Can the Measles Component of MMR Vaccine Infect Immune Cells?
Yes, the live attenuated measles virus in MMR vaccine can infect and replicate in immune cells, particularly in severely immunocompromised individuals, which is why MMR is contraindicated in this population. 1
Mechanism of Vaccine Virus Replication
- The measles component of MMR is a live attenuated virus that requires cellular infection and replication to generate immunity. 1
- Enhanced replication of vaccine viruses occurs in persons with immune deficiency diseases and in immunosuppressed individuals, allowing the vaccine strain to replicate beyond normal controlled levels. 1
- In severely immunocompromised patients, inadequate immune surveillance permits problematic vaccine virus replication that can lead to disseminated vaccine-strain measles infection. 2
Clinical Significance in Immunocompromised Patients
- Case reports have linked vaccine-associated measles infection to deaths in severely immunocompromised persons, demonstrating that the vaccine virus can cause clinically significant disease when immune function is inadequate. 1, 2
- The Advisory Committee on Immunization Practices (ACIP) explicitly states that MMR vaccine should not be administered to severely immunocompromised persons due to this risk. 1
- One documented case involved a 5-year-old child with prior hematopoietic stem cell transplant who developed measles illness 7-14 days after MMR vaccination, with genotyping confirming vaccine strain virus. 3
Who Is Considered Severely Immunocompromised?
The CDC defines severe immunocompromise as: 2
- Congenital immunodeficiency disorders
- HIV infection with severe immunosuppression (CD4 count criteria apply)
- Active hematologic or generalized malignancy
- Current therapy with alkylating agents, antimetabolites, or radiation
- High-dose corticosteroids: ≥2 mg/kg/day or ≥20 mg/day prednisone equivalent for ≥14 days 2
Important Exceptions Where MMR Can Be Given
- HIV-infected patients who are NOT severely immunosuppressed should receive MMR vaccine, as studies show variable antibody responses but not severe adverse events in this population. 1, 2
- Low-dose or short-term corticosteroids (<14 days, <2 mg/kg/day) do not contraindicate MMR administration. 1, 2
- Topical, aerosol, or intra-articular corticosteroid administration typically does not cause sufficient systemic immunosuppression to contraindicate MMR. 1
Safety in Immunocompetent Individuals
- In persons with normal immune function, the attenuated measles virus in MMR replicates in a controlled manner that stimulates immunity without causing disease. 1
- The vaccine has been used safely for decades in hundreds of millions of immunocompetent individuals worldwide. 4
- No cases exist linking MMR vaccine to subacute sclerosing panencephalitis (SSPE), which only develops from wild-type measles infection. 2
Critical Caveat: Protecting Immunocompromised Patients
- All household contacts and healthcare workers of immunocompromised patients should receive MMR vaccine to provide indirect protection through herd immunity. 1, 2
- Transmission of MMR vaccine strains from vaccinated contacts is extraordinarily rare, with no microbiologically confirmed secondary cases documented in most situations. 2, 3
- The risk of transmission from vaccine recipients to immunocompromised contacts is negligible compared to the risk from wild-type measles exposure. 3