---

WBUHS Professional Exam — Chapter 2: Principles of Epidemiology, Study Designs, Outbreak Investigation & Immunization

(Based on Park's Textbook of Preventive & Social Medicine)

---

SECTION A: EPIDEMIC & OUTBREAK INVESTIGATION

---

Q1. Define "Epidemic" [2 Marks]

Definition (Park's, p. 65):

"The occurrence in a community or region of cases of an illness, specific health-related behaviour, or other health-related events clearly in excess of normal expectancy. The community or region and the period in which the cases occur must be specified precisely."

• Key Points:
  • Epidemicity is relative to the usual frequency of the same disease in the same area, among the same population, at the same season of the year
  • A single case of a communicable disease long absent from a population OR first invasion of a disease not previously recognized requires immediate reporting and full field investigation
  • Two cases associated in time and place of such a disease may be sufficient to be considered an epidemic
  • The number of cases indicating the presence of an epidemic varies according to: the agent, size and type of population exposed, previous experience, time and place of occurrence

---

Q2. Define Outbreak. How does Outbreak differ from Epidemic? [1+1 Marks]

• The term outbreak is often used for a localized, geographically limited epidemic
• An outbreak generally refers to a smaller cluster compared to an epidemic
• The term "epidemic" tends to be reserved for larger-scale events crossing communities or regions

Park's Note: "Epidemic and outbreak are often used synonymously. However, 'outbreak' suggests a more localized event."

---

Q3. Steps in Investigation of an Epidemic [6-8 Marks] — (Tamralipto, Midnapore, Burdwan, IQ City, Barasat)

Steps in Epidemic Investigation (Park's, pp. 132-138):

Step 1: Verification of Diagnosis

• Examine a sample of cases clinically
• Collect appropriate specimens: stool/blood for culture, serology
• Send to laboratory for confirmation (Vibrio cholerae/Salmonella typhi/Rotavirus etc.)
• Epidemiological investigation should NOT be delayed until lab results are available
• Establish a case definition (clinical criteria for what constitutes a "case")

Step 2: Confirmation that an Epidemic Exists

• Compare current case frequency with baseline (same period in previous years)
• An epidemic exists when observed frequency exceeds expected frequency (baseline + 2 standard deviations)
• For common-source epidemics (like post-feast gastroenteritis) - the existence is obvious and easily recognized

Step 3: Characterization by Time, Place and Person

• TIME: Construct an Epidemic Curve (bar chart of cases vs. time of onset)
  • A point-source epidemic shows a single sharp peak followed by rapid decline
  • Incubation period helps identify the causative agent
  • Find the index case (first case) - helps trace the source
• PLACE: Draw a spot map; identify clustering of cases
  • Are cases concentrated around the feast venue?
  • Map common water sources, food preparation areas
• PERSON: Describe cases by age, sex, occupation
  • Who attended the feast? Who ate which food item?
  • Compare attack rates by food item (food-specific attack rates)

Step 4: Formulate a Hypothesis

• Based on the epidemic curve (point-source pattern after feast), suspect a common food/water source
• Hypothesis: "Contaminated food served at the community feast is the source"
• Identify the suspected vehicle: rice, fish curry, water, sweets?

Step 5: Test the Hypothesis

• Conduct an epidemiological study (preferably a retrospective cohort study or case-control study)
• Retrospective Cohort Study: Compare attack rates among those who ate each food item vs. those who did not
  • Calculate Food-specific Attack Rate = (Cases among those who ate food item / Total who ate that food item) × 100
  • Calculate Relative Risk (RR) for each food item
• This identifies the vehicle of infection

Step 6: Environmental Studies

• Inspect food preparation area, water source
• Test water for contamination (coliform count)
• Check refrigeration/storage of food
• Identify food handlers - take throat swabs/stool samples

Step 7: Further Descriptive/Analytical Studies

• If source not yet identified, conduct a more detailed case-control study
• Map secondary cases if any (person-to-person spread)

Step 8: Implement Control Measures (see below)

Step 9: Prepare a Report

• Document findings: magnitude of outbreak, cause, source, mode of transmission, control measures taken
• Submit to BMOH → CMO-H → State Epidemiologist

