Expansion
Live attenuated give strong lasting immunity; inactivated are safer but need boosters
Expansion
| Type | Examples | Notes |
|---|---|---|
| Live attenuated | MMR, BCG, oral polio, varicella, yellow fever | Strong, lasting, often single dose; contraindicated in immunosuppression and pregnancy |
| Inactivated | Injected polio, hepatitis A, rabies | Safe, needs boosters |
| Subunit or toxoid | Tetanus, diphtheria, hepatitis B, acellular pertussis | Very safe, needs adjuvant and boosters |
| Conjugate | Hib, pneumococcal, meningococcal | Protein conjugation converts a T-independent polysaccharide response into a T-dependent one, giving memory and efficacy in infants |
| mRNA and viral vector | COVID-19 | Rapid development |
The conjugate principle is the conceptually important one: pure polysaccharide vaccines do not generate memory and work poorly under 2 years, because infants respond poorly to T-independent antigens.
Passive immunisation with immunoglobulin gives immediate but temporary protection, and is used post-exposure for hepatitis B, tetanus, rabies and varicella.
Herd immunity protects those who cannot be vaccinated, and the threshold rises with transmissibility, which is why measles requires very high coverage.