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How Do Vaccines Work?

The story of Chip and Dale

To understand how vaccines work you need to understand the story of two 5-year-old children, Chip and Dale.


Chip plays with a child in his class who has measles. Ten days later, Chip develops high fever, runny nose, "pink eye” and a rash. The rash consists of red bumps that start on his face and work their way down to the rest of his body. After two more days, Chip starts to have trouble breathing. His breaths are short and rapid. Chip's mother takes him to the doctor where he gets an X-ray of his chest. The X-ray shows that Chip has pneumonia (a common complication of measles infection). Chip is admitted to the hospital where he stays for five days and finally recovers. After having fought off his measles infection, Chip will never get measles again. Or, said another way, Chip has immunity to measles. Chip is immune to measles because he has cells in his body that can make "antibodies" to measles virus. These cells, called "memory B cells,” developed during the infection, and will hang around for the rest of Chip's life.


Dale also plays with the child who has measles. However, Dale never develops symptoms of measles. He doesn't get fever, rash or pneumonia. Dale was infected with measles virus, but didn't get any of the symptoms of measles. This is called an "asymptomatic infection.” Because Dale, like Chip, also develops “memory B cells,” he too is immune to measles for the rest of his life.

The difference between Chip and Dale

Whereas Chip had to pay a high price for his immunity, Dale didn't. Dale was lucky. Although some children don't get severe infections when they are exposed to measles, most do. Before a measles vaccine was developed in 1963, measles would infect about 4 million children each year and kill 3,000.

Vaccines take the luck out of it

By also causing "asymptomatic infections,” vaccines mimic what happened to Dale. This allows children to benefit from the natural immunity that comes with infection without having to suffer the severe, and occasionally fatal, consequences of natural infection.

To see how vaccines cause immunity without causing disease, check out How Are Vaccines Made?.

Vaccines remove the element of luck by controlling:

View a video clip about how the pertussis vaccine works in a community»

Reviewed by: Paul A. Offit, MD
Date: April 2013

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.
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