Can Measles Virus Infect Neurons?
Yes, wild-type measles virus directly infects neurons and causes devastating CNS disease, while MMR vaccine-strain measles does not establish CNS infection and remains localized to peripheral tissues. 1, 2
Wild-Type Measles Neurotropism
Wild-type measles virus is definitively neurotropic and causes three distinct patterns of CNS infection:
Acute measles encephalitis occurs in approximately 1 per 1,000 infected persons, presenting with fever, altered mental status, and seizures during or shortly after acute infection (around 10 days post-infection), with potential for permanent brain damage in survivors. 1
Subacute sclerosing panencephalitis (SSPE) represents persistent mutant measles virus infection in the CNS, occurring years after initial infection in 4-11 per 100,000 measles-infected individuals (particularly those infected at young ages), and is invariably fatal. 1 This condition demonstrates insidious personality changes, intellectual decline progressing to dementia, myoclonic jerks with characteristic 1:1 EEG periodic complexes, motor deterioration, coma, and death. 3
Measles inclusion body encephalitis (MIBE) occurs in immunocompromised patients approximately six months after primary infection, with measles inclusion bodies found directly in brain tissue, often without the typical rash. 3
Mechanism of CNS Invasion
The cellular and molecular mechanisms governing CNS invasion by wild-type measles remain poorly understood, and the known measles entry receptors are not expressed in the CNS, making the exact route of brain entry unclear. 4 However, measles virus has been detected in cerebrospinal fluid, confirming that once in the lungs, the virus can spread throughout the body and eventually reach the CNS. 5
Recent advances suggest disruption of F protein function plays a critical role in measles spread within the nervous system, raising possibilities for treatment with fusion-inhibiting molecules. 6
MMR Vaccine-Strain Behavior: Critical Distinction
The MMR vaccine-strain viruses behave fundamentally differently from wild-type measles and do not establish CNS infection. 2
The MMR vaccine does not cross the blood-brain barrier, as it is administered subcutaneously and generates systemic immunity without requiring CNS penetration. 2
The vaccine contains live attenuated viruses that replicate only at the injection site and in regional lymphoid tissue, producing systemic antibody responses without CNS entry. 2
The vaccine produces an inapparent or mild, noncommunicable infection that remains localized to peripheral tissues. 2
Persons who receive MMR do not transmit vaccine viruses, indicating the vaccine produces only localized, self-limited infection. 2
Vaccine Safety Profile Regarding Neurological Events
The extraordinarily rare neurological events after MMR vaccination (approximately 1 per 2 million doses) occur at a rate not greater than the background incidence of CNS dysfunction in the general population (0.4 per million doses). 2, 7
The Advisory Committee on Immunization Practices definitively states that MMR vaccine does not increase SSPE risk, regardless of prior measles infection or vaccination history. 8
When SSPE has been reported rarely among children who received measles vaccine without documented natural measles, evidence indicates these children had unrecognized measles infection before vaccination, and the SSPE was directly related to the natural measles infection, not the vaccine. 2
Measles vaccination substantially reduces SSPE occurrence, with near elimination of SSPE cases after widespread vaccination. 2
Clinical Pitfall to Avoid
Do not confuse febrile seizures after MMR (1 per 3,000 doses, occurring 5-12 days post-vaccination) with encephalopathy—febrile seizures do not cause residual neurological disorders and carry no increased risk for subsequent epilepsy. 8, 2 True vaccine-strain measles encephalopathy occurs at approximately 1 case per 2 million doses, vastly lower than the 1 per 1,000 risk with wild-type measles infection. 8