Immunization & National Vaccine Schedule

1. What this chapter covers, and how NEET PG actually tests it

Immunisation questions ask for a schedule detail, a cold chain rule, a contraindication or the classification of an adverse event.

The organising principle is that one property decides almost everything: is the organism in the vaccine alive?

The last two rows are the ones candidates get wrong. Live vaccines are destroyed by heat, while adsorbed killed vaccines are destroyed by freezing, so the two halves of the cold chain protect against opposite dangers.

2. Types of immunity and vaccine

2.1 Active and passive

Active immunity is produced by the host's own immune response, takes days to weeks to develop, and lasts for years.

Passive immunity is transferred ready-made as antibody, acts immediately, and lasts weeks to months.

Passive immunity is used when there is no time to wait, as in post-exposure rabies or tetanus prophylaxis, and it is given alongside the vaccine rather than instead of it.

Maternal antibody crossing the placenta is natural passive immunity, and its persistence is the reason measles vaccine is not given before nine months.

2.2 Vaccine types

Conjugation exists to solve one specific problem. Polysaccharide capsules provoke a T-independent response, which is poor in children under two and generates no memory, so linking the polysaccharide to a protein converts it into a T-dependent response with memory.

3. India's national schedule

Pentavalent combines diphtheria, pertussis, tetanus, hepatitis B and Haemophilus influenzae type b in a single injection.

Vitamin A is given with the first measles dose and then six-monthly to five years, making nine doses in all.

Tetanus and adult diphtheria has replaced plain tetanus toxoid throughout, because diphtheria immunity in adolescents and adults had been waning.

3.1 Routes and sites

BCG goes intradermally into the left arm by convention, so that the scar can be found reliably when coverage is being assessed years later.

The anterolateral thigh is used in infants rather than the buttock, because gluteal injection risks sciatic nerve injury and deposits vaccine into fat where absorption is poor.

4. The cold chain

4.1 Temperatures

Vaccines are held at two to eight degrees Celsius in an ice-lined refrigerator at primary health centre level.

Oral polio vaccine is held in a deep freezer at minus fifteen to minus twenty-five degrees at district level and above.

Ice packs are frozen in the same deep freezer, and this is the reason freezers exist at that level at all once polio storage requirements changed.

4.2 What damages what

Oral polio vaccine is the most heat-sensitive vaccine in the programme, followed by measles and rubella.

Freezing damages adsorbed vaccines, because the aluminium adjuvant aggregates irreversibly and the antigen is no longer presented properly.

The shake test detects freeze damage: a frozen and thawed adsorbed vaccine settles rapidly into a granular sediment, while an undamaged vial stays uniformly cloudy.

The vaccine vial monitor is a heat-sensitive square inside a printed circle, and the rule is simple. While the square is lighter than the circle, the vaccine may be used; once it matches or is darker, the vial is discarded.

4.3 Open vial policy

Multi-dose vials of oral polio, hepatitis B, pentavalent and tetanus and adult diphtheria may be kept and reused for up to twenty-eight days if the cold chain has been maintained and the vaccine vial monitor is acceptable.

Reconstituted BCG, measles and rubella, and Japanese encephalitis must be discarded within four hours, because they contain no preservative and the diluent supports bacterial growth.

That distinction is between vaccines supplied as liquid with preservative and those requiring reconstitution, not between live and killed.

4.4 The equipment at each level

An ice-lined refrigerator holds temperature for many hours after a power cut, because the water jacket surrounding the chamber freezes and then releases its latent heat slowly, which is why it is chosen for places with unreliable electricity.

A vaccine carrier is packed with four conditioned ice packs and carries enough vaccine for one immunisation session.

Conditioning means leaving frozen ice packs out until water droplets appear, and it exists specifically to prevent freeze damage to adsorbed vaccines sitting against them.

5. Passive immunisation and combining vaccines

Immunoglobulin is given when protection is needed immediately and there is no time for an active response.

Vaccine and immunoglobulin are given at different sites with different syringes, because the antibody would neutralise the vaccine antigen if they mixed.

Two live parenteral vaccines are given either on the same day or at least four weeks apart, because the interferon response to the first blunts the take of a second given a few days later.

