1. Introduction to Epidemiology

2. Introduction
Edward Jenner ( ) developed a vaccine against small pox using cow pox, 160 years before virus was identified
John Snow ( ) described an association between dirty water and cholera, 44 years before Vibrio cholera was identified
Hill and Doll in a case control study demonstrated that smoking and lung carcinoma are related
Framingham study ( 50 years follow up)showed people a certain group of people were at risk

3. Intro. Fundamentals of epidemiology Sources of data Causation
Review of terms

4. Classically speaking Epi = upon Demos = people Ology = science
Epidemiology = the science which deals with what falls upon people…..
Bridge between biomedical, social and behavioral sciences

5. Definition
Study of the occurrence and distribution of health-related diseases or events in specified populations, including the study of the determinants influencing such states, and the application of this knowledge to control the health problem
(Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)

6. Who is an epidemiologist ?
A professional who strives to study and control the factors that influence the occurrence of disease or health-related conditions and events in specified populations and societies, has an experience in population thinking and epidemiologic methods, and is knowledgeable about public health and causal inference in health
(Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)

7. • Public health: because of the emphasis on disease prevention
Requirements in Epi
• Public health: because of the emphasis on disease prevention
•Clinical medicine: because of the emphasis on disease classification and diagnosis (numerators) 
• Pathophysiology: because of the need to understand basic biological mechanisms in disease (natural history)
• Biostatistics: because of the need to quantify disease frequency and its relationships to antecedents (denominators, testing hypotheses)
• Social sciences: because of the need to understand the social context in which disease occurs and presents (social determinants of health phenomena) 7

8. Purposes of Epi
To investigate nature / extent of health-related phenomena in the community / identify priorities
To study natural history and prognosis of health-related problems
To identify causes and risk factors
To recommend / assist in application of / evaluate best interventions (preventive and therapeutic measures)
To provide foundation for public policy 8

9. Traditional Epidemiology Triangle
Environment
Time
Host
Agent

10. Modern Epidemiology Triangle
Causative Factors
Time
Groups
of Populations
Physiological Factors,
Environment,
Behavior, Culture
Ecological Elements

11. Disease Stages in Individuals
Clinical disease: characterized by signs and symptoms
Nonclinical disease:
Preclinical: not yet clinically apparent
Subclinical: not clinically apparent and stays that way
Chronic: persists for years (can also refer to clinical disease

12. Disease Stages in Populations
Endemic: the level of a disease that is “normal” for a population
Epidemic: when the level of disease occurs in excess of the normal level for that population
Pandemic: an epidemic that occurs across many populations or even the entire world

13. Modes of Transmission Direct: person to person
Indirect: needs an intermediate item (water, food, flies, etc.)

14. Disease Transmission by People
Carriers: exposed and harbors disease
Active: can transmit even though they are better
Convalescent: transmit while convalescing
Healthy or passive: can transmit even though they never get sick
Incubatory: transmit before developing symptoms

15. Case Concepts Case: person with the disease/problem
Primary case: first case in a population
Index case: first identified case
Suspect case: all signs of disease but no diagnosis
Primary case: the individual who introduces the disease into the family or group under study, not necessarily the first diagnosed cases in a family or group.
Index case: First case that comes to the attention of investigators

16. Case Concepts Case definition: standard criteria for case
Case severity: length of hospital stay, disability, fatality, etc

17. Levels of Prevention Primary: prevent acquisition of disease
Secondary: screening for early detection of disease
Tertiary: prevention of disability from disease

18. Epidemiologic ( scientific ) Approach
1. Identify a PROBLEM : clinical suspicion ; case series ; review of medical literature
2. Formulate a HYPOTHESIS ( asking the right question ) ; good hypotheses are: Specific, Measurable, and Plausible
3. TEST that HYPOTHESIS ( assumptions vs. type of data )
4. always Question the VALIDITY of the result(s) : Chance ; Bias ; and Causality

19. Epidemiologic Study: threats to Validity
Chance : role of random error in outcome measure(s) ( p - value ; power of the study and the confidence interval ) --- largely determined by sample size
Bias : role of systematic error in outcome measure(s)
Selection bias - subjects not representative
Information bias - error(s) in subject data / classification
Confounding - 3rd variable (causal) assoc. w/ both X and Y

20. What is a hypothesis? An educated guess an unproven idea
based on observation or reasoning, that can be proven or disproven through investigation.

