Public Health and Preventive Medicine
Core concepts
Understand the natural history of disease and levels of prevention: primordial, primary, secondary, tertiary, and quaternary. Know the public health functions (assessment, policy development, assurance) and core functions of screening programmes (Wilson–Jungner criteria). Grasp herd immunity thresholds, outbreak investigation steps, and the epidemiologic transition. Familiarity with major global frameworks-sustainable development goals, universal health coverage-and the concept of burden of disease (DALYs, QALYs) is expected.
Traps
- Calling vaccination a secondary prevention measure-it is primary.
- Confusing screening (asymptomatic populations) with early diagnosis (symptomatic patients).
- Mixing up incidence-based and prevalence-based burden measures.
- Forgetting that herd immunity threshold depends on R0 and vaccine efficacy.
Worked example / rule of thumb
Herd immunity threshold = 1 − 1/R0. For measles R0 ≈ 12–18, threshold ≈ 92–95%. With a vaccine of 95% efficacy, coverage needed ≈ threshold/efficacy ≈ 97–100%. Rule of thumb: high-R0 diseases demand near-universal, highly efficacious coverage-explaining why measles outbreaks appear when coverage dips.