Vaccines provide immunity (protection) against severe diseases caused by certain viruses and bacteria.
The first time a germ enters the body, the immune system starts making antibodies (proteins that attach themselves to germs) and cells to fight that particular type of germ. That process takes time, so the body usually is not able to effectively fight an initial infection right away. During that time, the infection could make the person very ill.
But the immune system “remembers” germs that it has fought before. If a germ enters the body a second time, the immune system can respond without delay. The body starts fighting the infection before it has a chance to cause illness.
A vaccine contains weakened or killed germs or parts of germs that are not strong enough to cause disease. A child’s immune system reacts to a vaccine just like it would to an infection by making antibodies and cells that fight that type of infection. So a vaccine acts like an initial infection, except that it does not cause disease like a real infection could.
Infants and young children are more vulnerable to severe complications from certain diseases than older children or adults. This is why it is important to vaccinate children at an early age. The vaccination schedule is designed to provide immunity before children are exposed to life-threatening diseases.
Receiving more than 1 vaccine at a time will not overwhelm a child’s immune system. Every day, children are exposed to many more germs than are in all of the recommended vaccines put together.
Children who have a life-threatening allergy to an ingredient in a vaccine should not receive that vaccine. A child who has had a severe reaction to a vaccine in the past should not receive another dose of the vaccine.
Most vaccines are safe to give if a child has a cold or other mild illness. Children who are moderately or severely ill should usually wait until they recover to receive vaccines.
Be sure to tell your child’s doctor if your child has a weakened immune system. Illnesses like HIV/AIDS, leukemia, and lymphoma can weaken the immune system, and so can certain drugs such as long-term steroids and chemotherapy medications.
Tell your child’s doctor if your child has ever had a severe allergic reaction, any kind of cancer, or Guillain-Barré syndrome (a paralyzing illness) or if your child has recently received a blood transfusion or blood products.
All vaccines can cause side effects. Common side effects include mild fever, fussiness, and soreness, redness, or swelling where a vaccine shot was given. Moderate problems such as seizure due to fever are uncommon. Severe allergic reactions are very rare.
You should keep track of the vaccines your child receives. A printable vaccine tracker for children from birth to age 2 years is available at www.cdc.gov/vaccines/parents/downloads/milestones-tracker.pdf.
American Academy of Pediatricswww2.aap.org/immunization/families/families.html
Centers for Disease Control and Preventionwww.cdc.gov/vaccines/default.htm
To find this and previous JAMA Patient Pages, go to the Patient Page link on JAMA’s website at jama.com. Many are available in English and Spanish.
Sources: American Academy of Pediatrics, Centers for Disease Control and Prevention
Topic: Pediatric Care
Thompson AE. Childhood Vaccines. JAMA. 2015;313(19):1988. doi:10.1001/jama.2015.3465