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 Vaccine Safety Basics
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Frequently Asked Questions about Multiple Vaccinations and the Immune System

How many vaccines does CDC recommend for children?
Currently, CDC recommends vaccination against 14 vaccine-preventable diseases. Because some of these vaccines have to be administered more than once, a child may receive up to 23 shots by the time he or she is 2 years of age. Depending on the timing, a child might receive up to six shots during one visit to the doctor.

Why does CDC recommend that children receive so many shots?
Vaccines are our best defense against many diseases, which often result in serious complications such as pneumonia, meningitis (swelling of the lining of the brain), liver cancer, bloodstream infections, and even death. CDC recommends vaccination to protect children against 14 infectious diseases including measles, mumps, rubella (German measles), varicella (chickenpox), hepatitis B, diphtheria, tetanus, pertussis (whooping cough), Haemophilus influenza type B (Hib), polio, influenza (flu), and pneumococcal disease.

Why are these vaccines given at such a young age? Wouldn't it be safer to wait?
Children are given vaccines at a young age because this is when they are most vulnerable to certain diseases. Newborn babies are immune to some diseases because they have antibodies given to them from their mothers. However, this immunity only lasts about a year. Further, most young children do not have maternal immunity to diphtheria, whooping cough, polio, tetanus, hepatitis B, or Hib. If a child is not vaccinated and is exposed to a disease germ, the child's body may not be strong enough to fight the disease.

An infant's immune system is more than ready to respond to the very small number of weakened and killed antigens in vaccines. Babies have the capacity to respond to foreign antigens even before they are born. The human immune system has evolved since organisms began living on Earth and represents a culmination of the "best" of this experience. Just as babies are born with a full-length digestive system that simply stretches as the baby grows, they also are born with a well developed immune system that can produce a variety of needed antibodies. However, infants lack the memory cells trained to defend against specific diseases. Because of this, they are particularly susceptible to diseases such as diphtheria, whooping cough, polio, tetanus, hepatitis B, and Hib. This is an important reason why the recommended childhood vaccination schedule begins so early—to prevent the diseases that children are susceptible to at such a young age.

I've heard people talk about "simultaneous" and "combination" vaccines. What does this mean? Why are vaccines administered this way?
"Simultaneous vaccination" is when multiple vaccines are administered during the same doctor's visit, usually in separate limbs (e.g., one in each arm). A "combination vaccine" consists of two or more separate vaccines that have been combined into a single shot. Combination vaccines have been in use in the United States since the mid-1940s. Examples of combination vaccines in current use are: DTaP (diphtheria-tetanus-pertussis), trivalent IPV (three strains of inactivated polio vaccine), MMR (measles-mumps-rubella), DTaP-Hib, and Hib-HepB (hepatitis B).

Giving a child several vaccinations during the same visit offers two practical advantages. First, we want to immunize children as quickly as possible to give them protection during the vulnerable early months of their lives. Second, giving several vaccinations at the same time means fewer office visits, which saves parents both time and money and may be less traumatic for the child.

Is simultaneous vaccination with multiple vaccines safe? Wouldn't it be safer to separate combination vaccines and spread them out, vaccinating against just one disease at a time?
The available scientific data show that simultaneous vaccination with multiple vaccines has no adverse effect on the normal childhood immune system. A number of studies have been conducted to examine the effects of giving various combinations of vaccines simultaneously. These studies have shown that the recommended vaccines are as effective in combination as they are individually, and that such combinations carry no greater risk for adverse side effects. Consequently, both the Advisory Committee on Immunization Practices and the American Academy of Pediatrics recommend simultaneous administration of all routine childhood vaccines when appropriate. Research is underway to find methods to combine more antigens in a single vaccine injection (for example, MMR and chickenpox). This will provide all the advantages of the individual vaccines, but will require fewer shots.

Another advantage is that combination vaccines result in fewer shots and less discomfort for children. In addition, spreading out the administration of separate vaccines may leave children unnecessarily vulnerable to disease.

Can so many vaccines, given so early in life, overwhelm a child's immune system, suppressing it so it does not function correctly?
No evidence suggests that the recommended childhood vaccines can "overload" the immune system. In contrast, from the moment babies are born, they are exposed to numerous bacteria and viruses on a daily basis. Eating food introduces new bacteria into the body; numerous bacteria live in the mouth and nose; and an infant places his or her hands or other objects in his or her mouth hundreds of times every hour, exposing the immune system to still more antigens. An upper respiratory viral infection exposes a child to 4 to 10 antigens, and a case of "strep throat" to 25 to 50.

Adverse Events Associated with Childhood Vaccines, a 1994 report from the Institute of Medicine, states: "In the face of these normal events, it seems unlikely that the number of separate antigens contained in childhood vaccines ... would represent an appreciable added burden on the immune system that would be immunosuppressive."

Related Links

Vaccine Safety Information for Parents
The United States currently has the safest, most effective vaccine supply in history. Years of testing are required by law before a vaccine can be licensed.

How Do Vaccines Protect Children from Diseases?
When they are injected into fatty tissue or muscle, vaccine antigens are not strong enough to produce the symptoms and signs of the disease but are strong enough for the immune system to produce antibodies against them.1

Immunization Safety Review: Multiple Immunizations and Immune Dysfunction*
Although most people know that vaccines effectively protect against serious infectious diseases, many parents question: Can too many immunizations overwhelm an infant's immune system?


1Tortora GJ, Anagnostakos NP. Principles of Anatomy and Physiology, 3rd ed. Harper and Row Publishers, New York, 1981.

2Whitney EN, Hamilton EMN, Rolfes SR. Understanding Nutrition, 5th ed. West Publishing Company, St. Paul, Minn., 1990.

*Links to non-Federal organizations found at this site are provided solely as a service to our users. These links do not constitute an endorsement of these organizations or their programs by CDC or the Federal Government, and none should be inferred. CDC is not responsible for the content of the individual organization Web pages found at these links.

Page last reviewed: September 24, 2008
Page last updated: May 12, 2008
Content source: Immunization Safety Office

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