Discuss Types of Vaccines & Basis of Immunity — its causes, clinical features and management.

Basis — Active VS Passive Immunity

Classification of Vaccines

• Live attenuated — BCG, OPV, measles/MR/MMR, rotavirus, varicella, live JE, yellow fever.
• Inactivated/killed — IPV, whole-cell pertussis, hepatitis A, rabies, cholera.
• Toxoids — diphtheria and tetanus (inactivated toxins).
• Subunit / conjugate / recombinant — Hib, pneumococcal (PCV), meningococcal, recombinant hepatitis B, HPV, acellular pertussis.

Live VS Inactivated Vaccines

Herd Immunity

When a high proportion of a population is immunised, transmission is interrupted and even the unimmunised are protected — herd immunity — the basis of disease elimination/eradication programmes.

The Immune Response to Vaccination

• Primary response — after the first dose; slow, mainly IgM, low titre.
• Secondary (booster) response — rapid, high-titre IgG from memory cells; the basis of booster doses.

Adjuvants & Combination Vaccines

• Adjuvants (e.g. Aluminium salts) enhance the immune response to inactivated antigens.
• Combination vaccines (pentavalent, MMR) reduce the number of injections and improve compliance.

Routes of Vaccine Administration

• Oral (OPV, rotavirus), intradermal (BCG), subcutaneous (measles/MR), intramuscular (most inactivated/toxoid/conjugate).
• Correct route is essential for efficacy and safety.

Cold-chain & Stability Implications

The vaccine type determines storage needs — live vaccines are generally more heat-labile (careful cold chain, VVM monitoring), whereas adsorbed inactivated/toxoid vaccines must not be frozen; this links vaccine science directly to programme logistics.

Spacing & Co-administration

• Most vaccines can be co-administered at different sites on the same day.
• Two live parenteral vaccines, if not given together, are separated by ~4 weeks.

Vaccine Failure

• Primary failure — the vaccine does not induce an adequate initial response (e.g. Host/immune factors, or cold-chain breach).
• Secondary failure — immunity wanes over time (a reason for boosters).

WHY Vaccines Need Specific Schedules

Dosing schedules balance the infant's developing immune system, the decline of protective maternal antibodies, and the disease risk by age — hence, for example, measles vaccination at 9 months (when maternal antibody has waned) and the birth dose of hepatitis B (to pre-empt perinatal transmission).

Special Groups

• Immunocompromised children — avoid live vaccines; give inactivated vaccines (may need extra doses).
• Preterm infants are vaccinated by chronological age; pregnancy — inactivated vaccines/toxoids are safe, live vaccines avoided.