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
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.
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».
Related
- Vaccine types: live, inactivated, subunit, toxoid, mRNA, vector
- Colombia's PAI schedule (which vaccines, at what age)
- Live vaccines: who should not receive them
- Primary vs secondary response and memory
Microorganisms
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 (World Health Organization, 2020); How do vaccines work? (World Health Organization, 2020); ¿Cómo funcionan las vacunas? (Ministerio de Salud y Protección Social de Colombia, 2026); Niños y niñas menores de 6 años (Ministerio de Salud y Protección Social de Colombia (PAI), 2026).
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Representative model, for educational purposes only.