What is herd immunity?

Herd immunity (or population immunity) is the indirect protection that arises when a sufficiently large proportion of a population is immune to an infection, so transmission chains break down and even non-immune individuals are protected.

The mechanism. An infectious person must encounter susceptible people to pass on an infection. If most contacts are immune, each infected person passes it to fewer than one other on average, and an outbreak shrinks rather than grows.

The threshold depends on how contagious the disease is, expressed as R₀ — the average number of people one infected person would infect in a fully susceptible population. The approximate threshold is 1 − 1/R₀.

Measles has an R₀ of around 12–18, so roughly 95% immunity is required. This is why measles returns first and fastest when vaccination coverage slips — it has the highest bar of any common disease.

Polio, with a lower R₀, requires around 80%.

Who it protects. This is the central point. Some people cannot be vaccinated — newborns too young, people undergoing chemotherapy or immunosuppressive treatment, those with certain allergies or immune conditions. Vaccines also do not work perfectly in everyone. These people depend entirely on not encountering the infection, which requires everyone around them to be immune.

Important qualifications:

It is not a national average. Immunity clusters geographically and socially, so a country at 95% can contain communities at 60% where outbreaks occur readily.

It requires durable immunity. Where immunity wanes or the pathogen evolves to escape it, the threshold is a moving target rather than a state you reach and keep — which is why the concept applied awkwardly to COVID-19.

Achieving it through infection rather than vaccination means accepting the disease's full burden of death and disability, which for most diseases is substantial.

It does not apply to non-transmissible conditions, such as tetanus, acquired from the environment.