The Children's Hospital of Philadelphia
Vaccines and Your Baby - What is Immunity?

Paul A. Offit, MD: As parents we don't want our children to pay a high price for immunity. That's where vaccines come in. Vaccines give immunity without making children suffer the risks of natural infection.

Louis M. Bell, MD: Let's continue with our example of chickenpox. Chickenpox is caused by a virus. Children first come into contact with chickenpox virus when someone who is infected coughs or sneezes. The virus travels through the air in tiny droplets and lands on the lining of the nose or the back of the throat. The virus then begins to copy itself over and over again until it spreads into the bloodstream and eventually to the skin where it causes a rash.

However, as we've seen in some children, chickenpox virus doesn't grow very well. These children don't get very sick at all. But having been exposed to the virus, all of these children have one thing in common, they are now immune to the disease; and having this immunity means that they will not get a serious case of chickenpox again. Getting the chickenpox vaccine is just like getting a mild chickenpox infection. You get virtually no symptoms and long-lasting immunity without having to risk the occasionally severe consequences of natural infection.