Introduction To Epidemiology Mr. Manuel

What is Epidemiology?

Definition of Epidemiology Last JM: A Dictionary of Epidemiology The study of the distribution and determinants of health related states and events in populations and the application of this study to control of health problems

What Is The Unique Skill Of Epidemiologists? Measuring disease frequency in populations

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

Two Broad Types of Epidemiology Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of time, place, and person. Typical study design: community health survey (approximate synonyms - cross-sectional study, descriptive study) community health survey (approximate synonyms - cross-sectional study, descriptive study) Testing a specific hypothesis about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest. Typical study designs: cohort, case-control DESCRIPTIVE EPIDEMIOLOGYANALYTIC EPIDEMIOLOGY

The Basic Triad Of Descriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY ARE:  PERSON  PLACE  TIME

Person (characteristics?)  Age  Gender  Socio-economic status (education, occupation, income)  Marital status  Ethnicity/race/genetic profile  Behavior / habits

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

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

Data Collection Methods  Primary: where the investigator is the first to collect the data. Sources include: medical examinations, interviews, observations, etc. Pros: less measurement error, suits objectives of the study better. Cons: costly, may not be feasible. Cons: costly, may not be feasible.  Secondary: where the data is collected by OTHERS, for other purposes that those of the current study. Sources include: individual records (medical / employment); group records (census data, vital statistics) Sources include: individual records (medical / employment); group records (census data, vital statistics)

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 hypotheses compatible with laboratory evidence

A common error in epidemiology is moving to analytic epidemiology without having a solid base in the descriptive epidemiology of the condition is useless.

Epidemiologists are required to have some knowledge of the disciplines of the following: 1.public health, because of the emphasis on disease prevention. 2.clinical medicine, because of the emphasis on disease classification and diagnosis.

3.pathophysiology, because of the need to understand basic biological mechanisms in disease. 4.statistics, because of the need to quantify disease frequency and its relationships to antecedents. 5.social sciences, because of the need to understand the social context in which disease occurs and presents.

Purposes Of Epidemiology 1. Identify causes and risk factors for disease. 2. Determine the extent of disease in the community. 3. Study natural history and prognosis of disease. 4. Evaluate preventive and therapeutic measures 5. Provide foundation for public policy

MenWomenChildrenTotal 1 st class 67%3%038% 2 nd class 92%14%059% 3 rd class 84%54%66%62% Total82%26%48%62% EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC CHARACTERISTIC DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE WHAT CAUSE OF DEATH IS THIS?

The previous slide shows death rates by class of ticket on the Titanic, a large ocean liner that sank after colliding with an iceberg in 1912

Pandemic Influenza Phases  Time:Mondays and Wednesdays 4:10-5:20 p.m.  Office hours: BY ARRANGEMENT  Place: Room A -131 East Fee Hall –Department of Epidemiology classroom.  Person: Nigel Paneth, Instructor.  ;

WHO phases of pandemic alert - Phase 4  Verified human-to-human transmission of an animal or human-animal influenza reassortment virus able to cause “community-level outbreaks.”  The ability to cause sustained disease outbreaks in a community marks a significant upwards shift in the risk for a pandemic.  Indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion.

Phase 5  Human-to-human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage  Is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short.

Phase 6 (pandemic phase)  Community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5.  Indicate that a global pandemic is under way.

Differences Between Laboratory Sciences And Field Sciences In the Field:  Mostly observational  Mostly observational  Variables controlled by nature  Variables controlled by nature  Some variables unknown  Some variables unknown  Replication difficult; exact replication impossible  Replication difficult; exact replication impossible  Results often uncertain  Results often uncertain  Meaning of results for humans clear  Meaning of results for humans clear  Statistical control often very important  Statistical control often very important  Highly labor intensive

Differences Between Laboratory Sciences And Field Sciences In the Laboratory: Mostly experimental Mostly experimental Variables controlled by the investigator Variables controlled by the investigator All variables known All variables known Replication easy Replication easy Results valid Results valid Meaning of results for humans uncertain. Meaning of results for humans uncertain. Little need for statistical manipulation of data. Little need for statistical manipulation of data. Highly equipment intensive Highly equipment intensive

The Basic Triad Of Analytic Epidemiology THE THREE PHENOMENA ASSESSED IN ANALYTIC EPIDEMIOLOGY ARE: HOST ENVIRONMENTAGENT

Host  Age  Socio-economic status  Gender  Ethnicity/Race  Behavior

Agent  Living/Non-living  Species  Behavior  Habitat  If the agent is human, gender, race etc. are factors  Examples: Nutrients, Poisons, Allergens, Radiation, Physical trauma, Microbes, Psychological experiences

Host Factors  Genetic endowment  Immunologic state  Age  Personal behavior

Environment  Crowding  Atmosphere  Modes of communication – phenomena in the environment that bring host and agent together, such as: –Vector –Vehicle –Reservoir

Environment  Geographically restricted or widespread (pandemic)?  Geographically restricted or widespread (pandemic)?  Relation to water or food supply.  Relation to water or food supply.  Multiple clusters or one?

