Immunology and AllergyTier 2Topic

Immunisation

How vaccines work

Vaccination generates protective adaptive immunity by exposing the immune system to a pathogen antigen (or a component/inactivated form of it) without causing the disease itself
Vaccination programs have produced some of the largest reductions in infectious disease burden in medical history (smallpox eradication, near-elimination of polio)

Vaccine types

Vaccine type	Example	Key feature
Live attenuated	MMR, varicella, oral polio, yellow fever, BCG	Strongest, most durable immunity (mimics natural infection); contraindicated in significant immunosuppression and pregnancy
Inactivated (killed)	Injectable polio, hepatitis A	Safe in immunosuppression; generally needs boosters
Subunit/conjugate	Pneumococcal conjugate, Hib, HPV	Targets a specific antigen; conjugation to a carrier protein improves the immune response in infants
Toxoid	Tetanus, diphtheria	Targets the toxin, not the organism itself
mRNA	Some COVID-19 vaccines	Encodes the antigen for in vivo production; not a live product, safe in immunosuppression

Checking response

Vaccine response can be checked directly via specific antibody titres where relevant (e.g. hepatitis B surface antibody post-vaccination in at-risk groups)

Immunosuppressed patients

Live vaccines are generally contraindicated in significant immunosuppression (including high-dose corticosteroids, biologics, chemotherapy, and pregnancy) given the risk of vaccine-strain disease
Inactivated/subunit/mRNA vaccines are safe in immunosuppression but may produce a blunted response - timing vaccination before planned immunosuppression, when possible, improves efficacy

Herd immunity and cocooning

Herd immunity, cocooning strategies (vaccinating contacts of vulnerable individuals), catch-up vaccination schedules

Waning immunity

Vaccine-induced immunity can wane over time for some vaccines (pertussis, some inactivated products), requiring booster doses - distinct from the durable, often lifelong immunity from natural infection or live vaccines

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