---

Q4. Immediate Control Measures During Epidemic Investigation [2-3 Marks]

1. Notification - Report to CMO-H, State Surveillance Cell, IDSP
2. Isolation and Treatment - Admit severe cases, provide ORS/IV fluids, appropriate antibiotics as per protocol
3. Safe Water Supply - Chlorinate water source; distribute ORS/safe water
4. Food Safety - Stop consumption of suspected food item; condemn remaining contaminated food
5. Sanitation - Safe disposal of excreta, proper handwashing facilities
6. Surveillance - Active case search in the community; search for more cases
7. Health Education - Educate community on food and water hygiene, hand-washing with soap
8. Vaccination - If applicable (e.g., oral cholera vaccine)

---

Q5. Define Attack Rate and Calculate [4+1 Marks] — (Barasat GMC)

Attack Rate (Park's, p. 31):

• It is a type of incidence rate specific to epidemic situations
• Expressed as a percentage (not per 1,000)
• Time period = duration of the epidemic (limited and well-defined period)
• Used to measure the intensity of an epidemic

The number of cases of infection that occur among susceptible persons within the incubation period following exposure to a primary case in a household or its equivalent.

---

SECTION B: EPIDEMIOLOGICAL STUDY DESIGNS

---

Q6. Define Epidemiology. Classify Epidemiological Studies [2+3 Marks] — (MCK, Jagannath Gupta)

"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 of health problems." — Last, 1988

Classification of Epidemiological Studies:

Epidemiological Studies
│
├── 1. OBSERVATIONAL STUDIES (Nature takes its course)
│   ├── a. DESCRIPTIVE STUDIES
│   │   ├── (i) Ecological/Correlational (unit = population)
│   │   └── (ii) Cross-sectional/Prevalence (unit = individual)
│   │
│   └── b. ANALYTICAL STUDIES
│       ├── (iii) Case-Control/Case-Reference (unit = individual)
│       └── (iv) Cohort/Follow-up/Longitudinal (unit = individual)
│
└── 2. EXPERIMENTAL STUDIES (Active intervention)
    ├── a. Randomized Controlled Trial (unit = patients)
    ├── b. Field Trials (unit = healthy people)
    └── c. Community Trials (unit = communities)

---

Q7. Cross-Sectional Study — Prevalence of Obesity among Medical Students [2+13 Marks] — (Barasat GMC)

• The study needs to be completed within 3 months - making longitudinal studies impractical
• Objective is to measure prevalence of a condition at a point in time
• All measurements (exposure and outcome) done simultaneously

Steps of Cross-Sectional Study:

1. Define Objectives: To determine the prevalence of obesity among medical students of [Institution]
2. Define Study Population: All medical students of the institution enrolled in the current academic session
3. Develop Case Definition: Obesity = BMI ≥ 30 kg/m² (WHO cut-off); for Asian populations, BMI ≥ 25 kg/m²
4. Sampling: Stratified random sampling (stratify by year/batch); calculate sample size using: $$n = \frac{z^2 \times p(1-p)}{d^2}$$ where p = expected prevalence (~20-30%), d = precision (±5%), z = 1.96
5. Data Collection Tool: Structured questionnaire + physical measurements (height, weight, waist circumference)
6. Measurements: Weigh and measure height; calculate BMI; record dietary habits, physical activity, socioeconomic factors
7. Analysis:
   • Calculate prevalence of obesity = (Number obese / Total students examined) × 100
   • Cross-tabulation by sex, year of study
   • Calculate Prevalence Ratio (PR) for associated factors
8. Interpretation and Report Writing

---

Q8. Case-Control Study Design — Obesity and Osteoarthritis / OCP and Breast Cancer [8+4+3 Marks] — (PC Sen, ESI Joka)

Study Design: Case-Control Study

• Osteoarthritis and breast cancer are relatively rare diseases in general population
• Case-control is ideal for rare diseases and diseases with long latency
• Retrospective design - proceeds from effect → cause

Framework (Park's, p. 83):

