Epidemiology: Key Concepts and Definitions

Introduction to epidemiology L1 . . 2023-2024

Objectives At the end of this lecture the students will be able to understand : 1. Definition of epidemiology 2. Purposes of epidemiology 3. Risk, risk factors, relative risk and Causality 4. Epidemiological Triangle

Why does a disease develop in some people and not in others?

What does epidemiology mean ? The term epidemiology is derived from the Greek words epi, which means on or upon ; demos, which means the common people ; and logy, which means study. Putting these pieces together yields the following definition of epidemiology: the study of that which falls upon the common people.

Epidemiology is defined as: The study of the frequency , distribution and determinants of health-related states or events in the specified populations, and the application of this study to the control of health problems. John M Last

The definition of epidemiology has five key words or phrases: (1)population, (2) disease frequency, (3) disease distribution, (4) disease determinants, and (5) disease control

Population refers to a group of people with a common characteristic, such as place of residence, gender, age, or use of certain medical services. Disease frequency refers to quantifying how often a disease arises in a population. Disease distribution refers to the analysis of disease patterns characteristics of person, place, and time, in other words, who is getting the disease, where it is occurring, and how it is changing over time according to the

Disease determinants are factors that bring about a change in a person s health or make a difference in a person s determinants consist preventive factors. Determinants also include individual, environmental, characteristics. Disease Control Epidemiologists accomplish through epidemiological research, and through surveillance. health. causal Thus, and of both and societal disease control

Health related conditions: are conditions which directly or indirectly affect or influence health. These may be injuries, births, health related behaviors like smoking, unemployment, poverty etc.

Objectives of epidemiology : (1) To Study the natural course of disease from onset to resolution. (2) To Determine the extent of disease in a population. (3) To Identify patterns and trends in disease occurrence. (4) To Identify the causes of disease, and (5) To Evaluate the effectiveness of measures that prevent and treat disease.

Uses of epidemiology : Historical use: Study the history of the health of populations. Community health use: Diagnose the health of the community. Health services use: Study the working of health services. Risk assessment use: Estimate individuals risks of disease, accident, or defect. Disease causality use: Search for the causes of health and disease.

Sources of epidemiological information 1. Population census is collection of data from every member of a population; theoretically it should provide the most reliable data. 2. Registration of vital events Birth, death and marriage. 3. Hospital/health center records. 4. Disease registers. 5. Epidemiologic studies. 6. Publications, Electronic sources

Risk: A probability that an individual will become ill or die within a specified period of time or age. It is used to denote incidence rate Absolute Risk The incidence of a disease in a population is termed the absolute risk. Absolute risk can indicate the magnitude of the risk in a group of people with a certain exposure, but because it does not take into consideration the risk of disease in nonexposed individuals, it does not indicate whether the exposure is associated with an increased risk of the disease.

Nevertheless, absolute risk may have important implications in both clinical and public health policy: For example, a woman who contracts rubella in the first trimester of pregnancy and asks her physician, What is the risk that my child will be malformed? is given a certain number as an answer. On the basis of this information, she may decide to abort her pregnancy The woman is wondering not only what her risk is, but she is wondering how that risk compares with what it would have been had she not contracted rubella.

Relative Risk (RR): The relative risk can also be defined as the probability of an event (developing a disease) occurring in exposed people compared to the probability of the event in nonexposed people, or as the ratio of the two probabilities. The relative risk is important as a measure of the strength of the association, which is a major consideration in deriving causal inferences.

The relative risk is calculated by relating the incidence rate (IR) of the disease among those exposed to the risk factor to the incidence rate of the disease among those not exposed. Incidence rate among exposed Relative risk (RR) = ----------------------------------------- Incidence rate among non exposed

An Example Comparing Two Ways of Calculating Excess Risk POPULATION A 40 10 Incidence (%) In exposed In nonexposed B 90 60 Difference in incidence rate (%) 30 30 Ratio of incidence rates 4 1.5

Interpreting Relative Risk (RR)of a Disease If RR = 1 Risk in exposed equal to risk in nonexposed (no association) If RR > 1 Risk in exposed greater than risk in nonexposed (positive association; possibly causal) If RR < 1 Risk in exposed less than risk in nonexposed (negative association; possibly protective)

The attributable risk: which is defined as the amount or proportion of disease incidence (or disease risk) that can be attributed to a specific exposure. For example, how much of the lung cancer risk experienced by smokers can be attributed to smoking? Whereas the relative risk is important in establishing etiologic relationships, the attributable risk is in many ways more important in clinical practice and in public health, because it addresses a different question: How much of the risk (incidence) of disease can we hope to prevent if we are able to eliminate exposure to the agent in question?

An Example of an Attributable Risk Calculation for the Exposed Group =(Incidence in exposed group)- (Incidence in nonexposed group)

Definitions of a Cause Simple, pragmatic definition: something that makes a difference. According to the epidemiologist Susser, true causes have three association, time order, and direction. essential attributes:

Association: A statistical dependence between two or more variables (between the causal factor and effect). Variables are said to be associated if they tend to occur together more frequently than could be explained by chance. If no association is found, causality can be rejected. Epidemiolo gists quantify associations by making absolute or relative comparisons between two or more groups.

