exception of tics, in association with thimerosal exposure. For general developmental disorders, unspecified developmental delay, and attention deficit disorder, increasing thimerosal exposure had an apparent protective effect. Although the study was limited by an inability to adjust for several confounding factors, such as socioeconomic status and other medical conditions, in general, it had a sound methodology. GPRD is a good source of linked data that may be used to look at other aspects of the vaccination schedule in the United Kingdom. The aspect of the schedule covered by this study included the cumulative doses of thimerosal received by children immunized with DTP and DT and whether these were received, for example, on time or late.
Two studies examined aspects of the Danish immunization schedule. Hviid et al. (2003) studied the relationship between cumulative thimerosal exposure via the whole-cell pertussis vaccine and autistic spectrum disorder. The study included a cohort of children with a diagnosis of autistic spectrum disorder born between 1990 and 1996. The diagnoses were taken from the Danish Psychiatric Central Research Registry and linked with the immunization history of each child. The study covered a period (1990 to 1992) when only one thimerosal-containing vaccine was in use. The study found no association between a diagnosis of autism and the presence of thimerosal but noted that the incidence of autism may have been underascertained, especially in earlier birth cohorts. This study did not demonstrate a relationship between thimerosal administration via the pertussis vaccine and the development of autism in a small country (Denmark) with high immunization rates and a good system of record keeping. The only aspect of the schedule covered was thimerosal exposure specifically via the pertussis vaccine.
The other Danish study evaluating an association between immunization and PDD (Madsen et al., 2003) also used data from the Danish Psychiatric Central Research Registry. The authors sought to evaluate the vaccine history for all Danish children identified with autism between 1971 and 2000 to assess the incidence of autism among children between 2 and 10 years of age before and after the removal of thimerosal from vaccines in 1992. The annual incidence of autism increased rapidly starting in 1990 and continued to do so through 1999, even though thimerosal was eliminated from DTP in 1992. The study was limited, as was the study by Hviid et al. (2003), by the fact that before 1995, diagnoses of autism were made only for hospitalized patients, whereas after 1995, outpatient diagnoses of autism were included. This study failed to demonstrate a correlation between the discontinuation of thimerosal in DTP and the incidence of autism in Danish children. This was an ecological study and so it cannot confirm an association. The paper provided no real information about the immunization schedule.