Type of epidemiology and epidemiological methods
- 1. Principles of Epidemiology and Epidemiologic Methods
- 2. What is Epidemiology? • Epidemiology is the basic science of preventive and social medicine. • Progressed after 20th century • The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.
- 3. Principles of Epidemiology 1. Measurement of Disease Frequency • Incidence: • Measures the rate at which new cases of a disease occur in a population over a specific time. • Formula: • Helps in studying disease causation and evaluating the effectiveness of preventive measures.
- 4. Prevalence: • Measures the total number of existing cases (both new and old) in a population at a specific time. • Useful for assessing the disease burden in a community. Attack Rate: • A specialized incidence rate used during outbreaks, especially for acute diseases like foodborne illnesses.
- 5. 2. Distribution of Diseases • Epidemiology analyzes the person, place, and time dimensions of disease occurrence: • Person: • Age, sex, ethnicity, occupation, habits, socioeconomic status. • For example, the risk of heart disease increases with age and is more commonin males. • Place: • Geographical differences such as urban vs. rural or regional variations. • Example: Malaria is more prevalent in tropical regions due to favorable climatic conditions for mosquito breeding. • Time: • Includes seasonal variations (e.g., influenza peaks in winter) and secular trends (e.g., declining rates of tuberculosis over decades).
- 6. 3. Determinants of Health • Epidemiology examines factors that influence health and disease, grouped as: • Host factors: • Characteristics of individuals, such as immunity, age, genetics, and lifestyle. • Agent factors: • Biological (e.g., bacteria, viruses), chemical (e.g., toxins, pollutants), and physical (e.g., radiation, heat). • Environmental factors: • Social and physical surroundings, including climate, housing, sanitation, and socioeconomic status.
- 9. 4. Disease Causation • Understanding causation is crucial for disease prevention: • Epidemiologic Triad: • Interaction between agent, host, and environment determines disease occurrence. • Web of Causation: • Recognizes multiple interlinked factors, both direct and indirect, contributing to disease (e.g., social determinants of health in diabetes).
- 12. 5. Natural History of Disease • The course of disease progression without intervention: • Pre-pathogenesis phase: • The stage where the host, agent, and environment interact, but the disease has not developed (e.g., exposure to a pathogen without infection). • Pathogenesis phase: • Includes the subclinical stage (disease without symptoms) and clinical stage (symptoms appear). • Levels of Prevention: • Primary Prevention: Prevents disease before it occurs (e.g., vaccination, health education). • Secondary Prevention: Detects early disease (e.g., screening for hypertension). • Tertiary Prevention: Limits complications (e.g., rehabilitation for stroke).
- 15. Epidemiologic Methods Descriptive Epidemiology • Describes disease patterns in populations and forms hypotheses. • Time: • Observes trends like: • Secular trends: Long-term trends(e.g., increase in lifestyle diseases). • Seasonal variations: Short-term,recurrentpatterns(e.g., dengue outbreaksin monsoon). • Point epidemics: Sudden spikes due to exposure (e.g., food poisoning at an event). • Place: • Maps disease patterns geographically to identify hotspots. • Person: • Examines characteristicslike age, sex, and occupation.
- 16. Epidemiological Methods Analytical Epidemiology • Tests hypotheses to identify disease determinants. • Key study designs: • Case-Control Studies: • Retrospective studies comparing individuals with a disease (cases)to those without (controls). • Example: Studying smoking as a risk factor for lung cancer. • Cohort Studies: • Follows a group exposedto a risk factor to observe disease occurrenceover time. • Example: Observing people exposedto asbestos forthe developmentof mesothelioma. • Cross-Sectional Studies: • Examines exposureand outcomesimultaneously. • Example: Prevalence of obesity and hypertension at a point in time.
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- 18. Epidemiological Methods Experimental Epidemiology • Researcher intervenes and observes outcomes. • Randomized Controlled Trials (RCTs): • Participants are randomly assigned to intervention or control groups. • Example: Testing a new vaccine. • Community Trials: • Interventions implemented at the community level (e.g., mass deworming programs).
- 19. Association of Causation 1. Association • Definition: Association refers to a measurable relationship between two variables, where changes in one variable correspond to changes in the other. • Types of Association: • Positive Association: As one variable increases, the other also increases. For example: • Smoking is positively associated with lung cancer. • Negative Association: As one variable increases, the other decreases. For example: • Physical activity is negatively associated with obesity. • Strength of Association: • Measured using statistical methods such as: • Relative Risk (RR) in cohort studies. • Odds Ratio (OR) in case-control studies. • Correlation Coefficient in continuous data. • Important: Association does not imply causation. For example, ice cream sales and drowning incidents are associated (both increase in summer), but one does not cause the other.
- 20. Association of causation Causation • Definition: Causation means that a change in one factor (cause) directly leads to a change in another factor (effect). For example: • Smoking causes lung cancer through exposure to carcinogens in tobacco. • Determiningcausation is more complex and requires more robust evidence than merely identifyingan association. Epidemiology uses specific criteria to distinguish causation from mere correlation.
