How can I tell if my child is having a bad reaction to a vaccine?

Fortunately, most children have only mild reactions or no reaction at all to their childhood immunizations.

Rarely, an immunization causes a high fever, which could provoke a seizure, but has no long-term health implications for your child.

More severe reactions to vaccines such as difficulty breathing and fainting are very unusual, but they can happen.

Below are vaccine reaction symptoms to watch out for, and what to do if your child experiences them.

Mild reactions and what to do

The most common reactions to vaccinations include:

• Pain, redness, and swelling at the injection site
• Mild fever that lasts for a day or two
• Fussiness.

These symptoms will go away on their own, but if your child seems uncomfortable, you can give her ibuprofen (if she's at least 6 months old) or acetaminophen. Check with your doctor if you're concerned.

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.

Call the doctor if your child has these reactions

• Bleeding issues. Rarely, infants or children have an autoimmune response that causes them to have low platelets (blood cells that prevent bleeding). This can result in bleeding problems, such as easy bruising, bleeding of the gums, blood in stool or urine, or hard-to-stop bleeding from an injury. You might also notice a rash that looks like tiny red or purple dots/bruises (a result of bleeding in the skin). Call your doctor immediately if you suspect this problem.
• Seizure or a high fever (over 104 degrees Fahrenheit). If you can't reach your doctor right away, take your child to the closest emergency room or urgent care that treats children. Tell your healthcare provider which vaccines your child received and when, and describe all the symptoms you've observed.
• Rash, vomiting, swelling of the lymph nodes, and prolonged crying. While these reactions are unlikely to indicate a serious problem, call your doctor if you have concerns.

Call 911 if your child has any of these reactions

• Breaks out in hives
• Has difficulty breathing
• Becomes pale or weak, or
• Loses consciousness.

These are signs that she may be having a severe allergic reaction, which can lead to anaphylactic shock.

Serious anaphylactic reactions to vaccines are rare. For many vaccines, your child has only a 1 in a million chance of having this kind of reaction, according the Centers for Disease Control and Prevention (CDC)

Still, it's wise to keep an eye out for reactions after your child has been immunized, just in case.

Reporting adverse reactions to vaccines

Healthcare providers in the United States are required to report any and all suspected serious adverse reactions to the Vaccine Adverse Event Reporting System (VAERS), established by the government to monitor and investigate concerns about vaccine safety. This information helps us learn more about vaccines and enables researchers to identify patterns of adverse reactions so that vaccines can be reevaluated, as necessary.