In simple terms
Herd immunity, also called population immunity, is the indirect protection that reaches people once enough of those around them are immune to a disease.
The immunity itself comes from vaccination or from previous infection. WHO supports reaching it through vaccination, not by allowing a disease to spread through a population.
How it works
A disease keeps spreading while each infected person passes it on to more than one other person on average. As the immune share of a population rises, chains of transmission keep running into people who cannot pass the infection along, and outbreaks fade.
The share that has to be immune before this happens is called the herd immunity threshold. As the US Centers for Disease Control and Prevention puts it, if a high enough proportion of a population is resistant, the few who are susceptible are protected because the pathogen is unlikely to find them.
That threshold is not one number for all diseases. It depends on how easily the disease spreads. WHO puts the level for measles at about 95 percent of a population vaccinated, and for polio at about 80 percent.
Measles sits high because it spreads so readily: WHO notes that one infected person can generate up to 18 secondary infections.
Why it matters
Some people cannot be vaccinated, including very young babies and people with certain allergies or serious illnesses. They depend on the immunity of those around them.
Coverage that slips lets a disease return where it had nearly disappeared. WHO reported that 84 percent of children received a first dose of measles vaccine in 2025, below the 2019 level of 86 percent, and estimated 95,000 measles deaths in 2024.
A national average can also hide the real picture. CDC notes that in practice herd immunity has not prevented measles and rubella outbreaks in populations with immunization levels as high as 85 to 90 percent, because the remaining susceptible people tend to cluster in particular subgroups. If the pathogen reaches one of those groups, an outbreak can still follow.
Where you will see it
- National immunization coverage reports and dashboards.
- Outbreak investigations explaining why a disease returned.
- Public debates about vaccination requirements for schools.
- Modeling studies estimating thresholds for a new disease.
Example
Measles needs roughly 95 percent coverage. In a school where coverage has drifted to 85 percent, a single imported case can spread, even though most children there are vaccinated.
Often confused with
Herd immunity describes protection across a population. Individual immunity is protection for one person. Someone can be individually immune while the population around them sits well below its threshold.
Key facts
- WHO defines herd immunity, also called population immunity, as the indirect protection from an infectious disease that happens when a population is immune either through vaccination or through immunity developed by previous infection.1
- WHO supports achieving herd immunity through vaccination rather than by allowing a disease to spread through any segment of the population.1
- The threshold varies by disease: WHO puts it at about 95 percent of a population vaccinated for measles and about 80 percent for polio.1
- One person infected with measles can generate up to 18 secondary infections.2
- The proportion of children receiving a first dose of measles vaccine was 84 percent in 2025, below the 2019 level of 86 percent, and WHO estimated 95,000 measles deaths in 2024.2
- In practice herd immunity has not prevented measles and rubella outbreaks in populations with immunization levels as high as 85 to 90 percent, partly because susceptible people cluster in subgroups.3
This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.
Related concepts
Quick checkWhy is the herd immunity threshold higher for measles than for polio?Show answer
Because measles spreads more easily. WHO puts the measles level at about 95 percent of a population vaccinated and the polio level at about 80 percent.
Sources
- World Health Organization. Coronavirus disease (COVID-19): Herd immunity, lockdowns and COVID-19. 31 December 2020 (accessed 27 September 2026)
- World Health Organization. Measles fact sheet. 15 July 2026 (accessed 27 September 2026)
- US Centers for Disease Control and Prevention. Principles of Epidemiology in Public Health Practice, Lesson 1, Section 10: Chain of Infection. Undated, CDC archive (accessed 27 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 27, 2026Researched and drafted with AI assistanceReport an issue