21. What goes into a hypothesis?
Characteristics of the disease
The illness
Established modes of transmission
Distribution
In time
By place
By person

22. Hypothesis (1st Step in the Epidemiologic Approach)
Developed prior to a study to state research purpose
Is established to either accept or refute

23. Ideal Hypothesis Should include The population
The cause being considered
Expected effect
Dose-response relationship
Time response relationship

24. Sources of Data Vital records for epidemiologic study
Birth certificates
Death certificates
Marriage licenses

25. Determining Cause
Relationship
Association
Cause
Real or spurious??

26. Definition of Cause
Cause: an antecedent event, condition, or characteristic that was necessary for the occurrence of the disease at the moment it occurred given that other conditions are fixed

27. Types of Cause
Direct cause: factor causes outcome without any intermediate factor (without intermediate steps)
Indirect cause: factor causes disease but with an intermediate factor (with intermediate step or steps)

28. Evolution of Causal Theory
Koch’s Postulates
A. B. Hill’s Criteria for Causation (public health perspective)
Current - Epidemiologic specific guidelines for cause

29. Koch’s Postulates Causative organism is always present with disease
Causative organism is not found in any other disease
An organism isolated from a case is able to produce disease
Isolated organism can be grown in the laboratory

30. Hill’s Criteria of Causation
Consistency with other investigations
Strength of association
Specificity
Dose response relationship
Temporal relationship
Biologic plausibility
Coherence
Experiment
The association is consistent when results are replicated in studies
Defined by the size of the risk as measured by appropriate statistical tests
Established when a dingle putative cause produces a specific effect
An increasing level of exposure (amount or time) increases the risk.
Exposure always precedes the outcome. This is the only essential criterion.
Association agrees with currently accepted understanding of pathobiological processes.
Association should be compatible with existing theory and knowledge
The condition can be altered (prevented or ameliorated) by an appropriate experimental regimen

31. Epidemiologic-Specific Guidelines for Cause
Cause should be distributed the same as outcome
Incidence should be higher in exposed
Exposure should be higher in deceased
Temporal sequence
Dose response relationship
Cause and effect should be associated even with different study designs

32. Epidemiologic-specific Guidelines
high magnitude of association
Control methods should work
Individual control methods should result in a change in whole population
Consistency with animal studies
Biologic plausibility

33. Causal Relationships Necessary and sufficient
without the factor disease will not develop
with the factor disease always develops (HIV/AIDS)
Necessary but not sufficient
factor present, but low dose
part of a group of multiple factors
(carcinogens/cancers)

34. Causal Relationships Sufficient but not necessary
factor can act alone to cause disease, but other factors can cause the same disease
(radiation/leukemia)
Neither sufficient nor necessary
complex model
(hypertension)
Causal complement
set of conditions necessary and sufficient for a factor to produce disease, rapid, synergistic
(smoking & asbestos/lung cancer)

35. Broad Types of Epidemiology
DESCRIPTIVE EPI
ANALYTIC EPI
Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person. We try to formulate hypothesis, look into associations ?
Typical study design:
community health survey
(synonyms: cross-sectional study, descriptive study)
Testing a specific hypothesis about the relationship of a disease to a specific cause, by conducting an epidemiologic study that relates the exposure of interest to the outcome of interest (? Cause-effect relationship) Typical study designs: cohort, case-control, experimental design 35

36. To undertake an analytic epidemiologic study you must first:
Descriptive Epidemiology Is A Necessary Antecedent of Analytic Epidemiology
To undertake an analytic epidemiologic study you must first:
Know where to look
Know what to control for
Be able to formulate / test hypotheses compatible with a-priori lab / field evidence 36

37. Basic Triad of Descriptive Epidemiology
THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY ARE:
PERSON
PLACE
TIME 37

