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After an MMR vaccine, a sore arm, fever, or mild rash can occur; most people do not have serious problems. Some effects—especially joint symptoms and a small increase in fever-related seizure risk in young children—may appear days or weeks later. Call 911 for signs of a severe allergic reaction. For any other symptom that concerns you, contact a health care provider.
Common MMR vaccine side effects
Possible reactions include a sore arm where the shot was given, fever, and a mild rash. Swollen glands in the cheeks or neck may also occur. These are possible effects, not symptoms everyone will experience. CDC’s MMR vaccine safety information notes that most people do not have serious problems.
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Temporary joint pain or stiffness
Joint pain or stiffness is associated with the rubella component of MMR. It is particularly relevant to teenage and adult women who were not already immune to rubella. CDC says symptoms occur in up to 1 in 4 females past puberty who lacked prior rubella immunity; they generally begin 1 to 3 weeks after vaccination, are usually mild, last about 2 days, and rarely recur. This estimate does not apply to all MMR recipients.
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Fever-related seizures in young children
CDC reports a small increased risk of febrile seizures after MMR in children younger than 7: about 1 in 3,000 to 4,000 children. The risk period is approximate—about 8 to 14 days after vaccination in CDC’s overview and 6 to 14 days in its detailed safety information. A fever during that period does not mean a seizure will occur. CDC says these seizures are not associated with long-term effects. See the CDC’s detailed MMR safety data.
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Rare low platelet count
Rarely, a low platelet count can cause unusual bleeding or bruising. CDC cites one study estimating immune thrombocytopenic purpura after MMR at 1 case per 40,000 vaccinated children. That is a study estimate, not a universal current incidence rate; CDC notes the resulting bleeding disorder is usually not life-threatening.
When to call a doctor or get emergency help
Call 911 for possible severe allergic reaction
Severe allergic reactions are rare but can be life-threatening. The CDC MMR Vaccine Information Statement (VIS), dated January 31, 2025, says to call 911 and get emergency care for signs such as:
- Hives
- Swelling of the face or throat
- Difficulty breathing
- Fast heartbeat
- Dizziness or weakness
Contact a health care provider about other concerning signs
The VIS advises: “For other signs that concern you, call your health care provider.” It does not set a specific fever or duration threshold in this guidance. If you are unsure whether a symptom is urgent, contact a clinician for advice; do not delay emergency care for severe allergic symptoms.
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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 not sure the vaccine caused it. VAERS is an early-warning monitoring and reporting system co-managed by CDC and FDA; it does not provide medical advice. A report does not replace emergency care or contacting a health care provider. More information is available on the CDC VAERS page.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.MMR is not the same as MMRV
MMRV combines measles, mumps, rubella, and varicella (chickenpox) vaccines. Seizure-risk comparisons reported for MMRV in younger children should not be applied to MMR alone. The figures above describe MMR, as identified in CDC’s MMR safety information.
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