Medical Encyclopedia


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Immunizations - general overview

Contents of this page:


Hepatitis A immunization (vaccine)
Hepatitis A immunization (vaccine)

Alternative Names    Return to top


Definition    Return to top

Immunization (vaccination) is a way to trigger your immune system and prevent serious, life-threatening diseases.

Information    Return to top

Our bodies are designed to protect us from infections. When you are exposed to a virus or bacteria, your immune system actually learns from the experience. The next time your body is exposed to the same infection, your immune system often recognizes it and sets out to destroy it.

Immunization exposes you to a very small, very safe amount of the most important infections. This exposure helps your immune system recognize and attack the infection and prevent the disease it may cause. If you are exposed to the full-blown disease later in life, you will either not become infected or have a much milder infection. This is a natural way to deal with infectious diseases.

After immunizations were introduced on a wide scale, infections such as tetanus, diphtheria, mumps, measles, pertussis (whooping cough), and polio became rare. Newer immunization have also decreased certain types of meningitis, pneumonia, and ear infections in children.

Four different types of vaccines are currently available.


Many parents are concerned that some vaccines are not safe for their children. But a baby's immune system is designed to make antibodies to as many as 10,000 foreign proteins. If a baby were to receive all 11 available vaccines at once, this would engage only a tiny fraction of the immune system.


A small amount of mercury (called thimerosal) is a common preservative in multidose vaccines. Despite concerns, thimerosal-containing vaccines have NOT been shown to cause autism or ADHD. Nevertheless, if you have concerns about mercury, all of the routine vaccines are also available without added thimerosal.


The recommended immunization schedule is updated at least every 12 months by organizations such as the American Academy of Pediatrics. Consult your primary care provider about specific immunizations for you or your child. The current recommendations are available on the Centers for Disease Control and Prevention (CDC) website at At every doctor visit, ask about the next recommended immunizations.


Immunizations must be given as an injection (shot). The following tips can help make the experience easier for your child:

Some health care providers recommend giving your child one dose of acetaminophen (Tylenol) just before the vaccine is given. This may help avoid common, minor side effects. After the shot is given, a warm, damp cloth or a heating pad placed on the vaccine site may help reduce soreness. Frequently moving or using the arm or leg that has received the shot is also recommended to help reduces the soreness.


Immunizations are not only for children. Each year the CDC posts recommended adult immunizations on their website. Go there to learn about tetanus booster shots, the flu shot, hepatitis A and B vaccines, the pneumococcal vaccine, MMR, and immunizations for chickenpox and meningitis.


The CDC website ( gives travelers detailed information on immunizations and other precautions. Many immunizations should be obtained at least a month before travel.

Remember to take your immunization records with you when you travel internationally. Some countries require this documentation.

See also:

References    Return to top

American Academy of Pediatrics Committee on Infectious Diseases. Recommended immunization schedules for children and adolescents--United States, 2008. Pediatrics. 2008;121(1):219-220.

Centers for Disease Control. Recommended immunization schedules for persons Aged 0–18 Years--United States, 2008. MMWR. 2007;56(51 and 52);Q1-Q4.

Advisory Committee on Immunization Practices. Recommended adult immunization schedule: United States, October 2007-September 2008. Ann Intern Med. 2007;147(10):725-729.

Update Date: 6/18/2008

Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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