Slide 1
Protection can extend beyond one person
When immunity reduces transmission through a community, people without immunity may face fewer opportunities for infection. This indirect effect is called herd protection.
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It can matter especially to people who cannot receive a particular vaccine or do not develop a strong response. The protection comes from reduced exposure opportunities, not from transferring another person’s antibodies to them.
Slide 2
Transmission needs connected opportunities
An infection spreads when one person passes it to another, who may pass it on again. Immunity can break these chains and make continued spread harder.
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A person may still encounter infection even when the surrounding community has substantial immunity. Herd protection changes probabilities across a network; it is not an invisible barrier around each individual.
Slide 3
Local patterns matter
A high average level of immunity can hide groups with much lower protection. Who mixes with whom affects where transmission remains possible.
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Geographic or social clustering can leave connected groups susceptible. Two populations with the same overall vaccination percentage can therefore have different vulnerabilities if their patterns of contact differ.
Slide 4
There is no single universal threshold
The level of immunity needed to strongly limit spread differs among diseases and settings. Vaccine effects and how infection spreads between people both matter.
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Protection is also incomplete: no vaccine guarantees that every recipient becomes fully protected. A percentage quoted for one disease should not be carried over as a general rule for all infections.
Slide 5
Some diseases bypass this mechanism
Tetanus is acquired from bacteria in the environment rather than from another person with tetanus. Community immunity therefore does not interrupt its route in the same way.
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This exception explains why indirect protection must be discussed disease by disease. Knowing that vaccination protects an individual does not by itself establish that it creates the same community effect against every pathogen.
Sources
- Herd immunity (Herd protection)
University of Oxford, Vaccine Knowledge Project
- How do vaccines work?
WHO