Vaccines and Immunisation Pharmacology
On this page
Direct answer
Vaccines are biological preparations producing active immunity: live attenuated vaccines (BCG, oral polio, measles-rubella, rotavirus, varicella, yellow fever) replicate in the host, giving strong, durable, often single-dose immunity but are contraindicated in pregnancy and significant immunocompromise; killed or inactivated vaccines (injected polio, rabies, influenza), toxoids (tetanus, diphtheria) and subunit or conjugate vaccines (hepatitis B, Hib, pneumococcal) are safer in the immunocompromised but need multiple doses and boosters. Conjugation of polysaccharide to a carrier protein converts a T-cell-independent response into a T-cell-dependent one, creating memory and efficacy under two years — the rationale behind Hib and pneumococcal conjugate vaccines.
What you must remember
- Live vaccines: BCG, OPV, MMR, varicella, yellow fever and rotavirus; contraindicated in pregnancy and severe immunocompromise; different live injectable vaccines are spaced about four weeks apart or given the same day.
- Killed, toxoid and subunit vaccines: IPV, whole-cell or acellular pertussis components, rabies (cell culture), hepatitis B (recombinant surface antigen), tetanus and diphtheria toxoids — safe in immunocompromise but require complete schedules plus boosters.
- Conjugate versus polysaccharide: pure polysaccharide vaccines (23-valent pneumococcal) are T-independent — no memory, no mucosal herd effect, poor under two years; conjugate vaccines (PCV, Hib, meningococcal) are T-dependent, immunogenic in infants, and reduce carriage.
- Adjuvants: aluminium salts (depot effect, Th2 skewing), MF59, AS01 and CpG enhance immunogenicity, especially for subunit antigens; live vaccines need no adjuvant.
- Passive versus active: vaccines act after a lag but give durable memory; immunoglobulins give immediate, transient cover — rabies, tetanus and hepatitis B exposure use both.
- Indian UIP anchors: birth BCG, OPV zero dose and hepatitis B; pentavalent (DTP, Hib, hepatitis B) at 6, 10 and 14 weeks; PCV and rotavirus added nationally; measles-rubella at 9 to 12 months.
- Adverse-event logic: fever and local reactions are expected with aluminium-adjuvanted vaccines; rare serious events (anaphylaxis, intussusception with rotavirus in some settings) are tracked through AEFI surveillance, and contraindications such as severe egg allergy or prior anaphylaxis govern use.
Common confusion
OPV versus IPV is the most tested distinction: OPV is live, oral, induces gut (mucosal) immunity and herd protection but can revert to virulence (vaccine-associated and vaccine-derived polio), while IPV is killed, injected, systemic-only and safe in the immunocompromised host — the reason for the global switch to bivalent OPV plus IPV. Students also confuse herd immunity sources: conjugate vaccines reduce carriage (transmission), whereas toxoids do not prevent colonisation at all.
Exam-focused takeaway
Expect matching questions on vaccine type (live, killed, toxoid, conjugate, subunit), contraindication logic for the pregnant or immunocompromised patient, the immunological basis of conjugation, adjuvant mechanisms, and UIP schedule sequencing, such as which vaccines are given at birth. Stems describe a child on steroids or with HIV and ask which vaccines to withhold or give. The tetanus and rabies exposure pathways test combined active-passive immunisation timing.
Frequently asked questions
Why are live vaccines avoided in pregnancy and immunocompromise?
They replicate in the host, so theoretical fetal risk and uncontrolled infection in the immunodeficient outweigh benefit; killed and toxoid vaccines are used instead.
What advantage do conjugate vaccines have over polysaccharide ones?
Conjugation to carrier proteins makes the response T-cell dependent, generating memory, booster responses, infant efficacy and reduced nasopharyngeal carriage.
What is the role of an adjuvant?
To enhance the immunogenicity of the antigen — aluminium salts create a depot and stimulate innate immunity, allowing lower antigen doses and fewer injections.
Which hepatitis B vaccine type is used?
A recombinant subunit vaccine containing hepatitis B surface antigen produced in yeast, given in a three-dose schedule.
Can vaccines be given to a child on corticosteroids?
Killed and toxoid vaccines yes; live vaccines are deferred until immunosuppressive doses (such as prednisolone over 2 mg per kg for over two weeks) are stopped.