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Vaccines have contributed to a significant reduction in many childhood infectious diseases including diphtheria, measles, and Haemophilus influenzae type b (Hib). Some infectious diseases, such as polio and smallpox, have been eliminated in the United States due to effective vaccines. It is now rare for children in the United States to experience the devastating and often deadly effects of these diseases that were once common in the United States and other countries with high vaccination coverage.
The vast majority of vaccines are given to healthy babies, children and adults; therefore, it is critical that vaccines be demonstrated to be safe and effective. Ensuring the safety and effectiveness of vaccines is one of the Food and Drug Administration's (FDA) top priorities.
A vaccine is a medication. Like any medicine, vaccines have benefits and risks, and although highly effective, no vaccine is 100 percent effective in preventing disease or 100 percent safe in all individuals. Most side effects of vaccines are usually minor and short-lived. For example, a person may feel soreness at the injection site or experience a mild fever. Serious vaccine reactions are extremely rare, but they can happen.
For more information on potential adverse events or reactions, talk with your healthcare provider, and many vaccines also have FDA-approved labeling for the patient that can be a resource of information. It is important to discuss with your healthcare provider any prior reactions to vaccines and any adverse reactions following vaccination.
Vaccines work by preparing the body’s immune system for future attacks by a certain disease, caused by either viruses or bacteria. Vaccines contain weakened bacteria or viruses, or parts of bacteria or viruses, and mimic these disease-causing agents (which are called antigens). As a result of vaccination, the body’s immune system thinks the antigens from the vaccine are foreign and shouldn’t be in the body, but the antigens don’t cause disease in the person receiving the vaccine. After receiving the vaccine, if the virus or bacteria that cause the real disease then enters the body in the future, the immune system is prepared and responds quickly and forcefully to attack the disease-causing agent to prevent the person from getting sick. Vaccines are frequently given by injection (a shot), but some are given by mouth and one is sprayed into the nose.
Learn more about steps to take when your child is vaccinated and see the questions and answers from parents and caretakers on the new FDA’s Parents’ Guide to Kids' Vaccines