Steps of Case-Control Study:

1. Selection of Cases:
   • Include newly diagnosed cases of osteoarthritis of knee (35-65 years) confirmed by X-ray + clinical criteria
   • From outpatient departments, hospitals
   • Set inclusion/exclusion criteria (exclude secondary OA, other joint diseases)
2. Selection of Controls:
   • Hospital controls (from orthopaedics OPD without OA) or community controls
   • Matched with cases on age, sex, hospital (to control confounding)
   • Ratio: 1:1 or 1:2 (cases:controls)
3. Define Exposure:
   • Obesity = BMI ≥ 30 kg/m² (measured at time of study or using past records)
   • Questionnaire for past history of weight, dietary habits, physical activity
4. Data Collection:
   • Interviewer-administered questionnaire; review of medical records
   • Measure current BMI, collect information on confounders (age, sex, occupation, diet)
5. Analysis:
   • Calculate Odds Ratio (OR) = (a×d)/(b×c)
   • Logistic regression to control for confounders
   • If OR > 1 with confidence interval not crossing 1 → association established

Advantages of Case-Control Study:

1. Economical in time, money and subjects
2. Suitable for rare diseases
3. Can study multiple exposures (risk factors) for one disease
4. Quick - exposure and disease already occurred
5. No attrition problems
6. Ethical - no exposure needed

Disadvantages:

1. Recall bias - cases recall past exposure better than controls (memory bias)
2. Cannot calculate incidence or relative risk directly (only OR)
3. Cannot study rare exposures
4. Selection bias in choosing cases/controls
5. Temporal relationship difficult to establish
6. Confounding is difficult to control fully
7. Cannot study more than one disease at a time

Types of Bias in Case-Control Study:

1. Selection bias - improper selection of cases/controls; hospital-based controls may not represent general population (Berkson's bias)
2. Recall bias - cases tend to remember past exposures more vividly than controls (Information bias)
3. Interviewer bias - investigator might probe cases more thoroughly than controls
4. Confounding bias - a third variable is related to both exposure and disease (e.g., smoking as confounder in obesity-OA study)

---

Q9. Cohort Study — Radiation Exposure in Nuclear Plant Workers / Obesity-Hypertension / Smoking [2+8+5 Marks] — (JNM Kalyani, Deben Mahata, Bankura)

• Radiation exposure and disease outcome need to be followed over 5 years (forward in time)
• Cohort study establishes temporal relationship (cause precedes effect)
• Can calculate Relative Risk (RR) directly
• Suitable when exposure is well-defined (radiation dose) and incidence is the outcome

Definition of Cohort (Park's, p. 90):

"A cohort is a group of people who share a common characteristic or experience within a defined time period."

Framework of Cohort Study (Park's, p. 90):

Steps of Cohort Study:

1. Selection of Study Cohort:
   • Exposed cohort: All workers in nuclear plant with radiation exposure (dosimeter-verified)
   • Unexposed cohort: Administrative/clerical staff of same plant not exposed to radiation OR workers in a comparable non-nuclear industry
   • Ensure cohort is free of disease at the start (no pre-existing radiation-related disease)
2. Baseline Assessment:
   • Measure exposure level: radiation dose (milliSieverts/year)
   • Baseline health assessment, chest X-ray, blood counts
   • Record confounders: age, sex, smoking, alcohol, diet
3. Follow-Up:
   • Follow both groups for 5 years (regular annual health checks)
   • Ensure minimum drop-out; trace all participants
   • Record development of disease: leukemia, thyroid cancer, cataracts, etc.
   • Continue measuring exposure (cumulative radiation dose)
4. Analysis:
   • Calculate Incidence Rate in exposed and unexposed
   • Calculate Relative Risk (RR):
     • RR > 1 → positive association
     • RR = 1 → no association
     • RR < 1 → protective
   • Calculate Attributable Risk (AR) = Incidence exposed − Incidence unexposed
   • Calculate Population Attributable Risk (PAR)
5. Interpretation and Report

Indications for Cohort Study:

