Introduction à l'Épidémiologie

Introduction to Epidemiology

•  Epidemiology is a scientific discipline studying the distribution and determinants of health-related events and states in populations, aiming to control health problems.
• It's based on the assumption that disease occurrence isn't random, but rather determined by causal factors.
• Epidemiology deals with existing and occurring health phenomena.
• Key areas of study include behaviors, risk factors, disease causation, disease transmission, preventive factors, and effective disease control strategies.

Epidemiology Concepts

• Epidemiological studies can be descriptive or analytical (etiological).
• Descriptive epidemiology describes the distribution of health events or states in populations, focusing on time and location.
• Analytical epidemiology explores the relationship between exposures and health events or states, aiming for causal inference.
• Key epidemiological objectives include identifying risk factors, determining the relative importance of disease causes, and evaluating public health interventions and programs.

Incidence

• Incidence rate (IR) measures the frequency of new cases of a disease in a specific population over a given period.
• Person-time is a crucial concept in incidence calculation, representing the combined time spent at risk by all individuals in the population during the study duration.
• Methods for calculating incidence rate consider the number of new cases and the total person-time of individuals at risk.

Mortality Rate

• Crude mortality rate represents the rate of death in a population during a specific period.
• Specific mortality rate gives the mortality rate within a particular population subgroup (e.g., specific age, sex).

Measuring Risk

• Prevalence quantifies the proportion of individuals with a disease in a population at a specific point in time.
• Risk of disease is the probability of developing a disease within a defined time frame.
• Risk of death is the probability of dying within a defined time frame.

Standardization

• Direct standardization compares disease rates across populations with different age structures by applying a standard population's age distribution to the rates of the populations of interest.
• Indirect standardization compares disease rates using a standard set of mortality rates (e.g., population or region) as a reference to investigate differences in exposure.

Measures of Association

• Measures of association quantify the strength of the relationship between an exposure and a disease or health outcome.
• Examples include relative risk (RR), odds ratio (OR), and attributable fraction (AF).
• These methods provide quantitative information on the possible causal relationship between factors and disease.

Types of Epidemiological Studies

• Experimental studies are designed to establish cause-and-effect relationships between exposures and outcomes by randomly assigning individuals to intervention groups and control groups.
• Observational studies observe the relationship between exposures (e.g., lifestyle factors or environmental impacts) and health outcomes in populations.

Cohort Studies

• In cohort studies, researchers follow a group of individuals over time to assess the incidence of a disease related to a certain risk factor.
• Cohort studies include retrospective or historical studies where the exposures and outcomes are assessed in the past, and prospective studies are used to observe the exposure and outcomes in the future.

Case-Control Studies

•  In case-control studies, researchers compare the exposures of individuals with a disease (cases) to those without (controls) to identify potential risk factors.
• Case-control studies are retrospective and can be useful for investigating diseases with a long latency period between exposure and the onset of illness or rare diseases.

Cross-Sectional Studies

• In cross-sectional studies, researchers measure the prevalence of a disease and exposures simultaneously in a population at a specific point in time.
• These studies are useful for estimating prevalence rates and identifying potential associations between exposures and diseases.

Causality in Epidemiology

• Establishing causality is complex in epidemiology, requiring more than just correlation between an exposure and an outcome.
• Several criteria help assess causality (e.g., temporality, strength of association, dose-response relationship, consistency, specificity, plausibility).
• Epidemiological studies contribute crucial data used in assessing causality, while randomized controlled trials (RCTs) are considered the gold standard for establishing cause-and-effect relationships.

Determinants of Health

• The determinants of health include social, economic, and environmental factors that influence an individual's or population's health status.
• Different models, such as the Lalonde Report (1974), categorize determinants into various aspects like environment, lifestyle, genetics, biology, and the organization of health care.
• These models provide a framework for studying and addressing health disparities across populations.