What Are Vaccines, and How Do They Work?

The goal of vaccines is to greatly reduce the complications caused by childhood diseases. Although some diseases might be good for us to catch and leave us stronger, others cause more harm than good and leave us weaker. Vaccines prepare our bodies to fight against harmful diseases that we are likely to encounter in the world.

To understand how vaccines work, imagine that your immune system is a dictionary. For every substance that your body encounters, the immune system records a definition and an action. The definition is the description of the substance. The action tells the body what to do with the substance, like absorb it or attack it.

But, before your body knows what to do with something, it must identify the substance first. The way your body identifies something is based on the shapes found on the surface of that thing. These shapes are called antigens—physical characteristics of an object that the immune system can recognize. Vaccines help your body identify what infections look like so that the immune system can use its natural defenses to treat them.

For example, imagine a child eating a strawberry. It is important that the immune system has identified the strawberry as food and absorbs it. Now imagine a child who catches a cold. It is important that the immune system has identified the cold as harmful and attacks it. Vaccines don’t change how your body acts when it encounters something; they just help your body identify what it encountered.

Children are born with a bunch of blank definitions for serious childhood diseases like measles, pertussis (whooping cough), rotavirus, and others. If the child is unvaccinated and encounters one of those diseases, it takes time for the child’s immune system to examine the disease, record a description of it, and decide if the disease should be attacked. Unfortunately, this process takes so long that, by the time the definition and action are recorded, the disease may have already caused some permanent harm to the child. Vaccines teach our immune systems to fight diseases that otherwise would take them too long to recognize as harmful.

A vaccine for a particular disease may contain deactivated bacteria from that disease, or a deactivated or weakened live virus. The vaccine is given to a child with blank definitions. The child’s immune system can take its time defining what the disease looks like and noting to attack it. When encountering the live disease for the first time, the child’s immune system will be ready to fight it immediately and prevent serious illness or permanent harm.