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PSM: Infectious Epidemiology & Vaccines - NEET PG 2026

Source: Park's Textbook of Preventive & Social Medicine (gold standard for PSM)

PART 1: INFECTIOUS DISEASE EPIDEMIOLOGY

Key Definitions (High-Yield for MCQs)

Term	Definition	Exam Tip
Infection	Entry + development/multiplication of an infectious agent in a host	Not always = illness
Contamination	Presence of infectious agent on a body surface or inanimate object	Does NOT imply carrier state
Infestation	Lodgement of arthropods on body surface (lice, itch mite)	Surface only; gut worms = debated
Reservoir	Natural habitat where organism metabolizes, replicates, and is transmitted from	Man = reservoir of measles, typhoid
Source of infection	Person/animal/substance from which agent passes to host	Source ≠ Reservoir always

Chain of Transmission (Epidemiological Triad)

RESERVOIR → Modes of Transmission → SUSCEPTIBLE HOST

Agent - Host - Environment = Epidemiological Triad (also called the "triangle of epidemiology")

Modes of Transmission:

Direct - contact, droplet, transplacental
Indirect - vehicle-borne (water, food, air), vector-borne (mechanical/biological), fomites
Airborne - droplet nuclei (stay airborne >1m, e.g., TB, measles, chickenpox)

Carrier State (Frequently Asked)

Type	Description	Example
Incubatory carrier	Sheds agent during incubation period (infectious BEFORE illness onset)	Measles, hepatitis A, cholera
Convalescent carrier	Sheds agent after clinical recovery	Typhoid, diphtheria, dysentery
Healthy/Contact carrier	Never shows illness but transmits	Meningococcal, diphtheria
Chronic carrier	Carries for >6 months	Typhoid (Salmonella typhi), Hep B
Paradoxical carrier	Previously immune person who carries	-

Important Time Intervals (Very High Yield)

Incubation Period = Time from exposure to onset of symptoms

Generation Time = Interval from receipt of infection to MAXIMAL INFECTIVITY of that host

Generation time ≈ incubation period (but NOT the same)
Applies to both clinical AND subclinical infections (broader term)
In mumps: max communicability is 48 hours BEFORE swelling

Serial Interval = Time between onset of primary case and secondary case in a closed group (e.g., family)

Communicable Period = Time during which infectious agent can be transferred to another person

Secondary Attack Rate (SAR):

SAR = (Exposed persons developing disease within incubation period / Total exposed contacts) × 100

Primary case is excluded from BOTH numerator and denominator
Useful to measure transmissibility and test preventive measures

Surveillance (Park's Definition)

"Continuous analysis, interpretation, and feedback of systematically collected data, using methods distinguished by their practicality, uniformity, and rapidity (not accuracy or completeness)"

Key surveillance concepts:

Sentinel surveillance - selected sites report data; used when routine reporting is weak
Active vs. Passive surveillance
Sources: death certificates, hospital records, lab data, outbreak reports, vaccine uptake/side effects, serum banks

Eradication vs. Elimination vs. Control

Term	Meaning	Example
Eradication	Termination of ALL transmission worldwide; absolute, "all or none"	Smallpox (only one eradicated)
Elimination	Eradication from a large geographic region/country	Polio (India certified 2014)
Control	Reduction to acceptable levels	Most infectious diseases

Diseases amenable to eradication: Measles, diphtheria, polio, guinea worm (currently targeted)

Note: In smallpox eradication - it was NOT herd immunity alone that was decisive; it was surveillance and containment (ring vaccination strategy)

Types of Epidemics (Very Frequently Tested)

A. Common-Source Epidemics

(a) Point-source (single exposure) epidemic:

All exposure brief and simultaneous
All cases develop within one incubation period
Epidemic curve: sharp single peak, rises and falls rapidly, NO secondary waves
Classic example: Food poisoning outbreak at a wedding
Key concept: Median incubation period = time for 50% of cases to occur after exposure

(b) Continuous/multiple exposure epidemic:

Prolonged exposure from same contaminated source
Cases spread over longer period; plateau or multiple peaks
Example: Contaminated water supply

B. Propagated Epidemics

Person-to-person transmission
Gradual rise, slow tail-off over a longer period
Each wave is approximately one incubation period apart
Transmission continues until susceptibles depleted
Speed depends on: herd immunity, contact opportunities, SAR
Example: Hepatitis A, polio

C. Slow (Modern) Epidemics

Chronic diseases with long latency (tobacco-cancer, obesity epidemic)

PART 2: HERD IMMUNITY

Definition and Concept

Herd immunity = Protection conferred on unprotected individuals because a large proportion of the population is immune (vaccinated or previously infected).