1. When there is good prior evidence of association (from case-control or descriptive studies)
2. When exposure is rare but incidence of disease is high among exposed (e.g., occupational groups)
3. When attrition can be minimized (stable, cooperative, accessible cohort)
4. When ample funds are available

Advantages of Cohort Study:

1. Gold standard for establishing temporal relationship (cause → effect)
2. Can calculate true incidence and relative risk directly
3. Can study multiple outcomes from one exposure
4. Less susceptible to selection bias (exposure status determined before disease)
5. Can study the natural history of disease

Disadvantages of Cohort Study:

1. Expensive - requires large sample size and long follow-up
2. Time-consuming - not suitable when disease has long latency
3. Loss to follow-up is a major problem (attrition bias)
4. Not suitable for rare diseases (need very large sample)
5. Exposure status may change over time
6. Confounding may occur

Bias in Cohort Study:

• Selection bias: Healthy worker effect (workers may be healthier than general population)
• Information bias: Differential measurement of exposure over time
• Attrition bias/Loss to follow-up bias: Drop-outs may differ from those who stay
• Confounding

Calculation: RR from Obesity-Hypertension Data (Deben Mahata):

• Obese group: 200 cases out of 2000 → Incidence = 200/2000 = 0.10 (10%)
• Non-obese group: 100 cases out of 4000 → Incidence = 100/4000 = 0.025 (2.5%)
• RR = 0.10/0.025 = 4.0
• Interpretation: Obese individuals are 4 times more likely to develop hypertension compared to non-obese individuals. This is a strong positive association.

---

Q10. RR Calculation from Diamond Harbour Data (Silicosis Study)

---

Q11. Cohort vs Case-Control Comparison Table (Important for WBUHS)

---

Q12. Cross-Sectional Study — Screen Time and Mental Health (College of Medicine & Sagore Dutta)

• Since exposure (screen time in childhood) precedes outcome (mental disorders in adolescence), a retrospective cohort study or case-control study is appropriate
• If cohort records exist from childhood - use retrospective cohort
• If not - use case-control with recall of past screen time

• Recall bias (parents reporting past screen time)
• Confounding (socioeconomic status, parental education, physical activity)
• Selection bias
• Detection bias (diagnosed vs. undiagnosed mental disorders)

• Use validated questionnaires for screen time
• Match cases and controls for confounders
• Blinding of investigators
• Use objective data (screen time logs, device records)

---

SECTION C: IMMUNIZATION

---

Q13. National Immunization Schedule (NIS) for Infant — Age, Dose, Route & Site [KPCMCH]

• TT1: as early as possible in pregnancy
• TT2: 4 weeks after TT1 (protection for 3 years)
• TT Booster: if previously immunized within 3 years

⭐ KEY POINT: Hepatitis B birth dose MUST be given within 24 hours of birth to prevent perinatal/vertical transmission from HBsAg-positive mothers. This is the rationale for the birth dose even though subsequent doses are scheduled at 6 and 14 weeks.

⭐ KEY POINT: OPV is given at birth despite subsequent doses being scheduled because: (i) it colonizes the gut with vaccine virus, (ii) provides early protection, (iii) herd immunity benefit. The birth dose is a "zero dose" and does NOT replace subsequent doses.

---

Q14. AEFI — Classification, Investigation & Management [5+5 Marks] — (Malda, JMN)

Definition:

Classification of AEFI (WHO Classification):

1. Vaccine Product-related Reaction: Caused by the inherent properties of the vaccine (e.g., BCG lymphadenitis, MMR causing mild fever)
2. Vaccine Quality Defect-related Reaction: Due to a defect in the vaccine product (e.g., inadequate inactivation of toxin in DPT)
3. Immunization Error-related Reaction (Programme Error): Due to improper handling, prescribing or administration (e.g., reconstitution with wrong diluent, abscess due to subcutaneous DPT, toxic shock syndrome due to contaminated syringe)
4. Immunization Anxiety-related Reaction: From anxiety about the immunization (e.g., vasovagal syncope/fainting)
5. Coincidental Events: The event is related to underlying condition or illness that would have occurred regardless of vaccination

⭐ KEY POINT: 'Serious' vs 'Severe' AEFI are not the same:

