Fortunately, most children have only mild reactions or no reaction at all to their childhood immunizations. The most common reaction is soreness or swelling at the site, or a low-grade fever that lasts for a day or two. This type of reaction is quite common and nothing to be alarmed about, but check with your doctor if you're concerned.
© Dr. P. Marazzi / Science Source
Some vaccines are more likely to cause problems than others. The MMR, for example, can cause a fever and a rash seven to ten days after the injection. And the DTaP vaccine causes some babies (1 in 1,000) to cry inconsolably for several hours. When you take your child to be immunized, talk to your doctor about how your child might react.
Rarely, an immunization causes a high fever, which could provoke a seizure. This is scary but has no long-term health implications for your child. Call your doctor if your child has more than a mild reaction to a shot, or if you observe symptoms that concern you.
More severe reactions to vaccines are rare, but they can happen. Signs of a serious allergic reaction can include difficulty breathing, hoarseness, wheezing, hives, paleness, dizziness, fainting, or a rapid heartbeat. This type of reaction will occur a few minutes to a few hours after the shot. Call 911 immediately if your child shows any of these symptoms.
If your child has a moderate to severe reaction to a vaccine and you live in the United States, make sure your doctor files a report with the U.S. Center for Disease Control's Vaccine Adverse Event Reporting System (VAERS), or call VAERS yourself at (800) 822-7967.