Mechanism: High immunity → breaks chain of infection → susceptible individuals unlikely to contact infectious agent

Herd Immunity Threshold (HIT): The proportion immune ABOVE which disease cannot persist

R₀ - Basic Reproduction Number

R₀ = Number of secondary cases produced by ONE infectious case in a completely susceptible population

Disease	R₀
Measles	12-18 (highest among common infections)
Mumps	4-7
Rubella	5-7
Polio	5-7
Smallpox	5-7
Influenza	2-3
COVID-19 (original)	2-3

HIT formula:

HIT = 1 - (1/R₀)

For measles (R₀ = 15): HIT = 1 - (1/15) = 93% → Vaccination coverage needed >93%

Key facts for exam:

Herd immunity does NOT protect the individual against tetanus (tetanus is not spread person-to-person)
Smallpox eradication: surveillance-containment strategy was key (not just herd immunity)
For tetanus: individual immunity essential; herd immunity irrelevant

PART 3: VACCINES

Classification of Vaccines (Table 29, Park's)

A. Live Attenuated Vaccines

Prepared from attenuated (weakened) organisms - passed repeatedly in tissue culture/chick embryos
Advantages over killed vaccines:
1. Organisms multiply in host → larger antigenic dose
2. All major AND minor antigenic components present
3. Engage specific tissues (e.g., OPV engages intestinal mucosa)
4. Persistence of latent virus
Examples: BCG, OPV (oral polio), measles, MMR, yellow fever, varicella, rotavirus, typhoid (Ty21a oral)
Contraindications for live vaccines:
- Immunodeficiency states (HIV, leukemia, lymphoma, malignancy)
- Immunosuppressive therapy (corticosteroids, alkylating agents, antimetabolites, radiation)
- Pregnancy (unless risk of infection > risk to fetus)
Interval rule: Two live vaccines = give simultaneously at different sites OR with ≥3-week gap
Single dose usually sufficient (except OPV - needs ≥3 doses for effective mucosal immunity)

B. Killed/Inactivated Vaccines

Whole organisms killed by heat/chemicals
Examples: IPV (injectable polio), pertussis (whole cell), typhoid Vi (injectable), cholera, plague, rabies (HDCV), hepatitis A

C. Subunit Vaccines

Toxoids - inactivated toxins; stimulate antitoxin antibodies
- Diphtheria toxoid, tetanus toxoid
Protein vaccines - purified protein antigens
Recombinant protein vaccines - antigen produced by genetic engineering (e.g., Hepatitis B vaccine - HBsAg produced in yeast)
Polysaccharide-based vaccines - capsular polysaccharides
- Pure polysaccharide: pneumococcal (23-valent), meningococcal, typhoid Vi
Conjugated vaccines - polysaccharide + carrier protein = T-cell dependent response; immunogenic in infants
- Hib vaccine, pneumococcal (PCV13), meningococcal conjugate

Vaccine Excipients (Other Components)

Component	Role	Example
Adjuvant	Enhances immune response; allows dose reduction	Aluminium salts (Al hydroxide, Al phosphate)
Antibiotics	Prevent bacterial contamination during manufacturing	Neomycin in MMR and IPV (<25 µg/dose)
Preservatives	Prevent contamination of multi-dose vials	Thiomersal, formaldehyde
Stabilizers	Maintain vaccine stability	Gelatin, sorbitol, MgCl₂

Exam trap: Neomycin allergy = contraindication/caution for MMR and IPV

Combination Vaccines

Vaccine	Components
DPT	Diphtheria + Pertussis + Tetanus
MMR	Measles + Mumps + Rubella
DPTP	DPT + IPV
Pentavalent	DPT + Hep B + Hib
DT	Diphtheria + Tetanus (no pertussis)
Td	Tetanus + low-dose diphtheria (adult formulation)

Polyvalent vaccine = prepared from ≥2 strains of the SAME species (e.g., OPV types 1,2,3; influenza vaccine) Autogenous/auto vaccine = organism obtained from the SAME patient

Universal Immunization Programme (UIP) - India

Started as EPI (Expanded Programme on Immunization) in 1978
Converted to UIP in 1985
Initially covered: BCG, DPT, OPV, typhoid (urban), measles (added 1985), TT for pregnant women (1983)
Currently under Mission Indradhanush (2014) - target 90% coverage

Current UIP Schedule (NEET High Yield):

Age	Vaccines
Birth	BCG, OPV-0, Hepatitis B (birth dose)
6 weeks	OPV-1, Pentavalent-1 (DPT+HepB+Hib), IPV-1, Rotavirus-1, PCV-1
10 weeks	OPV-2, Pentavalent-2, Rotavirus-2, PCV-2
14 weeks	OPV-3, Pentavalent-3, IPV-2, Rotavirus-3, PCV-3
9-12 months	MR (Measles-Rubella), JE (endemic areas), Vitamin A (1st dose)
16-24 months	DPT booster-1, OPV booster, MR-2, Vitamin A (2nd dose)
5-6 years	DPT booster-2
10 years	TT
16 years	TT
Pregnancy	TT-1 (early), TT-2 (4 weeks later)

PART 4: COLD CHAIN

Definition

"A system of storage and transport of vaccines at low temperature from manufacturer to vaccination site"

6 Rights of Supply Chain: right vaccine, right quantity, right place, right time, right condition (no temperature break), right cost

Temperature Requirements

Vaccine	Storage Temperature	Key Point
OPV	−15°C to −25°C (freezer)	Most sensitive to heat
Varicella	−15°C to −25°C
MMR	+2°C to +8°C (short term) OR −20°C (long term)
BCG	+2°C to +8°C	Sensitive to heat AND light
DPT, Pentavalent, Hepatitis B, TT	+2°C to +8°C (NEVER freeze)	Freeze-sensitive!
Rotavirus, IPV	+2°C to +8°C

Critical exam fact: DPT, Hepatitis B, TT = freeze-sensitive vaccines. If frozen, they are destroyed and may cause local reactions if given.

