Question
Herd immunity
Answer
Herd immunity (community/population immunity) is the indirect protection conferred on susceptible individuals in a population when a sufficiently high proportion of that population is immune to an infectious agent (through vaccination or prior natural infection) — the resulting scarcity of susceptible hosts interrupts sustained chains of transmission, so even non-immune individuals benefit from a reduced probability of exposure.
Mechanism: for infection to spread, an infected person must, on average, transmit to at least one susceptible contact; if enough of the population is immune, most contacts an infected person encounters are already immune, so transmission chains die out even without the entire population being immunized.
Herd immunity threshold: the minimum proportion of a population that must be immune to interrupt sustained transmission — it depends directly on the pathogen’s basic reproduction number (R0, the average number of secondary cases generated by one infectious case in a fully susceptible population); pathogens with higher R0 (e.g., measles, R0 ≈ 12–18) require a much higher immunization coverage (~95%) to achieve herd immunity than less transmissible pathogens.
Public health significance:
- Protects individuals who cannot be vaccinated or mount an adequate immune response (neonates too young for a vaccine, immunocompromised patients, those with genuine vaccine contraindications).
- Underlies the rationale for high vaccination-coverage targets in national immunization programmes.
- Explains the resurgence of vaccine-preventable diseases (e.g., measles outbreaks) when vaccination coverage falls below the herd-immunity threshold in a community, even if most individuals remain vaccinated.
- Was central to the strategy behind the global eradication of smallpox, where sufficiently high immunization coverage eliminated ongoing transmission entirely.