Overview of Childhood Immunizations

Immunizations, also called vaccinations or inoculations, protect children against more than a dozen serious diseases that can cause severe illness, lifelong disabilities, or even death. The best time to immunize children is when they are young. This is because most of the diseases for which vaccinations are available are more common and cause more severe complications in young children. Children who are not immunized until they are older are at increased risk for exposure to disease without protection.

Immunizations work by introducing microorganisms (e.g., bacteria, virus) into the immune system, usually through an injection (shot). The immune system attacks the bacteria or virus, produces antibodies to prevent infection, and learns to recognize the bacteria or virus again if the child is exposed in the future.

Some vaccinations, such as those for measles or hepatitis B, provide lifelong immunity (protection from disease) and others, like tetanus or pertussis (whooping cough) must be periodically re-administered (called booster shots). Parents and caregivers should keep an accurate record of their child's immunizations so they know when he or she is due for a booster shot.

Due to widespread immunization, most children in the United States are protected against a number of dangerous diseases. In fact, because some of these diseases are now so rare in the United States, people may wonder if immunization is still necessary. The answer to this question is yes—even just a few unvaccinated children can open the door to widespread infection. Between 1989 and 1991, falling immunization rates among preschoolers in the United States led to an epidemic of measles that caused 55,000 cases and 136 deaths.

In many countries throughout the world, rates of immunization are lower than in the United States. Since diseases can spread very quickly, in part, due to air travel, and children may come into contact with people who have not been immunized, the best way to prevent many diseases is through vaccination.

If your child is not on schedule with his or her immunizations, a catch-up immunization schedule may be recommended. Catch-up immunizations often are recommended for children between the ages of 4 months and 18 years who begin receiving vaccinations late or who are more than 1 month behind in their immunizations. Ask your child's pediatrician for more information about catch-up immunizations.

Types of Vaccines

There are two general types of vaccines: live attenuated (weakened) vaccines and inactivated vaccines. Live attenuated vaccines are made using disease-causing microorganisms (e.g., bacteria, virus) that are modified and weakened in a laboratory, and inactivated vaccines are made using destroyed (killed) microorganisms or parts of destroyed microorganisms.

Live attenuated vaccines (e.g., chickenpox [varicella] vaccine, measles-mumps-rubella [MMR] vaccine), which use live, weakened viruses, may cause a child to develop a very mild form of the disease. In most cases, the illness is less severe than infection with the actual disease. Because live attenuated vaccines are similar to the actual disease, they often provide long-lasting immunity with a single dose (except for oral vaccines). Reactions to live attenuated vaccines may be systemic (widespread; e.g., rash, fever).

Inactivated vaccines, which use viruses that have been killed, usually require multiple doses to provide immunity. It is impossible to get any disease from an inactivated vaccine so a child will not get a mild form of the disease targeted for prevention, as may happen with a live, attenuated vaccine. Reactions from inactivated vaccines usually are localized (e.g., pain, swelling, redness at the injection site).

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 28 Aug 2008

Last Modified: 13 Oct 2011