Epidemiological Concepts

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your NEET PG exam.

Epidemiological Concepts — Key Facts

What is Epidemiology?

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 control health problems. John Snow (1854) is called the Father of Epidemiology for his work on cholera in London.

Key Terms

• Epidemic: Disease occurrence above normal expectancy
• Endemic: Constant presence in a geographic area
• Pandemic: Worldwide epidemic spread
• Incidence: New cases per population at risk per unit time
• Prevalence: All cases (old + new) at a point in time
• Attack Rate: (New cases / Total population) × 100

Critical Formulas

• Incidence = (New cases / Population at risk) × 1000
• Prevalence ≈ Incidence × Average Duration
• Secondary Attack Rate = (Secondary cases / Susceptible contacts) × 100
• Case Fatality Rate = (Deaths / Total cases) × 100

⚡ NEET PG High-Yield: Attack Rate uses percentage (×100), not per 1000 like other rates!

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🟡 Standard — Regular Study (2d–2mo)

Chapter: Epidemiological Concepts

1.1 Introduction to Epidemiology

Definition and Scope

Modern epidemiology is the basic science of public health. It deals with the frequency, distribution, and determinants of disease in populations.

Historical Landmark Studies

• John Snow (1854): Traced cholera to contaminated Broad Street pump water in London
• William Farr (1838): Developed systematic mortality reporting
• Joseph Goldberger (1915): Proved pellagra was dietary deficiency, not infectious

Key Functions of Epidemiology

1. Identifying disease etiology
2. Determining extent in community
3. Studying natural history and prognosis
4. Evaluating interventions
5. Developing health policy

1.2 Measurements in Epidemiology

Mortality Rates

Crude Death Rate (CDR):

• CDR = (Total deaths / Total population) × 1000
• Affected by age-sex structure

Age-Specific Death Rate (ASDR):

• Deaths in specific age group per 1,000 population in that age group

Infant Mortality Rate (IMR) — Very Important:

• IMR = (Deaths under 1 year / Live births) × 1000
• Best indicator of community health
• India’s IMR (2023): ~28 per 1,000

Maternal Mortality Ratio (MMR):

• MMR = (Maternal deaths / Live births) × 100,000
• India’s MMR: ~97 per 100,000 (2023)

Case Fatality Rate (CFR):

• CFR = (Deaths from disease X / Total cases) × 100
• Measures severity
• Rabies CFR: ~100%; COVID-19 CFR: ~1-2%

Morbidity Rates

Incidence Rate:

• New cases per population at risk per unit time
• Measures risk of developing disease

Prevalence Rate:

• All existing cases at a point in time
• Measures burden of disease
• Prevalence ≈ Incidence × Duration

Attack Rate:

• Cumulative incidence during an outbreak
• Used in food poisoning,传染病 outbreaks
• Formula: (New cases / Total population) × 100

Secondary Attack Rate:

• Person-to-person spread within households
• SAR = (Secondary cases / Susceptible contacts) × 100
• Denominator excludes the primary case

1.3 Screening Tests

Sensitivity and Specificity

Sensitivity = TP / (TP + FN) × 100

• Ability to correctly identify those WITH disease
• Also: 1 - False Negative Rate

Specificity = TN / (TN + FP) × 100

• Ability to correctly identify those WITHOUT disease
• Also: 1 - False Positive Rate

Positive Predictive Value (PPV):

• Probability person with positive test truly has disease
• PPV increases when prevalence increases

Negative Predictive Value (NPV):

• Probability person with negative test truly doesn’t have disease
• NPV increases when prevalence decreases

⚡ Important: Sensitivity and Specificity are intrinsic to the test (don’t change with prevalence). PPV and NPV do change with prevalence.

1.4 Epidemiological Triad

Agent → Host → Environment

Agent Properties:

• Infectivity: Ability to invade and multiply
• Pathogenicity: Ability to cause clinical disease
• Virulence: Severity of disease (Case Fatality Rate)
• Antigenicity: Ability to stimulate immune response
• Immunogenicity: Actual immune response produced

Host Factors: Age, sex, genetics, immunity, lifestyle, occupation

Environmental Factors: Physical (climate, housing), Biological (vectors, reservoirs), Social (SES, education)

Chain of Infection: Agent → Reservoir → Portal of Exit → Mode of Transmission → Portal of Entry → Susceptible Host

1.5 Disease Transmission

Direct Transmission:

• Direct contact (skin-to-skin)
• Droplet spread (sneezing, coughing) — travels <1 meter
• Vertical transmission (mother to fetus)
• Sexual transmission

Indirect Transmission:

• Vehicle-borne: Food, water, blood
• Vector-borne: Mosquito, flea, louse
• Airborne: Small particle aerosols (<5μm remain suspended)
• Fomite-borne: Contaminated objects

Terms:

• Zoonosis: Animal to human (rabies, plague)
• Anthroponosis: Human to human only
• Vector: Carrier (mosquito) — not the same as reservoir

1.6 Study Designs

Descriptive Studies:

• Case report, case series, cross-sectional (measures prevalence)

Analytical Studies:

• Cohort study: Exposed vs unexposed → follow forward → measure incidence → Relative Risk
• Case-control: Cases vs controls → look back → Odds Ratio

Experimental Studies:

• RCT (Gold Standard): Randomized, controlled, double-blind
• Phases: I (safety), II (efficacy), III (comparative), IV (post-marketing)

1.7 National Health Programs

National Health Mission (NHM):

• NRHM (Rural) + NUHM (Urban)

Immunization — UIP:

• BCG, OPV, DPT, Hepatitis B, Measles, Rotavirus, IPV, Pentavalent

Disease Programs:

• National TB Elimination Program (goal: eliminate by 2025)
• National Leprosy Eradication Program (elimination achieved: <1/10,000)
• NVBDCP: Malaria, Dengue, Chikungunya, JE, Filariasis

1.8 COVID-19 Pandemic

R₀ (Basic Reproduction Number):

• Average secondary cases from one primary case in susceptible population
• R₀ > 1: Epidemic grows
• R₀ < 1: Epidemic dies out
• COVID-19 R₀: 2-3

Herd Immunity Threshold:

• HIT = 1 - 1/R₀
• For COVID (R₀=2.5): HIT = 60%

1.9 NEET PG High-Yield Points

⚡ Commonly Asked:

1. John Snow = Father of epidemiology (cholera study)
2. Attack Rate = (New cases/Total population) × 100 (NOT per 1000)
3. Case Fatality Rate = Deaths/Total cases × 100
4. IMR = Best indicator of community health
5. Sensitivity = TP/(TP+FN); Specificity = TN/(TN+FP)
6. PPV increases with higher prevalence
7. Cohort study → Relative Risk; Case-control → Odds Ratio
8. R₀ = Average secondary cases per primary case
9. Chain of infection: 6 links
10. Screening criteria: Wilson and Jungner (1968)