What are the potential side effects of the Measles, Mumps, and Rubella (MMR) vaccine, particularly in pediatric patients with a history of allergic reactions or other medical conditions?

MMR Vaccine Side Effects

The MMR vaccine is extremely safe with serious adverse events being rare (≤6 per 100,000 doses), while common mild reactions include fever (5%), rash (4-5%), and injection site reactions that are self-limited and vastly outweighed by the risks of natural measles, mumps, and rubella infections. 1, 2

Common Side Effects (Non-Serious)

Fever and Rash

• Fever occurs in approximately 5% of children, typically appearing 7-12 days post-vaccination when viral replication peaks, coinciding with the normal immune response to the live attenuated viruses 3, 4
• Transient measles-like rash develops in about 4-5% of recipients, appearing 7-10 days after vaccination due to low-level viremia from vaccine virus replication 3, 4
• These symptoms are mild, brief, and represent normal vaccine response rather than adverse events requiring intervention 3

Local and Systemic Reactions

• Injection site reactions occur in approximately 157 per 100,000 doses (1.6%) in adolescents and adults 2
• Arthropathy (joint symptoms) is the most common non-serious outcome in adults, occurring in 263 per 100,000 doses (2.6%), with higher rates in females 2
• Transient lymphadenopathy sometimes occurs following rubella component replication 3
• Parotitis (salivary gland swelling) has been reported rarely from mumps component replication, occurring in only 3.4 per 100,000 doses 3, 2

Serious Adverse Events (Rare)

Febrile Seizures

• Febrile seizures have an attributable risk of approximately 1 per 1,700-2,600 doses, occurring during days 7-12 post-vaccination 1, 5
• This risk must be contextualized: febrile seizures normally occur in 2-4% of healthy children at least once before age 5 from any cause 5
• A personal or family history of seizures (sibling or parent) is a precaution for MMRV vaccine use; these children should receive separate MMR and varicella vaccines instead 1

Idiopathic Thrombocytopenic Purpura (ITP)

• ITP occurs at an attributable risk of approximately 1 case per 40,000 administered MMR doses 5
• The risk of ITP after vaccination is substantially smaller than after natural infection with measles or rubella (5 cases per 100,000 per year from natural infection) 5
• Children with a history of thrombocytopenia may be at increased risk for recurrence, but the benefits of immunization usually outweigh the risks, particularly given the even greater risk for thrombocytopenia after natural measles or rubella disease 1

Anaphylaxis

• Anaphylactic reactions to MMR are extremely rare, occurring at less than 1 case per million doses distributed 1
• Among 70 million doses distributed, only 11 cases met criteria for true anaphylaxis (immediate onset with consistent symptoms) 1

Other Serious Events

• Serious outcomes including encephalitis/myelitis, Guillain-Barré syndrome, meningitis, and seizures each have incidence ≤6 per 100,000 doses 2
• There is no evidence of association between MMR and encephalitis/encephalopathy (rate ratio 0.90,95% CI 0.50-1.61) 5
• Aseptic meningitis is associated with Urabe and Leningrad-Zagreb mumps strains, but NOT with Jeryl Lynn strain MMR vaccines used in the United States (rate ratio 1.30,95% CI 0.66-2.56) 5

Special Considerations for Allergic Reactions

Egg Allergy

• Egg allergy is NOT a contraindication for MMR vaccine 1, 3
• Anaphylactic reactions to MMR are not associated with egg antigens but with other vaccine components, primarily gelatin 1
• Skin testing for egg allergy is not predictive of vaccine hypersensitivity and is not required 1
• Special protocols and gradually increasing doses are not necessary for egg-allergic patients 1

Gelatin Allergy

• The gelatin component is the most frequently associated allergen with anaphylactic reactions after MMR vaccination 3, 6
• Extreme caution should be exercised when administering MMR to persons with a history of anaphylactic reaction to gelatin or gelatin-containing products 1
• Skin testing for gelatin sensitivity can be considered before vaccination, though no specific protocols have been published 1

Neomycin Allergy

• Persons who have experienced anaphylactic reactions to neomycin should not receive MMR vaccine (contains 25 μg trace amounts) 1
• However, contact dermatitis to neomycin (delayed cell-mediated response) is NOT a contraindication to MMR vaccination 1
• Neomycin allergy most often manifests as contact dermatitis appearing 48-96 hours after vaccination, not anaphylaxis 1

Contraindications and Precautions

Absolute Contraindications

• History of anaphylactic reaction to neomycin 1
• Allergic reaction to gelatin or other vaccine components 1
• Severe immunocompromise (congenital immunodeficiency, severe HIV, active malignancy, chemotherapy, high-dose corticosteroids ≥2 mg/kg/day or ≥20 mg/day prednisone for ≥2 weeks) 1, 3
• Pregnancy 1

Precautions (Not Contraindications)

• Moderate or severe acute febrile illness should prompt deferral until recovery from the acute phase to avoid superimposing vaccine effects on underlying illness 1
• Minor illnesses such as mild upper respiratory infection with or without low-grade fever are NOT contraindications 1
• Recent receipt of antibody-containing blood products (≤11 months, specific interval depends on dose) 1
• Personal or family history of seizures of any etiology (precaution for MMRV, not MMR) 1

Important Clinical Caveats

What MMR Does NOT Cause

• No evidence of association with autistic spectrum disorders (rate ratio 0.93,95% CI 0.85-1.01) 5
• No evidence of association with inflammatory bowel disease or Crohn's disease (odds ratio 1.42,95% CI 0.93-2.16) 3, 5
• No evidence of association with subacute sclerosing panencephalitis (SSPE), permanent neurological damage, type 1 diabetes, asthma, dermatitis/eczema, hay fever, leukemia, multiple sclerosis, or gait disturbance 3, 5

Tuberculosis Considerations

• Persons under treatment for tuberculosis have not experienced exacerbations when vaccinated with MMR 1
• Before administering MMR to persons with untreated active tuberculosis, initiating antituberculous therapy is advisable 1
• Tuberculin testing is not a prerequisite for routine MMR vaccination 1

Emergency Preparedness

• Although anaphylaxis after MMR is extremely rare, epinephrine should be immediately available for administration, as this adverse event can be life-threatening 1