Vaccines & Immunisation
Microbiology · Immunology · lean revision notes
Vaccines & Immunisation
Immunisation is the single most cost-effective public-health intervention after clean water. For NEET PG, this topic sits at the intersection of Microbiology, Immunology, and PSM/Community Medicine — questions reliably link a vaccine type to its organism, its schedule slot, its cold-chain temperature, or its contraindication in the immunocompromised. This note organises every high-yield fact into tables, criteria, and one-liners you can recall under exam pressure.
Definitions & basic concepts
- Active immunity — host's own immune system is stimulated to produce antibodies + memory cells; slow onset, long-lasting. Achieved by infection or by vaccines/toxoids.
- Passive immunity — preformed antibodies are transferred to the host; immediate onset, short-lived (weeks–months), no memory. Achieved by maternal IgG across placenta, breast milk IgA, immunoglobulins / antitoxins / antisera.
- Vaccine — a biological preparation that provides active acquired immunity against a particular infectious agent.
- Toxoid — an exotoxin inactivated (usually by formaldehyde) so it loses toxicity but retains antigenicity (e.g., tetanus, diphtheria).
- Antitoxin — preformed antibody against a toxin → passive protection (e.g., diphtheria antitoxin, tetanus immunoglobulin).
High-yield: Active immunity = slow onset + memory + long duration. Passive immunity = immediate + NO memory + short duration. This single distinction generates many one-liner MCQs.
Classification of vaccines
The most-tested table in this topic. Know the prototype organism for each category.
| Type | Mechanism | Examples (NEET high-yield) | Booster needed? |
|---|---|---|---|
| Live attenuated | Weakened organism replicates → robust humoral + cell-mediated immunity | BCG, OPV (Sabin), MMR, Varicella, Yellow fever, Rotavirus, Oral typhoid (Ty21a), Intranasal influenza | Usually single/few doses |
| Killed / inactivated | Whole dead organism; only humoral, weaker | IPV (Salk), Whole-cell pertussis, Cholera (killed oral), Rabies, Hepatitis A, Injectable influenza, JE (Vero-cell) | Multiple doses + boosters |
| Toxoid | Inactivated exotoxin | Tetanus, Diphtheria | Yes (10-yearly) |
| Subunit / purified protein | Specific antigen only | Acellular pertussis (aP), HBsAg (Hep B), HPV | Yes |
| Conjugate | Polysaccharide linked to carrier protein → T-cell help, works <2 yr | Hib, Pneumococcal (PCV13), Meningococcal conjugate, Typhoid conjugate (TCV) | Yes |
| Polysaccharide (plain) | Capsular polysaccharide; T-independent → poor in infants | Pneumococcal (PPSV23), older meningococcal, Vi typhoid | Poor memory |
| mRNA | Lipid-nanoparticle-encapsulated mRNA → host cells make antigen | COVID-19 (Pfizer–BioNTech, Moderna) | Booster(s) |
| Viral-vector | Harmless virus carries target gene | COVID-19 (Covishield/AstraZeneca, Sputnik), Ebola | — |
High-yield: Polysaccharide vaccines are T-independent → poor immunogenicity in children <2 years and no memory. Conjugation to a protein carrier converts them to T-dependent, generating memory and protecting infants — this is the entire rationale for Hib, PCV, and conjugate typhoid (TCV).
Classic mnemonic for live attenuated vaccines — "Live! See MR. BOY" roughly captures: BCG, OPV, Yellow fever, MMR, Rotavirus, Varicella, oral Typhoid. Live vaccines are the ones contraindicated in pregnancy and immunocompromise.
Why vaccine type dictates everything
Antigen → immune pathway → contraindications → storage is the logical chain:
- Live vaccine → replicates → strong & durable immunity → but can cause disease in immunocompromised/pregnant → contraindicated there.
- Killed/subunit → no replication → safe in immunocompromised → but weaker → needs adjuvant + boosters.
- Polysaccharide → T-independent → fails in infants → conjugate to fix it.
High-yield: Live vaccines given on the same day OR ≥28 days (4 weeks) apart. If you miss the same-day window, separate two live parenteral vaccines by at least 4 weeks, otherwise interference reduces the response to the second.
National Immunisation Schedule (India — UIP)
This is the PSM table examiners love. Memorise the slots, not just the names.
| Age | Vaccines |
|---|---|
| Birth | BCG, OPV-0 (zero dose), Hepatitis B (birth dose) |
| 6 weeks | OPV-1, Pentavalent-1 (DPT+HepB+Hib), fIPV-1, Rotavirus-1, PCV-1 |
| 10 weeks | OPV-2, Pentavalent-2, Rotavirus-2 |
| 14 weeks | OPV-3, Pentavalent-3, fIPV-2, Rotavirus-3, PCV-2 |
| 9–12 months | Measles–Rubella (MR)-1, JE-1, PCV-booster, Vitamin A (1st dose) |
| 16–24 months | MR-2, JE-2, DPT-booster-1, OPV-booster |
| 5–6 years | DPT-booster-2 |
| 10 years | Td (tetanus + reduced diphtheria) |
| 16 years | Td |
| Pregnancy | Td — two doses (or one booster if previously immunised) |
High-yield: BCG, OPV-0 and Hepatitis B are the only birth-dose vaccines in the UIP. BCG can be given up to 1 year of age if missed.