38. Personal Characteristics (whom)
Age
Gender
Socio-economic status (education, occupation, income)
Marital status
Ethnicity/race/genetic profile
Behavior / habits 38

39. Place (where ?)
Geographically restricted or widespread (outbreak, epidemic, pandemic)? Off-shore (tsunami…)
Climate effects (temperature, humidity, combined effects..)
Urban / sub-urban-squatter / rural  
Relation to environmental exposure
(water, food supply, etc)  
Multiple clusters or one? 39

40. Time (when ?) Changing or stable?
Clustered (epidemic) or evenly distributed (endemic)?
Time-trends: Point source, propagated, seasonal, secular, combinations 40

41. What designs do epidemiologists use ?
Qualitative designs
Quantitative designs
Observational
Experimental
Building evidence

42. What measures do epidemiologists use ?
Frequency measures
Effect measures
Impact fractions

43. Measuring Disease Frequency
Classifying and categorizing disease
Deciding what constitutes a case of disease in a study
Finding a source for ascertaining the cases
Defining the population at risk of disease
Defining the period of time of risk of disease
Obtaining permission to study people
Making measurements of disease frequency
Relating cases to population and time at risk 43

44. Agents Biological (micro-organisms)
Physical (temperature, radiation, trauma, others)
Chemical (acids, alkalis, poisons, tobacco, others)
Environmental (nutrients in diet, allergens, others)
Psychological experiences 44

45. Host Factors Genetic endowment Immunologic status
Personal characteristics
Personal behavior
Definitive versus intermediate (in vector-borne diseases) 45

46. Environment
Living conditions (housing, crowding, water supply, refuse, sewage, etc)
Atmosphere / climate
Modes of communication: phenomena in the environment that bring host and agent together, such as: vector, vehicle, reservoir, etc) 46

47. Epidemiology as a problem solving discipline: Integrating principles
The first integrating principle is that epidemiology is an information science.
The second integrating principle is that epidemiology operates within an environment of complex systems.
Third integrating principle is that epidemiology is not just a scientific discipline but a professional practice area.

48. Epidemiology is an information science
Data generated by epidemiologists is to be used for decision making.
Epidemiology is purposive: methods and knowledge are to be used for the ultimate purpose of prevention of disease, disability and death
Epidemiology is under public scrutiny.
Information affects decisions at the public policy level, at the level of individuals, and by health professionals. A social responsibility.

49. EPIDEMIOLOGIC  PROCESS OF  INTERVENTION
INFORMATION  DECISION  ACTION
GENERATION PROCESS
EPIDEMIOLOGIC  PROCESS OF  INTERVENTION
METHODS INFERENCES

50. Prevalence
How many cases of disease already exist in a population at a specific time
Proportion
Static measure; time is frozen
Why would you want to know about prevalence of a disease?

51. Calculation of Prevalence
# of current cases ( old and new)
Size of population at risk at that time
Prevalence is a proportion and can never exceed 100%

52. Expressing Prevalence
Prevalence of diabetes in Clinic X:
116/184 =0.63
63%
630 per 1000
6300 per 10,000

53. Incidence
The rate at which new cases of disease develop in a population over time
e.g. how many new cases of diabetes were diagnosed in one year at Clinic X?
When would you want to know incidence versus prevalence?

54. Incidence Calculation
# of new cases or number of events
population at risk during the same period
(for a long period use the midpoint of the population level)
Incidence is a measure of risk
All persons should be disease-free at the beginning of the interval*
*in practice they aren’t always excluded

55. Calculating Incidence
Among 60 people attending a 12 month residential detoxification program, 50 tested HIV negative at the start of the program in Jan At the end of the program in Dec 1998, 3 of the 50 tested positive for HIV.
What is the cumulative incidence of HIV infection?
3/50 = 6% per year
It is important to pay attention to the time period. A cumulative incidence of 6% per month is a much greater rate than 6% per year.

56. Calculating Incidence
3/50 = 6% per year
It is important to pay attention to the time period. A cumulative incidence of 6% per month is a much greater rate than 6% per year