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After an MMR vaccine, a sore arm, fever, or mild rash may occur; swollen glands or temporary joint pain and stiffness are also possible. Most people do not have serious problems. Call 911 for signs of a severe allergic reaction, and contact a health care provider about other symptoms that concern you.
Common MMR vaccine side effects
Possible reactions include soreness where the shot was given, fever, and a mild rash. Swelling of glands in the cheeks or neck can also occur. These effects do not happen to everyone, and CDC does not give one exact onset or duration that applies to every person.
Temporary joint pain or stiffness
Joint symptoms are linked to the rubella component of the vaccine and are more likely in teenage or adult women who were not already immune to rubella. CDC says they occur in up to 1 in 4 females past puberty in that specific group—not in 1 in 4 of all people vaccinated. Symptoms generally begin 1 to 3 weeks after vaccination, are usually mild, last about 2 days, and rarely recur. CDC: MMR vaccine safety
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Febrile seizures in young children
CDC reports an increased risk of febrile seizures in the days after MMR vaccination among children younger than 7 years: about 1 in 3,000 to 4,000 children. The risk period is approximate—CDC describes it as about 8 to 14 days after vaccination in its overview and 6 to 14 days in detailed safety information. A fever during this period does not mean a child will have a seizure. CDC says these febrile seizures are not associated with long-term effects. CDC: MMR vaccine safety
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Low platelet count
Rarely, a low platelet count can cause unusual bleeding or bruising. CDC cites one study estimating immune thrombocytopenic purpura at 1 case per 40,000 vaccinated children. This is an estimate from that study, not a universal current incidence rate; CDC notes the condition is usually not life-threatening. CDC: MMR vaccine safety
When to call 911 or a health care provider
Call 911 for possible severe allergic reaction
Severe allergic reactions are rare but can be life-threatening. If the vaccinated person has hives, swelling of the face or throat, difficulty breathing, a fast heartbeat, dizziness, or weakness, call 911 and get emergency care. These are the symptoms listed in the CDC MMR Vaccine Information Statement (VIS), dated January 31, 2025.
Contact a clinician about other concerning symptoms
The CDC MMR VIS advises: “For other signs that concern you, call your health care provider.” It does not set a universal fever or duration threshold for when to call, so seek advice based on the person’s symptoms and your concern.
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Patients and caregivers can report a concerning health problem after vaccination to the Vaccine Adverse Event Reporting System (VAERS), even when they are unsure whether vaccination caused it. VAERS is an early-warning monitoring and reporting system co-managed by CDC and FDA; it does not provide medical advice. Reporting does not replace emergency care or contacting a clinician. CDC: Vaccine Adverse Event Reporting System
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.MMR is not the same as MMRV
MMR protects against measles, mumps, and rubella. MMRV also includes varicella (chickenpox), and its seizure-risk comparisons in younger children are specific to that combination vaccine. Do not apply MMRV figures to MMR; the febrile-seizure estimate above is CDC’s figure for MMR. CDC: MMR vaccine safety
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