainty and expressed as a probability.

• That probability is best estimated by means of past experience with similar patients.

• Clinical true natures may be distorted or misled by subjective observation

• To deal with this misleading, clinical observation should be based on sound scientific principles, which requires an understanding of the design of human research.

• Because clinical observation also can be influenced by the play of chance, interpretation of the observation requires an understanding of statistics.

• Understanding these principles is as important to clinicians who whish to be self-sufficient in judging clinical information as it is to researches who will produce research.

• Although the principles of clinical scientific methods are implicit in a great deal of current clinical teaching, they are not a formal part of most medical curricula.

• Epidemiology – The logic of modern medicine

• Epidemiology is the methodology of medicine

Epidemiology & Clinical Practice

• The practice of medicine is dependent on population data

• Diagnosis – population-based process• Prognosis – the prediction of disease; population-

based• Selection of therapy – population-based;

randomized clinical trials– Based on how groups of people respond to certain

therapies

Students should be able to:

• Identify and interpret the meaning of risk factors for health problems

• Use comparative reasoning to evaluate risks for health conditions

• Apply scientific method of investigating a disease outbreak to real-life situations affecting health

Students should be able to:

• Understand and use the basic concepts of mathematics and statistics in assessing health risks

• Develop an epidemiological case definition• Design a basic case-control study for investigating

a disease outbreak

Students should be able to:

• Define and use selected medical and epidemiologic terms

• Describe the epidemiology and clinical features of selected health problems