High-yield: India switched from TT (tetanus toxoid alone) to Td (which adds reduced-dose diphtheria) to address waning diphtheria immunity in adolescents/adults.
Note: fIPV = fractional inactivated polio vaccine (intradermal, 0.1 mL), introduced as part of the polio endgame after OPV2 withdrawal (switch from trivalent → bivalent OPV in 2016).
Cold chain — temperatures & the "freeze-sensitive vs heat-sensitive" trap
The cold chain is the system of storing and transporting vaccines within a safe temperature range from manufacture to administration.
| Level | Equipment | Temperature |
|---|---|---|
| Primary/regional store | Walk-in cooler (WIC) | +2°C to +8°C |
| Walk-in freezer (WIF) | −15°C to −25°C | |
| District/PHC | ILR (Ice-Lined Refrigerator) | +2°C to +8°C |
| Deep freezer (DF) | −18°C to −20°C (makes ice packs, stores OPV) | |
| Field/transport | Vaccine carrier (4 ice packs) | up to ~12 h |
| Cold box | larger, longer transport |
High-yield: In the Deep Freezer keep OPV and Measles/MR. In the ILR (+2–8°C) keep all others, and critically the freeze-sensitive ones: DPT, TT/Td, Hepatitis B, IPV, PCV, Pentavalent, dilutents must NOT freeze.
Mnemonic for most heat-sensitive → least: OPV > Measles/MR > BCG. The "shake test" detects a DPT/Hep-B/TT vaccine that has been frozen and thawed (flocculation/sedimentation appears faster than an undamaged comparator) → discard if damaged.
- VVM (Vaccine Vial Monitor): heat-sensitive square on the label. Use vaccine while the inner square is lighter than the outer ring; discard once the square matches or is darker than the ring.
- Open Vial Policy: opened multi-dose vials of OPV, DPT, TT, Hep B, Pentavalent may be used in subsequent sessions up to 4 weeks if VVM ok, no contamination, expiry not crossed. Reconstituted BCG and Measles/MR must be discarded within 4 hours.
Herd immunity
Herd (community) immunity = indirect protection of susceptibles when a sufficient proportion of the population is immune, breaking transmission chains.
- Herd immunity threshold (HIT) = 1 − 1/R₀ (proportion that must be immune).
- Higher R₀ → higher threshold.
| Disease | R₀ (approx) | Herd immunity threshold |
|---|---|---|
| Measles | 12–18 | ~92–95% (highest) |
| Pertussis | 12–17 | ~92–94% |
| Diphtheria | 6–7 | ~85% |
| Polio | 5–7 | ~80–86% |
| Mumps/Rubella | 4–7 | ~75–86% |
| Influenza | 1.5–2 | ~33–50% |
High-yield: Measles needs the highest herd-immunity threshold (~95%) because of its very high R₀ — this is why measles outbreaks reappear first when coverage dips. Herd immunity does NOT apply to tetanus (acquired from soil/environment, not person-to-person).
Contraindications & special situations
| Situation | Avoid | Notes |
|---|---|---|
| Pregnancy | All live vaccines (MMR, varicella, BCG, OPV, yellow fever) | Td, influenza (inactivated), Tdap are recommended |
| Immunocompromised / HIV with low CD4 / on chemo / high-dose steroids | Live vaccines generally | Inactivated vaccines safe |
| Egg allergy (severe) | Caution: Yellow fever, influenza | MMR is grown in chick-embryo fibroblast — generally safe even with egg allergy |
| Anaphylaxis to prior dose | That vaccine | Absolute contraindication |
| Encephalopathy within 7 days of pertussis | Further pertussis component | Use DT instead of DPT |
High-yield: HIV-infected children CAN receive BCG only if asymptomatic; symptomatic HIV/AIDS is a contraindication to BCG (risk of disseminated BCG-osis). WHO now advises against BCG in known HIV-positive infants unless on ART and immunologically stable.
High-yield: OPV (live) is replaced by IPV (killed) in immunocompromised children and their household contacts to avoid vaccine-associated paralytic polio (VAPP) and shedding of live virus.
Not true contraindications (common exam distractors — vaccination should proceed): mild illness/low-grade fever, current antibiotic use, mild local reaction to previous dose, prematurity (vaccinate by chronological age), breastfeeding, malnutrition.
Adverse events following immunisation (AEFI)
- BCG: local ulcer at ~2–3 weeks, regional (axillary) lymphadenitis; disseminated BCG in immunocompromised.
- OPV: VAPP (≈1 per 2.7 million doses) and circulating vaccine-derived poliovirus (cVDPV) — the reason for the global switch toward IPV.
- DPT (whole-cell pertussis): fever, prolonged crying, hypotonic-hyporesponsive episodes, rarely encephalopathy → acellular pertussis (aP) chosen in many countries to reduce reactogenicity.
