Master Epidemiology & Public Health
for PLAB 1

Core Concepts

Epidemiology is 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. Public Health aims to improve health and prevent disease through organized community efforts.

• Measures of Disease Frequency:
  • Incidence: Number of new cases of a disease in a population over a specified period. Measures risk.
  • Prevalence: Total number of existing cases (new and old) in a population at a specific point in time or over a period. Measures burden.
  • Mortality Rate: Number of deaths from a disease in a population over a specified period.
  • Case Fatality Rate (CFR): Proportion of individuals with a disease who die from it.
  • Standardised Mortality Ratio (SMR): Compares observed deaths in a population to expected deaths (adjusting for age/sex).
• Study Designs:
  • Descriptive: Case reports, case series, cross-sectional (snapshot of prevalence).
  • Analytical:
    • Case-Control: Retrospective, compares exposure history in cases vs. controls. Calculates Odds Ratio (OR). Good for rare diseases.
    • Cohort: Prospective or retrospective, follows exposed vs. unexposed groups over time to observe outcome. Calculates Relative Risk (RR) and Absolute Risk Reduction (ARR). Good for rare exposures.
  • Interventional:
    • Randomised Controlled Trial (RCT): Gold standard for causality, randomly assigns participants to intervention or control. Reduces confounding. Calculates NNT (Number Needed to Treat).
• Bias: Systematic error leading to incorrect estimates.
  • Selection Bias: Differences between comparison groups (e.g., healthy worker effect).
  • Information Bias (Observation/Measurement Bias): Errors in data collection (e.g., recall bias in case-control).
  • Confounding: Third variable independently associated with both exposure and outcome, distorting the true relationship. Can be controlled for in design (randomisation, matching) or analysis (stratification, regression).
• Causality (Bradford Hill Criteria): Strength, Consistency, Specificity, Temporality (exposure before outcome - *essential*), Biological gradient, Plausibility, Coherence, Experiment, Analogy.
• Screening: Early detection in asymptomatic populations.
  • Sensitivity: Proportion of true positives correctly identified (TP / (TP + FN)). High sensitivity = few false negatives (good for ruling out).
  • Specificity: Proportion of true negatives correctly identified (TN / (TN + FP)). High specificity = few false positives (good for ruling in).
  • Positive Predictive Value (PPV): Probability of having the disease given a positive test (TP / (TP + FP)). Varies with prevalence.
  • Negative Predictive Value (NPV): Probability of not having the disease given a negative test (TN / (TN + FN)). Varies with prevalence.
• Levels of Prevention:
  • Primary: Preventing disease onset (e.g., vaccination, health education).
  • Secondary: Early detection and treatment to halt progression (e.g., screening tests, early medication).
  • Tertiary: Reducing impact of established disease, improving quality of life (e.g., rehabilitation, chronic disease management).
• Social Determinants of Health: Non-medical factors influencing health outcomes (e.g., socio-economic status, education, housing, access to healthcare).

Clinical Presentation (Application in Public Health Scenarios)

• Outbreak Investigation: Rapid increase in cases of a disease (e.g., food poisoning, infectious disease), requiring identification of source, mode of transmission, and affected population.
• Interpretation of Screening Results: Understanding when a positive or negative test result truly indicates disease presence or absence, especially in low vs. high prevalence settings.
• Identifying Risk Factors: Linking patient history (lifestyle, occupation, exposures) to known epidemiological associations (e.g., smoking and lung cancer, asbestos and mesothelioma).
• Health Inequalities: Recognising disproportionate disease burden or poorer health outcomes in specific demographic groups or geographical areas.
• Evidence-Based Medicine: Applying findings from epidemiological studies (e.g., RCTs) to individual patient care and public health policy decisions.

Diagnosis (Gold Standard Methodologies)

In Epidemiology and Public Health, "diagnosis" refers to identifying health problems, determining their determinants, and assessing intervention effectiveness.

• Identifying Disease Cause: Randomised Controlled Trials (RCTs) are the gold standard for establishing causal links between an intervention and an outcome.
• Disease Surveillance: Robust, continuous data collection systems (e.g., national disease registries, mandatory reporting) are gold standard for monitoring disease trends and detecting outbreaks.
• Evaluating Interventions: Meta-analysis and systematic reviews of high-quality RCTs provide the strongest evidence for intervention effectiveness.
• Defining Disease Outbreaks: Clear, consistent case definitions are critical for accurate diagnosis and monitoring of outbreaks.

Management (First Line Public Health Interventions)

• Outbreak Control:
  • Identification & Isolation: Promptly identify and isolate cases to prevent further spread.
  • Contact Tracing: Identify and monitor individuals exposed to cases.
  • Treatment & Prophylaxis: Administer appropriate medications or vaccinations.
  • Environmental Control: Address source of infection (e.g., food recall, water purification).
  • Public Health Messaging: Clear communication to inform and guide the public.
• Disease Prevention:
  • Immunisation Programs: Widespread vaccination to achieve herd immunity.
  • Health Education & Promotion: Campaigns encouraging healthy lifestyles (e.g., anti-smoking, healthy diet, physical activity).
  • Legislation & Policy: Seatbelt laws, clean air acts, food safety regulations.
• Screening Programs: Organised population-level screening (e.g., cervical, breast, bowel cancer screening) for early detection.
• Addressing Health Inequalities: Targeted interventions and policies for vulnerable populations.