Do You Know How Vaccines Work? Here’s a Simple Analogy.

This blog post is adapted from our Parents’ Guide to Children’s Vaccines.

Maybe you think of vaccines as those shots your baby needs to take every few months, or the shots you have to take before you travel out of the country. You may know that they provide immunity against diseases, but how do they work?

The goal of vaccines

Vaccines greatly reduce the complications caused by harmful diseases that you’re likely to encounter in the world. While some diseases might be good for us to catch and can leave us stronger, others cause harm, sickness, or even death.

Imagine that your immune system is a dictionary. Every time that you encounter a substance, bacteria or virus, your immune system catalogs a “definition” and an “action.” The definition helps describe the substance, based on the shapes on its surface. These shapes are called antigens.

The action tells your body what to do, like absorb the substance, or attack it.

Vaccines help your body spot infections so that your immune system can use its natural defenses to treat them.

The immune system as dictionary

Think about your child eating a strawberry. His immune system catalogs it as food, so his body absorbs it. Then, think about when your child catches a cold. His body catalogs ist as harmful and attacks it to help him get better.

Vaccines simply help your body identify what it’s encountering; they don’t change how your body acts when it encounters those things.


Children are blank slates. They enter the world with only partially-filled immune dictionaries. Their bodies don’t know the definitions for serious and harmful diseases like measles, pertussis (whooping cough), rotavirus, varicella (chicken pox), and others.

If an unvaccinated child encounters those diseases, her immune dictionary combs its entries to find a fitting definition. It takes a long time for her body to examine the disease, figure out a description for it, and decide if it should be absorbed or attacked.

Unfortunately, by the time all of this is done, the disease may have already cause permanent harm to the child.

When a child is vaccinated, on the other hand, their body receives those definitions. The vaccine may contain deactivated bacteria from the disease, or a deactivated or weakened live virus, which helps give the body a “dictionary entry” for the disease.

So if the child encounters the live disease out in the world, their immune system is primed to identify it and fight it immediately — thus preventing serious illness or permanent harm.

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