One in six children will get a low fever and one in 20 will get a mild rash. In rare cases, your child may have swelling in the glands in her neck or cheeks. Perhaps one in a hundred children will experience joint pain. If these problems occur, they usually happen within seven to 12 days of getting the shot. And your child is less likely to have these symptoms after her second dose.
Moderate reactions are less common.
About one in 30,000 will have a temporarily low platelet count, which may cause bleeding problems. Serious reactions are extremely rare. About one in a million children will have a serious allergic reaction, marked by difficulty breathing, hoarseness or wheezing, hives, paleness, weakness, a racing heartbeat, or dizziness. Children have allergic reactions within a few hours of getting the shot if they have them at all. And while more than 70 million doses of the MMR vaccine have been distributed in the United States since 1990, only 11 children have had a severe allergic reaction to it, and all of them recovered.