Oral live vaccines and inactivated vaccines carry no such restriction and may be given at any interval.

Immunoglobulin delays the response to live parenteral vaccines for several months, so measles vaccine after blood products must be deferred, though this does not apply to oral polio or yellow fever.

6. Vaccines outside the national programme

Several vaccines are recommended by professional bodies but are not supplied free under the national programme, and the exam expects the distinction.

Typhoid conjugate vaccine, varicella, hepatitis A, influenza and measles-mumps-rubella all fall into this category.

The absence of mumps from the national programme is the one most often asked about, since India uses measles and rubella rather than the triple vaccine.

Judging any candidate vaccine for programme inclusion turns on disease burden, vaccine efficacy, cost per case averted, cold chain feasibility and the ability to sustain supply, not on efficacy alone.

7. Adverse events following immunisation

The World Health Organization classification has five categories, and the value of it is that only two of them reflect a problem with the vaccine itself.

Immunisation error is the largest preventable category, covering wrong diluent, contaminated multi-dose vials, wrong route and reuse of syringes, and it is the category a district investigation is most likely to find.

Immunisation anxiety includes vasovagal fainting and hyperventilation, which cluster in adolescents and can spread through a group.

Coincidental events matter disproportionately, because a vaccine given to millions of healthy children will inevitably be followed by unrelated illness, and confusing that with causation has repeatedly damaged coverage.

Every serious event is reported and investigated at district level, and the investigation asks specifically whether other children from the same vial or session were affected, since clustering points to an error or a quality defect rather than to the product itself.

8. Special situations

Live vaccines are contraindicated in significant immunodeficiency, in high-dose corticosteroid therapy and in pregnancy.

Human immunodeficiency virus infection is a partial exception, and asymptomatic children still receive measles vaccine because measles in an infected child is far more dangerous than the vaccine.

Bacille Calmette-Guerin is withheld in symptomatic infection because it is a live bacterial vaccine capable of disseminating.

Mild illness, low-grade fever, malnutrition, breastfeeding and antibiotic treatment are not contraindications, and treating them as such is a major cause of missed opportunity.

An interrupted schedule is resumed rather than restarted, because immunological memory persists, and no dose already given is wasted.

Preterm infants are immunised according to chronological age rather than corrected age, since the immune response depends on time since birth rather than on gestational maturity.

A previous severe allergic reaction to a vaccine component is a genuine contraindication to further doses of that vaccine, and it is one of very few absolute ones.

Encephalopathy within seven days of a pertussis-containing vaccine is the classical contraindication to further pertussis doses, and the acellular preparation is used instead where available.

Egg allergy no longer excludes measles-containing vaccines, since these are grown in chick embryo fibroblast culture and contain negligible egg protein.

9. Programmes and current position

Mission Indradhanush was launched in 2014 to reach children who had been missed, and successive intensified rounds have extended the target age and added district-level micro-planning.

India was certified polio-free in 2014, and injectable polio vaccine was introduced alongside the oral vaccine to eliminate the residual risk of vaccine-derived paralytic disease.

The measles and rubella elimination target is 2026, pursued through a national campaign seeking coverage above ninety-five per cent in every district.

India launched a nationwide human papillomavirus vaccination programme in February 2026, offering a single dose free of charge to girls aged fourteen, with roughly 1.15 crore girls targeted each year.

The single-dose schedule reflects evidence that one dose gives protection comparable to two in this age group, and it makes a programme of this scale logistically feasible.

The vaccine is delivered through government facilities including community health centres, district hospitals and government medical colleges.

9.1 Measuring coverage and efficacy

Coverage is assessed by the thirty-cluster survey, in which thirty clusters are chosen with probability proportional to population size and seven children are surveyed in each.

The design exists because a full census of immunisation status in a district is impossible, and simple random sampling across a scattered rural population is logistically unworkable.

Vaccine efficacy is measured by comparing attack rates in vaccinated and unvaccinated groups.

