I am 29 weeks pregnant, received the measles‑mumps‑rubella (MMR) vaccine 12 years ago, and have been exposed to a patient with suspected rubella; what is my risk of acquiring rubella?

Risk of Rubella Infection After Exposure at 29 Weeks Pregnant with Prior MMR Vaccination

Your risk of acquiring rubella is extremely low—approximately 5% or less—because your MMR vaccination 12 years ago provides long-term, likely lifelong protection against both clinical rubella and viremia. 1

Understanding Your Protection Level

Vaccine-Induced Immunity Duration

• Greater than 90% of vaccinees maintain protection against both clinical rubella and viremia for at least 15 years after a single dose of MMR vaccine, and available follow-up studies indicate vaccine-induced protection is long-term, probably lifelong 1
• Your vaccination 12 years ago falls well within this protective window 1
• A history of vaccination can be considered presumptive evidence of immunity according to ACIP guidelines 1

Mechanism of Protection

• Vaccine-induced immunity usually protects against both clinical illness and viremia after natural exposure, even though vaccine-induced antibody titers are generally lower than those from natural infection 1
• Vaccinees demonstrate resistance to reinfection similar to the resistance that follows natural infection 1

Small Risk of Breakthrough Infection

Rare Reinfection Possibility

• A small number of reports indicate that viremic reinfection following exposure may occur in vaccinated individuals with low levels of detectable antibody 1
• The frequency and consequences of this phenomenon are currently unknown but believed to be rare 1
• These rare breakthrough cases are expected because there are also rare reports of clinical reinfection and fetal infection following disease-induced immunity 1

Context at 29 Weeks Gestation

• At 29 weeks (third trimester), the risk of congenital rubella syndrome is substantially lower than first trimester exposure, where the risk is ≥20% with maternal infection 1
• Even if breakthrough infection occurred (which is rare), third trimester rubella poses minimal risk for congenital defects compared to first trimester infection 1

Recommended Clinical Actions

Immediate Steps

• No intervention is required if you have documented evidence of MMR vaccination received after your first birthday 1
• Immune globulin (IG) is not recommended for routine postexposure prophylaxis of rubella in pregnancy, as it does not prevent infection or viremia and may only create an unwarranted sense of security 1
• Infants with congenital rubella have been born to women who received IG shortly after exposure, demonstrating its ineffectiveness 1

Optional Serologic Testing

• If you want definitive confirmation of immunity, serologic testing for rubella IgG antibody can be performed, though this is not necessary given your documented vaccination history 1
• Laboratory evidence of rubella immunity (IgG antibody) is the only truly reliable proof of immunity, but documented vaccination is considered acceptable presumptive evidence 1

Critical Pitfalls to Avoid

• Do not rely on clinical diagnosis of rubella in the exposed patient, as clinical diagnosis is unreliable 1
• Do not administer immune globulin, as it will not prevent infection and provides no proven benefit 1
• Do not consider MMR revaccination during pregnancy, as live rubella vaccine is absolutely contraindicated in pregnancy 1
• Do not assume you need pregnancy termination or special interventions based solely on this exposure, given your vaccination history 1, 2

Bottom Line

With documented MMR vaccination 12 years ago, you have presumptive evidence of immunity and are highly protected against rubella infection. 1 The combination of long-lasting vaccine-induced immunity (>90% protection at 15 years) and your third trimester gestational age means both your risk of infection and risk of fetal complications are minimal. 1 No specific interventions are warranted beyond routine prenatal care. 1, 2