Time order means that the cause must precede the effect Direction, the third essential attribute of a cause, means that there is an asymmetrical relationship between the cause and effect. In other words, one must demonstrate that a change in an outcome is a conse- quence of change in an antecedent factor so that a symmetrical relation- ship is noncausal.

Considerations for causation: A statistical association is likely to be causal if the following criteria are fulfilled: A-Epidemiological Hill s Guidelines for Assessing Causation B-Biological criteria ( Koch's Postulates).

During the 1960s, Hill described a set of nine guidelines to help determine whether associations are causal. These guidelines include : Hill s Guidelines forAssessing Causation Strength of association Consistency Specificity Temporality Biological gradient ( dose-response ) Plausibility Coherence Experiment Analogy

A-Epidemiological criteria(Bradford Hill criteria) for Considerations of causation 1. Strength of the association : What is the strength between the cause and the effect? (relative risk) For example, there is a strong association between smoking and the risk of developing lung cancer. The risk among smokers is about 20 times greater than that among those who never smoked. In addition, the risk further increases with the number of cigarettes smoked per day and the duration of smoking. Such strong associations are likely to be causal because they are unlikely to be accounted for entirely by alternative explanations such as bias and confounding.

2- Consistency : Have similar results been shown in other studies? if they are observed repeatedly by different persons, in different places, circumstances and times. 3- Specificity :The concept of Specificity, which was first developed for infectious diseases, means that a cause should lead to a single effect and vice versa. 4-Temporality :Time order, or temporality, means that the cause must precede the disease.

5- Doseresponse relationship : In other words, an association is more likely to be causal if its strength increases as increases. Hill gives as an example the linear rise in lung cancer death rates with the number of cigarettes smoked, stating that this evidence adds a very great deal to the simpler evidence that cigarette smokers have a higher death rate than non- smokers. the exposure level

6-Plausibility: there should be an existing biological or social model to explain the association. For example, it is biologically plausible that smoking causes lung cancer because cigarettes contain many carcinogenic substances. 7- Coherence is very close to biological plausibility. According to Hill, it means that the cause-and-effect interpretation of our data should not seriously conflict with generally known facts of the natural history and biology of the disease.

8-Experiment: Experiment is not a causal guideline in its strictest sense; rather, it is a method for testing a causal hypothesis. 9-Analogy:Hill suggested that epidemiologists use an analogy or a similarity between the observed association and any other associations

B-Biological criteria ( Koch's Postulates). Koch stated that four postulates should be met before a causal relationship can be accepted between a particular bacteria, parasite (or disease agent) and the disease in question. These are: 1. The agent must be shown to be present in every case of the disease by isolation in pure culture. 2. The agent must not be found in cases of other disease. 3. Once isolated, the agent must be capable of reproducing the disease in experimental animals. 4. The agent must be recovered from the experimental disease produced.

The Epidemiological Triangle The epidemiological triangle, is a model that scientists have developed to describe the relationship among three key factors (vertices) in the occurrence of disease or injury: agent, environment, and host. The mission of an epidemiologist is to break at least one of the sides of the Triangle, disrupting the connection between the environment, the host, and the agent, and stopping the continuation of disease.

Human disease does not arise in a vacuum. It results from an interaction of the host (a person), the agent (e.g., a bacterium), and the environment (e.g., a contaminated water supply). Although some diseases are largely genetic in origin, virtually all disease results from an interaction of genetic and environmental factors, with the exact balance differing for different diseases

The Epidemiologic Triangle

Disease has been classically described as the result of an epidemiologic triad. According to this diagram, it is the product of an interaction of the human host, an infectious or other type of agent, and the environment exposure. A vector, such as the mosquito or the deer tick, is often involved. For such an interaction to take place, the host must be susceptible. that promotes the

Human susceptibility is determined by a variety of factors including genetic background and nutritional and immunologic characteristics. The immune status of an individual is determined by many factors including prior experience both with natural infection and with immunization. The factors that can cause human disease include biologic, physical, and chemical factors as well as other types, such as stress, that may be harder to classify

Factors That May Be Associated with Increased Risk of Human Disease Host Characteristics Age ,Sex , Race , Religion, Customs, Occupation Genetic profile , Marital status, Family background, Previous diseases, Immune status Types of Agents and Examples Biologic Bacteria, viruses Chemical Poison, alcohol, smoke Physical Trauma, radiation, fire Nutritional Lack or excess

Environmental Factors Temperature, Humidity, Altitude, Crowding Housing, Neighborhood, Water, Milk, Food Radiation, Air pollution, Noise

Quiz 1- Define epidemiology. 2- What are the objectives of epidemiology 3-Enumerate the sources of epidemiological information 4- Enumerate Biological criteria ( Koch's Postulates) Considerations for causation . 5- Enumerate Epidemiological criteria(Bradford Hill criteria) Considerations for causation 6- Write short notes about the epidemiological triangle regarding covid-19. 7- Enumerate the Key Characteristics of Causes

Thank you Gordis L. Epidemiology. 5th Edition. Philadelphia,Elsevier Saunders . 2014 Ann Aschengrau and George R. Seage III. Essentials of epidemiology in public health, edition. United States of America, Jones & Bartlett Learning 2020 4th