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[Immunology](https://durgical.com/en/immunology/) · [Vaccines](https://durgical.com/en/immunology/#a-vaccines)

# Herd immunity and outbreak control

- Topic

When nearly everyone is vaccinated, the germ cannot find anyone to infect, and that also protects people who cannot be vaccinated.

Updated 2026-11-01

## Key points

- Threshold ≈ 1 − 1/R0; measles ≈ 95 %.
- Herd immunity protects those who cannot be vaccinated.
- Pockets of low coverage allow outbreaks even when the average is high.
- Tetanus has no herd immunity.

## What it is

Herd or population immunity is the indirect protection that appears when enough of a population is immune that chains of transmission break. The threshold depends on how contagious the disease is: the higher the basic reproduction number (R0), the higher the coverage needed. It explains why falling coverage brings back diseases such as measles and pertussis.

## Key facts

- Approximate herd immunity threshold: 1 − 1/R0.
- Measles: about 95 % coverage; polio: about 80 %.
- It protects very young infants, immunosuppressed people and those with contraindications.
- No vaccine protects 100 %, so high and even coverage matters more than the national average.
- Tetanus gives no herd immunity: it does not spread between people.

## Step by step

- A suspected case of a vaccine-preventable disease is detected and reported.
- It is confirmed by the laboratory and contacts are traced.
- Susceptible people around the case are vaccinated (ring or block vaccination).
- A house-to-house sweep finds people with incomplete schedules.
- Schedule ages can be moved earlier, such as the measles «Zero Dose».
- Surveillance continues until incubation periods pass with no new cases.

## Memory aid

Every vaccinated person is a wet match: with enough of them, the fire cannot jump.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does measles need coverage of about 95 % for herd immunity?

Because it is extremely contagious (high R0); the approximate threshold is 1 − 1/R0, which comes close to 95 %.

## Why does tetanus vaccine not produce herd immunity?

Because tetanus does not pass from person to person: the spore is in the environment. Each person depends on their own vaccination.

## Which vaccination measures are used in a measles outbreak?

Ring vaccination around the case, a house-to-house sweep to complete schedules and, if decided, earlier ages such as the infant «Zero Dose».

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## Related

- [Vaccine types: live, inactivated, subunit, toxoid, mRNA, vector](https://durgical.com/en/immunology/vaccine-types-live-inactivated-subunit-toxoid-mrna-vector/)
- [Colombia's PAI schedule (which vaccines, at what age)](https://durgical.com/en/immunology/colombia-s-pai-schedule/)
- [Live vaccines: who should not receive them](https://durgical.com/en/immunology/live-vaccines-who-should-not-receive-them/)
- [Primary vs secondary response and memory](https://durgical.com/en/immunology/primary-vs-secondary-response-and-memory/)

## Microorganisms

- [Measles virus](https://durgical.com/en/microbiology/measles-virus/)
- [Bordetella pertussis](https://durgical.com/en/microbiology/bordetella-pertussis/)
- [Yellow fever virus](https://durgical.com/en/microbiology/yellow-fever-virus/)
- [Rubella virus](https://durgical.com/en/microbiology/rubella-virus/)

[← All of immunology](https://durgical.com/en/immunology/)

Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: [Coronavirus disease (COVID-19): Herd immunity, lockdowns and COVID-19](https://www.who.int/news-room/questions-and-answers/item/herd-immunity-lockdowns-and-covid-19) (World Health Organization, 2020); [How do vaccines work?](https://www.who.int/news-room/feature-stories/detail/how-do-vaccines-work) (World Health Organization, 2020); [¿Cómo funcionan las vacunas?](https://vacunacion.minsalud.gov.co/Vacunas/Paginas/como-funcionan-las-vacunas.aspx) (Ministerio de Salud y Protección Social de Colombia, 2026); [Niños y niñas menores de 6 años](https://vacunacion.minsalud.gov.co/EV/Paginas/ninos-y-ninas-menores-de-6-anos.aspx) (Ministerio de Salud y Protección Social de Colombia (PAI), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/herd-immunity-and-outbreak-control/