What are the risks associated with the Measles, Mumps, and Rubella (MMR) vaccine?

MMR Vaccine Safety Profile

Yes, there is a small risk from MMR vaccination, but these risks are minimal, well-characterized, and far outweighed by the protection against serious diseases. The vaccine has an excellent safety profile with over 40 years of global use, and serious adverse events are rare 1, 2.

Common Adverse Reactions

The most frequent side effects are mild and self-limited 1:

• Transient rashes occur in approximately 5% of vaccinated persons, typically appearing 7-10 days after vaccination 1
• Fever (≥102°F) occurs in approximately 15% of recipients 1
• Transient lymphadenopathy, sore throat, and headache are reported but generally mild 1
• Local injection site reactions including redness and pain occur in approximately 11-12% of recipients 3

Rare but Serious Adverse Events

Febrile Seizures

• Risk: Approximately 1 additional febrile seizure per 2,300-2,600 doses when MMRV vaccine is used for the first dose in children aged 12-23 months 1
• Timing: Occurs 5-12 days after vaccination during peak measles virus replication 1
• Context: Febrile seizures normally occur in 2-4% of healthy children before age 5; the vaccine adds minimal additional risk 4
• Clinical significance: These seizures typically resolve spontaneously without long-term sequelae 1

Thrombocytopenic Purpura (ITP)

• Risk: Approximately 1 case per 40,000 MMR doses administered 4
• Timing: Within 6 weeks after vaccination 5
• Context: The risk of ITP after natural measles or rubella infection (5 cases per 100,000 per year) is significantly higher than after vaccination 4

Aseptic Meningitis

• Only associated with Urabe and Leningrad-Zagreb mumps strains (not used in current U.S. vaccines) 1, 5
• No evidence of association with Jeryl Lynn strain used in current MMR vaccines 4

Anaphylaxis

• Risk: Approximately 1.0-3.5 occurrences per million doses 1
• Contraindication: History of anaphylactic reaction to neomycin or gelatin 1
• Note: Contact dermatitis to neomycin is NOT a contraindication 1

Joint Symptoms (Rubella Component)

• Arthralgia develops in approximately 25% of rubella-susceptible postpubertal females 1
• Acute arthritis-like symptoms occur in approximately 10% of this population 1
• Duration: Symptoms generally persist 1 day to 3 weeks and rarely recur 1
• Chronic joint symptoms are very rarely reported, if they occur at all 1

What MMR Does NOT Cause

Strong evidence demonstrates no association between MMR vaccination and 1, 5, 4:

• Autism spectrum disorders (rate ratio 0.93,95% CI 0.85-1.01) 4
• Encephalitis or encephalopathy (rate ratio 0.90,95% CI 0.50-1.61) 4
• Hearing loss 1
• Type 1 diabetes 1, 4
• Crohn's disease or inflammatory bowel disease 1, 4
• Guillain-Barré Syndrome 1
• Asthma 4
• Multiple sclerosis 4
• Cognitive delay 4

Special Populations

Pregnancy

• Theoretical risk to the fetus exists; women should avoid pregnancy for 28 days after vaccination 1
• However: Among 321 rubella-susceptible women vaccinated within 3 months before or after conception, none of the 324 infants had malformations compatible with congenital rubella syndrome 1
• Estimated risk of serious malformations: 0% to 1.6% 1
• Not a reason to terminate pregnancy if inadvertently given 1

Immunocompromised Persons

• Contraindicated in severely immunocompromised individuals due to risk of vaccine-associated measles infection 1
• Can be given to asymptomatic HIV-infected persons without severe immunosuppression 1

Personal or Family History of Seizures

• This is a precaution (not contraindication) for MMRV vaccine 1
• Recommendation: Use separate MMR and varicella vaccines instead of MMRV in children with personal or family history of seizures 1

Clinical Bottom Line

The MMR vaccine's safety profile is excellent with over 40 years of documented use in more than 75 countries 2. The attributable risks are minimal: approximately 1 febrile seizure per 2,300 doses and 1 case of ITP per 40,000 doses 1, 4. These small risks are substantially lower than the risks from natural measles, mumps, or rubella infection, which can cause serious complications, disability, and death 5, 4, 6.