- MMR: fever + rash ~5–12 days, transient thrombocytopenia; NOT linked to autism (Wakefield study retracted/fraudulent — a favourite MCQ).
- Rotavirus: small risk of intussusception.
- Yellow fever: rare viscerotropic/neurotropic disease.
High-yield: AEFIs are classified as vaccine-product-related, vaccine-quality-defect-related, immunisation-error-related (programmatic), immunisation-anxiety-related, and coincidental. "Immunisation error" (e.g., wrong diluent, contamination, frozen vaccine) is the preventable category emphasised in exams.
Specific vaccine pearls (organism links)
- BCG — live attenuated Mycobacterium bovis; protects best against disseminated TB & TB meningitis in children, less so against adult pulmonary TB. Given intradermally over left deltoid.
- OPV — Sabin; trivalent → bivalent (type 1 & 3) after type-2 withdrawal; provides intestinal (mucosal IgA) immunity → interrupts community transmission.
- IPV — Salk; no mucosal immunity but no VAPP risk.
- Hib (conjugate) — against Haemophilus influenzae type b capsular polysaccharide (PRP).
- Hepatitis B — recombinant HBsAg; birth dose within 24 h prevents perinatal transmission.
- HPV — recombinant L1 capsid VLP; quadrivalent/9-valent; prevents cervical cancer (types 16, 18).
- Rabies — modern cell-culture vaccines (HDCV, PCEC, PVRV); post-exposure may add Rabies Immunoglobulin (RIG) = passive + active.
- Typhoid — Vi polysaccharide (≥2 yr), live oral Ty21a, and the newer Typhoid Conjugate Vaccine (TCV) usable from 6 months.
Active + passive combined
Some situations need both preformed antibody (immediate cover) and vaccine (durable cover):
- Rabies PEP (category III) → wound RIG + vaccine series.
- Tetanus-prone wound in under-immunised → TIG + Td.
- Hepatitis B exposure / neonate of HBsAg+ mother → HBIG + Hep B vaccine.
High-yield: Give the immunoglobulin and vaccine at different sites so the passive antibody does not neutralise the active antigen.
Key differentials / commonly confused pairs
| Confused pair | Discriminator |
|---|---|
| OPV vs IPV | OPV live, oral, mucosal immunity, VAPP risk; IPV killed, IM, no mucosal immunity |
| Salk vs Sabin | Salk = killed (IPV); Sabin = live oral (OPV) |
| Toxoid vs antitoxin | Toxoid = active (tetanus toxoid); antitoxin = passive (diphtheria antitoxin) |
| PPSV23 vs PCV13 | PPSV = plain polysaccharide (no infant memory); PCV = conjugate (works in infants) |
| TT vs Td vs Tdap | TT = tetanus only; Td = tetanus + low-dose diphtheria; Tdap adds acellular pertussis |
Recently asked / exam angle
- Vaccine–organism matching: "Which vaccine is a conjugate?" (Hib, PCV, meningococcal conjugate, TCV). "Which is a toxoid?" (tetanus, diphtheria).
- Storage temperature MCQs: ILR = +2 to +8°C; deep freezer = −18 to −20°C; which vaccines go in the deep freezer? → OPV & Measles/MR. Which must never be frozen? → DPT, Hep B, TT, IPV, PCV.
- Cold chain damage: "Shake test positive" → frozen DPT/Hep-B → discard. VVM interpretation.
- Schedule slots: birth-dose vaccines; age of first measles/MR; when fIPV is given.
- Herd immunity: formula 1 − 1/R₀; which disease needs the highest threshold (measles).
- Immunocompromise/pregnancy: which vaccines are contraindicated (all live); BCG in HIV.
- mRNA vaccine mechanism (lipid nanoparticle delivering mRNA → endogenous antigen synthesis → both arms of immunity) — increasingly asked post-COVID.
- MMR & autism myth — choose "no causal association."
- Open vial policy and 4-hour discard rule for reconstituted BCG/measles.
Rapid revision
- Active = memory + slow + durable; passive = immediate + no memory + short.
- Live vaccines: BCG, OPV, MMR, varicella, yellow fever, rotavirus, oral typhoid — contraindicated in pregnancy & immunocompromise.
- Toxoids = tetanus & diphtheria; conjugates = Hib, PCV, meningococcal, TCV.
- Polysaccharide vaccines fail in <2 yr (T-independent); conjugation fixes this.
- Birth doses (UIP): BCG + OPV-0 + Hepatitis B only.
- ILR = +2 to +8°C; Deep freezer = −18 to −20°C. OPV & Measles/MR in deep freezer.
- Never freeze: DPT, TT/Td, Hep B, IPV, PCV — detect freezing with the shake test.
- Most heat-sensitive order: OPV > Measles/MR > BCG; use VVM to judge heat exposure.
- Herd immunity threshold = 1 − 1/R₀; measles needs the highest (~95%).
- Two live parenteral vaccines: same day or ≥4 weeks apart.
- Salk = killed IPV; Sabin = live OPV; IPV used in immunocompromised to avoid VAPP.
- Reconstituted BCG/Measles → discard in 4 hours; other opened vials → up to 4 weeks (open vial policy).