Vaccination (immunisation)

Vaccines exploit immunological memory. Since Jenner's use of cowpox to protect against smallpox (1796), vaccination has eradicated smallpox (declared 1980) and brought polio, measles, diphtheria, tetanus and Hib disease under control.

Vaccine types. Live attenuated vaccines contain weakened, replicating organisms and induce strong, long-lasting antibody and T-cell immunity, often after one or two doses — e.g. MMR, varicella, yellow fever, BCG, rotavirus, oral typhoid and the oral (Sabin) polio and intranasal influenza vaccines — but they are contraindicated in significant immunosuppression and in pregnancy. Inactivated whole-organism vaccines (injectable Salk polio, hepatitis A, rabies) are safe but need boosters. Subunit, recombinant and virus-like-particle vaccines use purified antigens (hepatitis B surface antigen, HPV L1 VLPs, acellular pertussis, recombinant zoster glycoprotein E). Toxoids are inactivated toxins (tetanus, diphtheria) that induce neutralising antitoxin. Polysaccharide vaccines are T-independent, so they give short-lived IgM-dominated responses and work poorly under 2 years; conjugate vaccines link the polysaccharide to a protein carrier (e.g. tetanus toxoid, CRM197), recruiting T-cell help for class-switched IgG and memory, and are effective in infants (Hib, pneumococcal, meningococcal). Nucleic-acid vaccines deliver mRNA (in lipid nanoparticles) or DNA in a viral vector so that host cells make the antigen (COVID-19, Ebola).

Adjuvants (aluminium salts, oil-in-water emulsions such as MF59, TLR agonists such as monophosphoryl lipid A and CpG, saponin QS-21) enhance immunogenicity by activating innate immunity and dendritic cells. Herd (community) immunity arises when enough of a population is immune to stop sustained transmission; the threshold is approximately 1 − 1/R₀ — about 92–95% for measles (R₀ ≈ 12–18), lower for less transmissible infections — protecting infants and immunocompromised people who cannot be vaccinated.

Passive immunisation provides preformed antibody for immediate, temporary protection without memory: naturally (maternal IgG across the placenta — boosted by vaccinating pregnant women against pertussis, influenza and RSV — and breast-milk IgA) or artificially (immunoglobulins, antitoxins, monoclonal antibodies).