- 21. Criteria for Causation (Bradford Hill’s Criteria) • Strength of Association: Strong associations are more likely to be causal than weak ones. Example: The relative risk of lung cancer in smokers compared to non-smokers is very high (>10). • Consistency: The association is observed repeatedly in different studies, populations, and circumstances. Example: Smoking has been consistently linked to lung cancer across multiple studies worldwide. • Specificity: A specific cause leads to a specific effect. While not always applicable in multifactorial diseases, specificity strengthens causation. Example: Smoking specifically increases the risk of lung cancer. • Temporality: The cause must precede the effect in time. This is an essential criterion for causation. Example: Smoking must occur before the development of lung cancer
- 22. • Biological Gradient (Dose-Response Relationship):Increased exposure leads to increased risk. Example: Heavier smoking (more cigarettes per day) increases the risk of lung cancer. • Plausibility: The association must make biological sense based on current knowledge. Example: Tobacco smoke contains carcinogens that cause genetic mutations, leading to cancer. • Coherence: The association should align with existing knowledge and data. Example: The relationship between smoking and lung cancer is supported by experimental and epidemiological studies. • Experiment: Evidence from experiments (e.g., randomized controlled trials or natural experiments) supports the causal relationship. Example: Reducing smoking in populations leads to a decrease in lung cancer incidence. • Analogy: Similar known associations can support causation. Example: Exposure to other carcinogens (e.g., asbestos) also leads to cancer, analogous to smoking.
- 23. Types of Causal Relationships • Causation can be direct or indirect: • Direct Causation: The cause directly produces the effect without any intermediate factors. • Example: A sharp object causes a cut. • Indirect Causation: The cause leads to the effect through one or more intermediate steps. • Example: Poor sanitation → exposure to contaminated water → diarrheal diseases. • Epidemiology often deals with multi-causal diseases where multiple factors interact (e.g., genetic predisposition + environmental exposure).
- 24. Measures of association • Relative Risk (RR): Used in cohort studies to compare disease risk in exposed and unexposed groups. • Odds Ratio (OR): Used in case-control studies as an estimate of relative risk.
- 26. Epidemiological Surveillance • Systematic collection and analysis of health data. • Types: • Active Surveillance: • Health workers actively collect data (e.g., during an epidemic). • Passive Surveillance: • Data is reported by healthcare providers to public health authorities. • Example: Reporting notifiable diseases like tuberculosis.
- 27. Applications of Epidemiology • Identifying Risk Factors: • Epidemiology helps determine causes of diseases (e.g., smoking and lung cancer). • Public Health Planning: • Used to allocate resources effectively (e.g., vaccination programs). • Policy Formulation: • Guides health policy decisions, such as air quality regulations. • Outbreak Investigation: • Determines the source and control measures for outbreaks (e.g., cholera).
- 28. Screening • Screening is a public health strategy aimed at the early detection of diseases or risk factors among apparentlyhealthy individuals. "The search for unrecognized disease or defect by means of rapidly applied tests, examinations, or other procedures in apparently healthy individuals.” • The goal of screening is to detect diseases at an early stage when interventions can be more effective, thereby reducing morbidity, mortality, and disability.
- 29. Characteristics of Screening • Target Population: Screening is conducted among asymptomatic individuals who are at risk or within a defined group. • Not Diagnostic: Screening identifies individuals who may have the disease, but confirmation requires diagnostic tests. • Population-based Approach: Often involves large groups or communities. • Purpose: To improve prognosis through early detection and prompt treatment.
- 30. Types of Screening • K. Park classifies screening into different types based on the purpose, population, and timing of the screening procedure: 1. Mass Screening • Definition: Screening of an entire population or a large group, irrespective of their risk status. • Example: Screening all adults in a community for hypertension or diabetes. • Advantages: • Covers a large population, identifying more cases. • Disadvantages: • Expensive, resource-intensive, and may include people at low risk of disease.
- 31. Types of Screening 2. High-Risk (Selective) Screening • Definition: Screening only specific high-risk groups who are more likely to develop the disease. • Example: Screening smokers for lung cancer or individuals with a family history of heart disease for hypercholesterolemia. • Advantages: • Cost-effective and focused. • Higher yield of positive cases. • Disadvantages: • May miss cases among low-risk individuals.
- 32. Types of Screening 3. Multiphasic Screening • Definition: Conducting multiple screening tests simultaneously on the same individual during one screening session. • Example: Health check-ups for employees, including blood pressure, blood sugar, vision tests, etc. • Advantages: • Saves time and resources by combining tests. • Useful for periodic health examinations. • Disadvantages: • Increased risk of false positives due to multiple tests. • Logistically complex to manage.
- 33. Types of Screening 4. Opportunistic (Case-Finding) Screening • Definition: Screening conducted during routine health visits or for unrelated reasons. • Example: Screening for hypertension during a visit for flu symptoms. • Advantages: • Utilizes existing healthcare visits, requiring no special effort. • Inexpensive and convenient. • Disadvantages: • Limited coverage, as it depends on who visits healthcare facilities.
- 34. Types of Screening 5. Periodic Screening • Definition: Screening at regular intervals to detect conditions that develop slowly or have a long latent phase. • Example: Annual mammography for women above 40 years of age. • Advantages: • Helps detect diseases at the earliest stages. • Disadvantages: • Requires systematic follow-up and resources.
- 35. Criteria for a Successful Screening Program • K. Park emphasizes the following criteria to determine whether a screening program is appropriate and effective: • The Disease: • Must have a significant burden on public health. • Should have a detectable preclinical phase where interventions can change the disease outcome. • The Screening Test: • Must be simple, safe, reliable, and acceptable to the population. • Should have high sensitivity (to detect true positives) and specificity (to rule out false positives).
- 36. Criteria for a Successful Screening Program • The Population: Should be at a sufficient risk level to justify screening. There must be access to follow-up diagnostic services and treatment. • Cost-Effectiveness: The benefits of screening (e.g., reduced disease burden, improved prognosis) should outweigh the costs.
- 37. Examples of Screening Programs • Universal Screening: • Neonatal screening for congenital hypothyroidism. • Tuberculosis screening in endemic regions. • Targeted Screening: • Cervical cancer screening (Pap smear) for sexually active women. • Mammography for breast cancer in women above a certain age. • Opportunistic Screening: • Checking blood pressure during outpatient visits. • Screening for diabetes in individuals presenting with obesity.