• Serious AEFI = meets specific regulatory criteria: hospitalization, life-threatening, permanent disability, death, congenital anomaly - regardless of clinical severity
• Severe AEFI = clinically intense/significant reaction but may not meet the regulatory definition of "serious"

AEFI Scenario (JMN - 9-month-old child after MR vaccine):

• Confirm information in the report
• Clinical description, immunization history, previous allergies
• Check all 5 children who received vaccine from same session
• Inspect vaccine (lot number, expiry, storage condition, vaccine vial monitor status)
• Cold chain assessment
• Review reconstitution process and syringes used

• Keep adrenaline (1:1000) and resuscitation equipment ready at all immunization sessions
• Observe all children for 30 minutes post-vaccination
• Screen for egg allergy before MMR
• Hub cutter (needle cutter) must be present to prevent needlestick injury
• Trained vaccinators; proper supervision

1. Local reactions: Pain, swelling, redness at injection site
2. Systemic reactions: Fever, malaise
3. Serious reactions: Anaphylaxis, encephalopathy, febrile convulsions
4. Very rare reactions: SSPE (post-measles vaccine), intussusception (post-rotavirus)

---

Q15. Immunization Drop-out and Left-out Children [4 Marks] — (Jhargram, RGMCH)

⭐ KEY POINT: Left-out and Drop-out are NOT the same:

• Distance from health facility, poor transport
• Fear of AEFI
• Cultural/religious beliefs
• Misinformation
• Missed sessions, unavailability of vaccine

• Regular immunization sessions at sub-centre
• Outreach sessions in remote areas
• Tracking defaulters through ANMOL/RCH portal
• Intensified Mission Indradhanush (IMI)
• Health education to mothers
• ASHA workers motivation

---

Q16. Purpose of Vaccination is Not Only Individual Protection — Herd Immunity [ESI Joka]

1. Individual Protection: Direct immunity in vaccinated person
2. Herd Immunity (Community Protection):
   • When a sufficiently large proportion of a population is immune, the pathogen cannot spread even to unvaccinated individuals (interruption of transmission)
   • Herd Immunity Threshold (HIT) = 1 - 1/R₀
   • For measles (R₀ = 12-18): HIT ≈ 92-95%
   • For polio (R₀ = 5-7): HIT ≈ 80-85%
3. Eradication Goal: High vaccination coverage → interruption of transmission → eventual eradication

---

SECTION D: SHORT NOTES (Important Points)

---

Bias and Confounding Are Not Synonymous (CNMC)

---

Sentinel Surveillance (MCK)

• A surveillance system that uses a selected network of reporting sites (sentinel sites) rather than universal reporting
• Designed to detect early signals of disease outbreaks
• Useful for: Monitoring trends in influenza, measles, AFP (polio), AEFI
• Advantage: More sensitive for early outbreak detection than passive surveillance
• Limitation: May not be representative of the entire population

---

Cohort Study is Gold Standard but Inappropriate for Rare Disease (College of Medicine & Sagore Dutta)

• Gold standard for temporal association because it follows people from exposure to outcome - clearly establishing that exposure precedes disease
• Inappropriate for rare disease because:
  • Rare diseases have very low incidence
  • Need an extremely large sample size to observe sufficient cases
  • Practically and economically not feasible
  • Case-control study is preferred for rare diseases (assembles existing cases, more efficient)

---

Randomization and Blinding Are Not Used for Same Purpose (Deben Mahata)

"Randomization is the heart of clinical trial" — ensures comparability of groups at baseline. Blinding ensures that subsequent outcome assessment is not influenced by knowledge of group assignment.

---

Incidence is Preferred Over Prevalence in Studying Disease Causation (JNM Kalyani)

• Incidence measures new cases in a defined time period → reflects the risk of developing disease
• Prevalence measures existing cases at a point in time → affected by both incidence AND duration of disease
• For studying causation (aetiology), we need to know who newly developed the disease in relation to exposure → incidence
• Prevalence-based studies cannot establish temporality (which came first, exposure or disease?)

