Measles Encephalitis in Children Before MMR Vaccination in the USA
Before the measles vaccine was licensed in 1963, approximately 3,500 children developed encephalitis from measles annually in the United States.
Historical Disease Burden
The pre-vaccine era measles burden was substantial:
Approximately 3.5 million measles cases occurred annually in the United States before 1963, representing essentially an entire birth cohort as virtually all children acquired measles 1
Encephalitis occurred in approximately 1 per 1,000 reported measles cases, meaning the rate was consistent across the disease spectrum 1, 2
With 3.5 million annual cases and a 1:1,000 encephalitis rate, this translates to approximately 3,500 cases of measles encephalitis per year in children before vaccine introduction 1
Clinical Consequences of Measles Encephalitis
The neurological impact was severe:
Survivors of measles encephalitis often had permanent brain damage and mental retardation, making this a devastating complication beyond the acute illness 1
Encephalitis was one of the two most common causes of death from measles, along with pneumonia, contributing to the overall case-fatality rate of 1-2 per 1,000 cases 1
Symptoms typically appeared during or shortly after the acute measles illness, with neurological signs occurring around 10 days after initial infection 2
Additional Long-Term Neurological Burden
Beyond acute encephalitis, another devastating complication existed:
Subacute sclerosing panencephalitis (SSPE) occurred in approximately 4-11 per 100,000 measles-infected individuals, particularly those infected at young ages, representing an invariably fatal late complication appearing years after initial infection 2, 3
With 3.5 million annual measles cases, this translated to approximately 140-385 children developing SSPE annually from their measles infections 2
Impact of Vaccination
The introduction of measles vaccine dramatically altered this landscape:
During the first 20 years of vaccine licensure (1963-1983), vaccination prevented an estimated 17,400 cases of mental retardation from measles complications, primarily encephalitis 4
Measles vaccination has essentially eliminated SSPE from the United States in countries with high vaccination coverage 1, 2
The reported incidence of measles decreased by greater than 99% following widespread vaccination programs 1
Critical Context for Modern Practice
Understanding the pre-vaccine burden is essential for counseling:
The risk of encephalopathy from MMR vaccine is approximately 1 per 2 million doses, vastly lower than the 1 per 1,000 risk with wild-type measles infection 2, 3, 5
There is no evidence of an association between MMR immunization and encephalitis or encephalopathy in modern surveillance (rate ratio 0.90,95% CI 0.50 to 1.61) 6, 7
The only proven prevention strategy for SSPE is measles vaccination, as SSPE is caused by persistent wild-type measles virus infection, not by vaccination 5