Herd Immunity

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Did you know that vaccination not only protects you from disease but can also help protect people in your community who are not vaccinated. This phenomenon is known as 'herd immunity.' We'll explain what all these red and blue circles mean. Just click 'Next' to see a simple demonstration of how herd immunity works ... and how it can fail.

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Replay

In this demonstration, light blue circles represent healthy people who are not vaccinated. In pictures to come, dark blue circles will represent healthy people who get vaccinated, and red circles will be the people who become sick.

Sick

How disease spreads

Unvaccinated

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Vaccinated

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How vaccination protects a community

Vaccination protects against disease by stimulating the immune response against a specific germ. In this case, most of the people in one area -- the gray area in the center -- chose to be vaccinated. They're the dark blue circles. None of the other people outside the gray area got vaccinated: they're all light blue. The people in the gray area make up the herd community."

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As the infection spreads, the unvaccinated people in the herd community -- the community where a lot of people got vaccinated -- escape the infection because they are protected by the vaccinated people who surround them. But the people outside the herd community where no one got vaccinated, all get sick.

Herd immunity works only to a point. In this situation, too few people in the herd community have been vaccinated to create the herd immunity effect.

When herd immunity fails

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When disease strikes this time, those people in the gray area who are vaccinated are protected but there are not enough of them to protect people who are not vaccinated -- most of them get infected. Herd immunity has failed to occur.

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