Cold Chain Equipment (UIP)

Equipment	Temperature	Level	Use
Walk-in Freezer (WIF)	−15°C to −25°C	National/State/Regional	Bulk storage of OPV; ice pack preparation
Walk-in Cooler (WIC)	+2°C to +8°C	State/Regional/District	All UIP vaccines except OPV bulk
Deep Freezer (DF)	−15°C to −25°C	District level and above	OPV storage (3 months); freezing ice packs
Ice-Lined Refrigerator (ILR)	+2°C to +8°C	District/PHC level	Most UIP vaccines; better hold-over time than DF
Cold box	Non-electrical	Transport	3-day transport of vaccines
Vaccine carrier	Non-electrical	Field level	Outreach/session sites; ≤1 day

ILR vs DF: ILR has BETTER hold-over time than DF (ice lining retains cold during power cuts)

Vaccine Vial Monitor (VVM)

A heat-sensitive label attached to every vaccine vial
Color changes progressively with heat exposure
Inner square DARKER than outer circle = vaccine should NOT be used
Useful for: checking heat exposure during cold chain
Does NOT detect freeze damage in freeze-sensitive vaccines

PART 5: PASSIVE IMMUNIZATION (Antisera)

Disease	Preparation	Dose
Diphtheria	Antitoxin (horse serum)	500-1000 IU IM; lasts only 2-3 weeks
Tetanus	ATS (horse)	1500 units SC/IM after injury
Rabies	Antirabies serum	40 IU/kg body weight IM within 72 hrs (part applied locally to wound)
Botulism	Polyvalent antitoxin	10,000 units q3-4h
Gas gangrene	Polyvalent antitoxin	Cl. perfringens 10,000 IU + Cl. septicum 5,000 IU + Cl. oedematiens 10,000 IU

PART 6: AEFI (Adverse Events Following Immunization)

Classification	Description	Example
Vaccine-induced	Due to inherent properties of vaccine	OPV-associated polio (VAPP)
Vaccine-potentiated	Pre-existing condition triggered by vaccine	Febrile seizure in predisposed child
Programme error	Wrong technique/dose/storage	Sterile abscess due to wrong route
Coincidental	Temporal association only; no causal link	Fever from another infection post-vaccination

VAPP (Vaccine-Associated Paralytic Polio): ~1 in 500,000 first doses of OPV; reason India switched to IPV in UIP (2015)

PART 7: HIGH-YIELD MNEMONICS & EXAM TRAPS

Live Vaccines Memory Aid: "BCG My OPV YeaR VaTa"

BCG, Measles, OPV, Yellow fever, Rotavirus, Varicella, Typhoid (oral Ty21a)

Freeze-Sensitive Vaccines: "DHHT"

DPT, Hepatitis B, Hib, TT - all stored at +2 to +8°C, NEVER freeze

Key Numbers

Measles R₀ = 12-18 (highest of common infections)
Herd immunity for measles = >93%
Rabies antirabies serum = 40 IU/kg
VVM: inner square darker than outer = discard
WIF/Deep Freezer temperature = −15°C to −25°C
ILR/WIC temperature = +2°C to +8°C
Diphtheria antitoxin lasts only 2-3 weeks
Serial interval = time between primary and secondary case onset
Generation time = time from infection to maximal infectivity
In mumps = max communicability 48 hours BEFORE parotid swelling

Commonly Tested Exam Traps

Tetanus - herd immunity does NOT protect (soil reservoir; not person-to-person)
Smallpox eradication - surveillance-containment (ring vaccination), NOT mass immunization
OPV vs IPV - OPV produces mucosal (secretory IgA) + systemic immunity; IPV produces only systemic immunity
Eradication vs Elimination - Only smallpox has been ERADICATED globally
Common-source vs Propagated epidemic - Common-source: ONE incubation period, sharp peak; Propagated: gradual rise, multiple waves
Generation time ≠ Incubation period - Generation time is broader (includes subclinical); incubation only for manifest disease
Contamination ≠ Carrier state - Contamination on body surface does NOT imply carrier state
Aluminium salts as adjuvant - In DPT, hepatitis B, pneumococcal vaccines; enhances antibody response

Sources: Park's Textbook of Preventive and Social Medicine (24th ed.) - the authoritative text for PSM/Community Medicine in NEET PG. All definitions, classifications, and data points above are directly drawn from Park's.

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