Introduction to Epidemiology

Introduction Edward Jenner (1749-1823) developed a vaccine against small pox using cow pox, 160 years before virus was identified John Snow (1813-1858) 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

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

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

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)

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)

• 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

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

Traditional Epidemiology Triangle Environment Time Host Agent

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Determining Cause Relationship Association Cause Real or spurious??

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

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)

Evolution of Causal Theory 1880 - Koch’s Postulates 1965 - A. B. Hill’s Criteria for Causation (public health perspective) Current - Epidemiologic specific guidelines for cause

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

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

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

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

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)

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)

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

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

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?

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%

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

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?

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

Calculating Incidence Among 60 people attending a 12 month residential detoxification program, 50 tested HIV negative at the start of the program in Jan 1998. 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.

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