Epidemiology · Reference
What is herd immunity?
Herd immunity is the indirect protection from an infectious disease that arises when a sufficiently high proportion of a population is immune, so that the chain of transmission is interrupted and even non-immune individuals are protected. It is a population-level concept, not an individual one.
How herd immunity works
Infectious diseases spread when an infected person passes the agent to susceptible contacts. As the proportion of immune people rises, an infected person is increasingly likely to meet someone who cannot be infected, so the chain of transmission breaks. This indirect protection is why even those who are not themselves immune gain some protection once the surrounding population is largely immune. The concept applies to diseases transmitted person to person; it does not apply in the same way to conditions that are not passed between people.
The herd immunity threshold
The herd immunity threshold is the proportion of a population that must be immune for transmission to decline. It depends on how contagious the agent is, captured by the basic reproduction number (R0). A standard approximation is that the threshold equals 1 − 1/R0: the more transmissible the disease (higher R0), the larger the immune fraction required. For example, a disease with an R0 of 4 implies a threshold of about 75%. These are simplified estimates that assume a well-mixed population and uniform immunity, so real-world thresholds vary.
Why the concept matters
Herd immunity explains why immunisation programmes can protect a whole community, including those who cannot be immunised, such as some infants or people with particular medical conditions, by reducing the amount of disease circulating. It is central to setting population-level immunisation goals and to understanding why coverage gaps can allow a previously controlled disease to return. The World Health Organization frames herd immunity as a population concept best achieved through immunisation rather than by allowing widespread infection, given the harm uncontrolled spread can cause.
Limits and assumptions
Herd immunity is an idealisation. The simple threshold formula assumes the population mixes evenly and that immunity is complete and lasting — assumptions rarely fully met. If immunity wanes over time, or if a pathogen changes so that prior immunity is less protective, the effective threshold shifts and protection can erode.
Pockets of low immunity, uneven contact patterns and new variants can all allow transmission even when overall immunity looks high. For these reasons, herd immunity is best treated as a useful population-level model rather than a precise guarantee for any individual or community.
Key facts
At a glance
- Definition: Indirect, population-level protection from immunity
- Also called: Population immunity
- Threshold ≈: 1 − 1/R0 (the higher the R0, the higher the threshold)
- Depends on: How transmissible the agent is, and immunity duration
- Key body: World Health Organization (WHO)
Common questions
FAQ
What is the herd immunity threshold?+
It is the proportion of a population that needs to be immune before sustained transmission of a disease declines. As an approximation it equals 1 minus 1 divided by the basic reproduction number (R0): more transmissible diseases require a higher immune fraction. It assumes a well-mixed population and uniform immunity, so real thresholds vary.
Does herd immunity protect everyone?+
Herd immunity is a population-level effect that reduces how much disease circulates, which indirectly protects people who are not themselves immune. It does not guarantee protection for any individual, and uneven mixing, waning immunity or pockets of low immunity can allow transmission to continue.
How is herd immunity achieved?+
Population immunity can build through prior infection or through immunisation. The World Health Organization describes immunisation as the preferred route because reaching the threshold by allowing widespread infection can cause substantial harm. This page describes the concept and does not offer health advice.
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