$\text{Vaccine efficacy}=\frac{\text{ARU}-\text{ARV}}{\text{ARU}}\times 100$

Here ARU is the attack rate in the unvaccinated and ARV is the attack rate in the vaccinated, so the expression states the proportion of disease prevented among those vaccinated.

A common and instructive trap follows from this. In a population with very high coverage, most cases of a disease will occur in vaccinated people simply because almost everyone is vaccinated, and this does not mean the vaccine has failed.

The herd immunity threshold is the proportion of a population that must be immune to interrupt transmission, and it rises with the basic reproduction number.

Measles, being among the most transmissible infections known, requires coverage above ninety-five per cent, which is precisely why elimination campaigns set that figure as their target.

10. Worked examples

Example 1. A vial of pentavalent vaccine has been kept in the freezer compartment overnight. Can it be used?

No. Pentavalent is an adsorbed vaccine and freezing irreversibly aggregates the aluminium adjuvant. The shake test would confirm it, showing rapid granular sedimentation instead of a uniform suspension.

Example 2. A vaccine vial monitor shows the inner square lighter than the outer circle. What is the action?

Use the vaccine. The monitor is read by comparison, and only when the square matches or becomes darker than the circle has cumulative heat exposure made the vial unusable.

Example 3. A child received the first two pentavalent doses eight months ago and has missed the third. Should the course be restarted?

No. Resume where the schedule was interrupted and give the third dose. Immunological memory persists, so no previously given dose is wasted and restarting only delays protection.

Summary

Ask first whether the organism is alive, because that decides doses, route, contraindications and storage.

Live vaccines are destroyed by heat and killed adsorbed vaccines by freezing, so the cold chain guards against opposite dangers.

Active immunity is slow and lasting; passive is immediate and temporary, and the two are given together after exposure.

Maternal antibody is why measles vaccine waits until nine months.

Conjugation converts a T-independent polysaccharide response into a T-dependent one with memory, which is why it works under two years.

Pentavalent covers diphtheria, pertussis, tetanus, hepatitis B and Haemophilus influenzae type b.

Vitamin A begins with the first measles dose and continues six-monthly to five years, nine doses in all.

Tetanus and adult diphtheria has replaced plain tetanus toxoid because adult diphtheria immunity was waning.

BCG is 0.1 mL intradermal into the left arm, halved under one month, so the scar can be found later.

Measles and rubella is subcutaneous into the right arm; pentavalent is intramuscular into the anterolateral thigh.

The gluteal site is avoided in infants because of sciatic nerve risk and poor absorption from fat.

Vaccines are stored at two to eight degrees, and oral polio at minus fifteen to minus twenty-five.

Oral polio is the most heat-sensitive vaccine, followed by measles and rubella.

Pentavalent, hepatitis B, tetanus and adult diphtheria, injectable polio and pneumococcal must never be frozen.

The shake test detects freeze damage by rapid granular sedimentation.

A vaccine vial monitor is usable while the inner square remains lighter than the outer circle.

Liquid multi-dose vials last twenty-eight days; reconstituted vaccines are discarded within four hours.

Adverse events fall into five categories, and immunisation error is the largest preventable one.

Coincidental events are inevitable at population scale and have repeatedly been mistaken for vaccine harm.

Live vaccines are avoided in immunodeficiency and pregnancy, but asymptomatic children with human immunodeficiency virus still receive measles vaccine.

Mild illness, fever, malnutrition and breastfeeding are not contraindications.

An interrupted schedule is resumed, never restarted.

India was certified polio-free in 2014 and targets measles and rubella elimination by 2026.

A nationwide single-dose human papillomavirus programme for fourteen-year-old girls began in February 2026.

Two live parenteral vaccines go on the same day or four weeks apart; oral live and inactivated vaccines have no such restriction.

Vaccine and immunoglobulin are given at separate sites with separate syringes, since antibody would neutralise the antigen.

Ice packs are conditioned until droplets appear, specifically to protect adsorbed vaccines from freezing against them.

Coverage is assessed by the thirty-cluster survey, and vaccine efficacy compares attack rates in the unvaccinated and vaccinated.

In a highly vaccinated population most cases occur in vaccinated people, which reflects arithmetic rather than vaccine failure.