What Is Herd Immunity?

Herd immunity, also called population immunity, is the indirect protection from an infectious disease that occurs when a large enough proportion of a population is immune to it. When many people can no longer catch or pass on a pathogen, the chains of transmission are interrupted, and the disease struggles to spread. As a result, even those who are not immune, such as newborns or people who cannot be vaccinated, are less likely to be exposed. The protection is indirect: it comes not from an individual’s own immunity but from the immunity of those around them.

How herd immunity works

Infectious diseases spread from person to person. If someone who is infectious is surrounded mostly by immune people, the infection has few places to go and tends to die out rather than grow. Immunity in a population comes from two routes: recovering from a natural infection, or being vaccinated. Vaccination works by prompting the body’s defences to recognise a pathogen without causing the disease, a process explained further in how vaccines train the immune system. As the immune share of a community rises, each infectious person passes the disease to fewer others, and outbreaks shrink.

The threshold varies by disease

There is no single figure for herd immunity. The proportion of a population that needs to be immune depends on how contagious the disease is: the more easily it spreads, the higher the share required. Highly transmissible diseases such as measles need a very large majority of a population to be immune before spread is reliably halted, while less contagious diseases need a lower share. Other factors, including how immunity is distributed across a community and how long protection lasts, also affect whether the threshold holds in practice. Because these variables differ, the herd immunity threshold is specific to each disease and setting.

Vaccination versus natural infection

Herd immunity can in principle be reached either by vaccination or by large numbers of people becoming infected and recovering. Public health authorities, including the World Health Organization, favour the vaccination route. Relying on natural infection means allowing a disease to spread widely, which causes illness, complications and deaths that vaccination is designed to prevent. Vaccination builds population immunity without people having to fall ill first. For some diseases, immunity may also wane over time or the pathogen may change, which is why booster doses or updated vaccines are sometimes needed to maintain protection.

Why the concept matters

Herd immunity explains why vaccination is often described as protecting not only the individual but the wider community. People who cannot be vaccinated, such as very young infants or those with certain medical conditions, depend on high immunity around them to keep their risk low. It also explains why vaccination coverage falling in a community can allow a previously controlled disease to return, even among those who could not have been protected directly. Understanding herd immunity as a population-level effect, rather than a personal one, is central to how societies manage infectious disease. This article is intended for general understanding and does not constitute medical advice.