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 have 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. Allergic reactions occur within a few hours of getting the shot if they occur 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.