Thimerosal exposure in infants and developmental disorders: a retrospective cohort study in the United kingdom does not support a causal association.
OBJECTIVE
After concerns about the possible toxicity of thimerosal-containing vaccines in the United States, this study was designed to investigate whether there is a relationship between the amount of thimerosal that an infant receives via diphtheria-tetanus-whole-cell pertussis (DTP) or diphtheria-tetanus (DT) vaccination at a young age and subsequent neurodevelopmental disorders.


METHODS
A retrospective cohort study was performed using 109 863 children who were born from 1988 to 1997 and were registered in general practices in the United Kingdom that contributed to a research database. The disorders investigated were general developmental disorders, language or speech delay, tics, attention-deficit disorder, autism, unspecified developmental delays, behavior problems, encopresis, and enuresis. Exposure was defined according to the number of DTP/DT doses received by 3 and 4 months of age and also the cumulative age-specific DTP/DT exposure by 6 months. Each DTP/DT dose of vaccine contains 50 microg of thimerosal (25 microg of ethyl mercury). Hazard ratios (HRs) for the disorders were calculated per dose of DTP/DT vaccine or per unit of cumulative DTP/DT exposure.


RESULTS
Only in 1 analysis for tics was there some evidence of a higher risk with increasing doses (Cox's HR: 1.50 per dose at 4 months; 95% confidence interval [CI]: 1.02-2.20). Statistically significant negative associations with increasing doses at 4 months were found for general developmental disorders (HR: 0.87; 95% CI: 0.81-0.93), unspecified developmental delay (HR: 0.80; 95% CI: 0.69-0.92), and attention-deficit disorder (HR: 0.79; 95% CI: 0.64-0.98). For the other disorders, there was no evidence of an association with thimerosal exposure.


CONCLUSIONS
With the possible exception of tics, there was no evidence that thimerosal exposure via DTP/DT vaccines causes neurodevelopmental disorders.
Andrews
Nick
N
Statistics Unit, Health Protection Agency, Communicable Disease Surveillance Centre, London, United Kingdom.
Miller
Elizabeth
E
Grant
Andrew
A
Stowe
Julia
J
Osborne
Velda
V
Taylor
Brent
B
D001289:Attention Deficit Disorder with Hyperactivity / Q000139:chemically induced; D001321:Autistic Disorder / Q000139:chemically induced; D015331:Cohort Studies; D002658:Developmental Disabilities / Q000139:chemically induced*; D022422:Diphtheria-Tetanus Vaccine / Q000008:administration & dosage / Q000009:adverse effects*; D015721:Diphtheria-Tetanus-Pertussis Vaccine / Q000008:administration & dosage / Q000009:adverse effects*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007223:Infant; D007806:Language Disorders / Q000139:chemically induced; D008297:Male; D015999:Multivariate Analysis; D011310:Preservatives, Pharmaceutical / Q000008:administration & dosage / