Increase in autism due to change in definition, not MMR vaccine

Changes in the definition of autism, rather than use of the measles, mumps, and rubella vaccine (MMR vaccine), led to increased diagnosis of autism in the United States and probably in Europe, says a study from the Mayo Clinic in Rochester, Minnesota (Archives of Pediatrics and Adolescent Medicine 2005;159:37-44).

Before the late 1980s and early 1990s, these children were diagnosed as retarded or having developmental delay, lead author William Barbaresi, a developmental paediatrician, told the BMJ.

The prevalence of autism increased more than eightfold in the United States, from 5.5 (95% confidence interval 1.4 to 9.5) per 100 000 children in 1980-3 to 44.9 (32.9 to 56.9) per 100 000 in 1995-7.

The diagnosis of autism became more precise with the Diagnostic and Statistical Manual of Mental Disorders, third edition, revised (DSM-III-R) in 1987 and with the publication of a subsequent edition (DSM-IV) in 1994. Similar changes were made at about the same time in the International Classification of Diseases.

In 1991, the US federal law providing special educational services for children with disabilities was broadened to include more services and awareness of the problem increased. Diagnoses of autism increased substantially after 1988 in the United States, although diagnosis of other problems increased more slowly. Dr Barbaresi said that “we can't completely exclude an environmental factor” for the increase in the incidence of autism; the increase is “a correlation, not cause and effect.”

He said that the incidence of autism increased in Minnesota children who received a vaccine containing thiomersal (known as thimerosal in the United States), however, and also in Danish children who received a thiomersal-free vaccine during the same time period (New England Journal of Medicine 2002;347:1477-82, BMJ 2002;325:1134).

Before children start school, Minnesota has required them to be immunised against measles since 1967, against rubella since 1973, and against mumps since 1978. The MMR vaccine was licensed in 1971 and is the usual vaccine used in Minnesota.

In Denmark, Dr Barbaresi noted, thiomersal was removed from vaccines in 1991, but the increase in autism continued until 2000. In the United States, vaccines containing thiomersal continued to be used.

Researchers at the Mayo Clinic were able to study a population in relatively isolated Olmsted County in southeastern Minnesota, which has unusually good and complete medical records. More than 95% of medical care is provided by the Mayo Clinic and Olmsted Medical Center. People in the county have almost universal access to high quality medical care and computerised complete medical records are available for more than 95% of residents. Few people move in or out of the county.

Dr Barbaresi and his colleagues looked at the incidence of autism year by year among about 36 000 children aged under 21 of the county between 1976 and 1997. Working retrospectively, the researchers correlated the definition of autism used in DSM-IV with symptoms described in medical and school records, using a list of 80 developmental, psychiatric, and neurological diagnoses.

They identified 124 children with autism as diagnosed by DSM-IV criteria. Most had not been diagnosed as having autism, but rather as having developmental delay, delayed speech and language development, attention deficit or hyperactivity disorder, and mental retardation. Boys outnumbered girls by more than three to one. The first symptom was noted at about 2½ by medical professionals (48.1%), school staff (30.4%), and parents or care-givers (20.9%).