---

RR and AR Are Not Synonymous (Midnapore)

---

HPV Vaccination (CNMC, Midnapore, Santiniketan, Burdwan)

• Type: Recombinant virus-like particle (VLP) vaccine
• Types available: Bivalent (16,18), Quadrivalent (6,11,16,18), Nonavalent (6,11,16,18,31,33,45,52,58)
• Target: Girls 9-14 years (before sexual debut, before HPV exposure)
• India Context: Protects against cervical cancer (2nd most common cancer in Indian women after breast cancer)
• India programme: Two doses for girls 9-14 years; three doses for older girls (>15 years)
• Rationale for adolescent vaccination: Maximum immunogenicity before HPV exposure; school-based delivery feasible

---

Open Vial Policy (OVP) (ICARE)

• WHO policy allowing multi-dose vaccine vials to be used in subsequent sessions (not discarded after first use) if:
  1. The cold chain has been maintained (2-8°C)
  2. Vaccine vial monitor (VVM) has not crossed discard threshold
  3. Not past expiry date
  4. No contamination (aseptic technique maintained)
  5. Not a reconstituted vaccine (reconstituted vaccines must be discarded within 6 hours/end of session)
• OVP does NOT apply to: BCG, measles, measles-rubella, JE, meningococcal, yellow fever (reconstituted vaccines)
• Benefit: Reduces vaccine wastage

---

Cold Chain in UIP (Malda)

1. National level: Walk-in coolers/freezers (−20°C, 2-8°C)
2. State level: Walk-in coolers/freezers
3. District level (CMO-H): Deep freezers + ILR
4. CHC/PHC: Ice-lined refrigerator (ILR) + Deep freezer
5. Sub-centre/Field: Vaccine carrier with ice packs

---

Epidemic Curve — Draw and Interpret

• X-axis: Time (date/time of onset)
• Y-axis: Number of cases
• Point source epidemic (common-source, single exposure): Single sharp peak, rapid rise and fall within one incubation period (e.g., post-feast gastroenteritis)
• Propagated/Progressive epidemic (person-to-person): Multiple peaks at intervals of one incubation period; gradually increasing then decreasing
• Continuous common source: Extended plateau during continued exposure (e.g., contaminated water supply)

The epidemic curve identifies: type of epidemic, probable time of exposure, incubation period, and expected end of epidemic.

---

Population Attributable Risk (PAR) (KPC Medical College)

• Measures the excess disease in the population attributable to the exposure
• Useful for public health planning — tells what proportion of disease would be eliminated if the exposure were removed from the whole population

---

⭐ IMPORTANT POINTS SUMMARY FOR WBUHS EXAM

1. Epidemic = cases clearly in excess of normal expectancy; relative to area, season, population
2. Outbreak = localized epidemic; 85 cases of diarrhoea in a village after a feast = outbreak (also qualifies as epidemic)
3. Attack Rate = incidence during epidemic; expressed as percentage
4. First step in epidemic investigation = Verification of Diagnosis
5. Epidemic curve identifies type of epidemic and probable time of exposure
6. Case-control = retrospective; OR; rare diseases; multiple risk factors
7. Cohort = prospective; RR; rare exposure; multiple outcomes; Gold standard for temporality
8. Cross-sectional = prevalence; quick; short studies
9. RCT = gold standard for intervention studies; Randomization is heart of RCT
10. Hepatitis B birth dose = within 24 hours; prevents vertical transmission
11. OPV birth dose = zero dose; does not replace schedule doses; colonizes gut
12. AEFI classification = Product-related, Quality defect, Programme error, Anxiety-related, Coincidental
13. Anaphylaxis post-vaccination = adrenaline 0.5 ml (1:1000) IM immediately
14. Left-out ≠ Drop-out; Serious ≠ Severe AEFI; Bias ≠ Confounding; RR ≠ AR
15. Incidence preferred over prevalence for studying disease causation
16. Framingham Study = prospective cohort study (since 1948); risk factors for cardiovascular disease
17. Open Vial Policy does NOT apply to reconstituted vaccines
18. Sentinel surveillance = early warning system; selected sentinel sites

---