An analysis of data covering young children in California does not suggest any association between measles-mumps-rubella (MMR) immunization and an increase in the occurrence of autism.
The results are reported in an article in today's issue of the Journal of the American Medical Association (JAMA). They counter the continuing rumors of some link between the MMR vaccine and autism, based on a 1990s hypothesis by one investigative group in the UK.
Autism is a disorder that involves impairment in children's social interactions and interpersonal communication, along with repetitive and stereotyped activities.
Loring Dales, M.D., and colleagues from the Immunization Branch, California Department of Health Services, Berkeley, conducted a retrospective study to determine if a correlation exists between trends of MMR immunization among young children and the occurrence of autism.
They examined MMR immunization coverage rates among children born in 1980-1994 who were enrolled in California kindergartens, using survey samples of 600 to 1,900 children each year.
Immunization records were reviewed to determine the age at which the children first received MMR immunization. The authors also analyzed caseload data among children born in 1980-1994 who were diagnosed with autism and were enrolled in California Department of Developmental Services regional service centers.
The authors point out that autism is an incompletely understood developmental disorder or group of related disorders that vary in clinical presentation, making diagnosis less than straightforward.
A medical investigative group in Great Britain speculated in the mid to late 1990s that measles immunization and/or immunization with measles, mumps, and rubella vaccines given combined at the same time, or in succession over a relatively short time period, might be a cause of autism.
There has been strong interest in and debate over this hypothesis. Some people cite a 1999 report from the California Department of Developmental Services showing an increase within the past two decades in its caseload of children with autism, wondering if increasingly widespread combined MMR immunization of young children might have been responsible for this increase.
The authors measured MMR coverage rates as of ages 17 months and 24 months, and the numbers of Department of Developmental Services system enrollees diagnosed with autism, grouped by year of birth.
"Essentially no correlation was observed between the secular trend of early childhood MMR immunization rates in California and the secular trend in numbers of children with autism enrolled in California's regional service center system," they write.
"For the 1980-1994 birth cohorts, a marked, sustained increase in autism case numbers was noted, from 44 cases per 100,000 live births in the 1980 cohorts to 208 cases per 100,000 live births (a 373 percent relative increase) between 1980 and 1994, but changes in early childhood MMR immunization coverage over the same time period were much smaller and of shorter duration," they report.
"Immunization coverage by the age of 24 months increased from 72 percent to 82 percent, a relative increase of only 14 percent, over the same time period," they continue.
The authors point out that a similar lack of correlation between the trends in early childhood MMR immunization rates and autism occurrence has been noted in Great Britain and in Sweden.
"In conclusion, to date, published observations based on empirical evidence do not suggest that increased MMR immunization among young children is associated with secular increases in cases of autism," they conclude.
(Reference: JAMA. 2001; 285:1183-1185)