Concept
Herd immunity
Indirect protection at high population immunity
High-yield clue
When enough people are immune, chains of transmission break and even non-immune people are indirectly protected.
Overview
The indirect protection of susceptible individuals that occurs when a high enough proportion of a population is immune, interrupting chains of transmission. It explains why vaccination coverage protects even the unvaccinated.
Classification
- Public-health concept
- Population-level immunity
- Transmission interruption
Lab & identification clues
- Herd immunity threshold rises with higher contagiousness (R0)
- Highly transmissible pathogens like measles need very high coverage
- Depends on immune fraction, not any single test
Associations
- Vaccine coverage gaps allow outbreaks to return
- Protects those who cannot be vaccinated (vocabulary)
- Ties epidemiology to vaccination policy
Commonly confused with
- Herd immunity vs individual immunity
- Herd immunity threshold vs R0
Your notes
Original microbiology concept summary. Sources checked: OpenStax Microbiology 2e, NCBI Bookshelf Medical Microbiology, and CDC/WHO topic pages where applicable; reviewed 2026-06. Educational only; no diagnosis, treatment selection, infection-control instructions, or specimen-handling guidance.