Analytical Epidemiology

Study Development Process Analyze results and retest Model Building and hypothesis formulation Analytic Studies for Hypothesis testing Descriptive Studies: Data Collection and Analysis

Study Designs - Analytic Epidemiology  Experimental Studies –Randomized controlled clinical trials –Community trials  Observational Studies –Group data  Ecologic –Individual data  Cross-sectional  Cohort  Case-control

Experimental Studies  treatment and exposures occur in a “controlled” environment  planned research designs  clinical trials are the most well known experimental design. Clinical trials use randomly assigned data.  Community trials use nonrandom data

Observational Studies  non-experimental  observational because there is no individual intervention  treatment and exposures occur in a “non-controlled” environment  individuals can be observed prospectively, retrospectively, or currently

Cross-sectional studies  An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population) time Study only exists at this point in time

Cross-sectional Studies  Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions)  It measures prevalence, not incidence of disease  Example: community surveys  Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression

Cross-sectional Design time Study only exists at this point in time Study population No Disease Disease factor present factor absent factor present factor absent

Epidemiologic Study Designs  Case-Control Studies –an “observational” design comparing exposures in disease cases vs. healthy controls from same population –exposure data collected retrospectively –most feasible design where disease outcomes are rare

Case-Control Studies Cases: Disease Controls: No disease

Case-Control Studies  Strengths –Less expensive and time consuming –Efficient for studying rare diseases  Limitations –Inappropriate when disease outcome for exposure is not known at start of study –Exposure measurements taken after disease occurrence –Disease status can influence selection of subjects

Case-Control Design Study population Cases (disease) Controls (no disease) factor present factor absent factor present factor absent present past time Study begins here

Epidemiologic Study Designs  Cohort Studies –an “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposure –looking for a difference in the risk (incidence) of a disease over time –best observational design –data usually collected prospectively (some retrospective)

Cohort Design time Study begins here Study population free of disease Factor present Factor absent disease no disease disease no disease present future

Timeframe of Studies  Prospective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future time Study begins here

Prospective Cohort study Measure exposure and confounder variables Exposed Non-exposed Outcome Baseline time Study begins here

Timeframe of Studies  Retrospective Study - “to look back”, looks back in time to study events that have already occurred time Study begins here

Retrospective Cohort study Measure exposure and confounder variables Exposed Non-exposed Outcome Baseline time Study begins here

Cohort Study  Strengths –Exposure status determined before disease detection –Subjects selected before disease detection –Can study several outcomes for each exposure  Limitations –Expensive and time-consuming –Inefficient for rare diseases or diseases with long latency –Loss to follow-up

Experimental Studies  investigator can “control” the exposure  akin to laboratory experiments except living populations are the subjects  generally involves random assignment to groups  clinical trials are the most well known experimental design  the ultimate step in testing causal hypotheses

Experimental Studies  In an experiment, we are interested in the consequences of some treatment on some outcome.  The subjects in the study who actually receive the treatment of interest are called the treatment group.  The subjects in the study who receive no treatment or a different treatment are called the comparison group.

Epidemiologic Study Designs  Randomized Controlled Trials (RCTs) –a design with subjects randomly assigned to “treatment” and “comparison” groups –provides most convincing evidence of relationship between exposure and effect –not possible to use RCTs to test effects of exposures that are expected to be harmful, for ethical reasons

Epidemiologic Study Designs  Randomized Controlled Trials (RCTs) –the “gold standard” of research designs –provides most convincing evidence of relationship between exposure and effect  trials of hormone replacement therapy in menopausal women found no protection for heart disease, contradicting findings of prior observational studies

Experimental Design time Study begins here (baseline point) Study population Intervention Control outcome no outcome outcome no outcome baseline future RANDOMIZATION

Randomized Controlled Trials  Disadvantages –Very expensive –Not appropriate to answer certain types of questions  it may be unethical, for example, to assign persons to certain treatment or comparison groups

Credits All information modified from these sources:  Nigel Paneth at the University of Pittsburgh  Thomas Songer, PhD - University of Pittsburgh Modified by Supercourse team  Ahmed Mandil, MBChB, DrPH Prof of Epidemiology High Institute of Public Health University of Alexandria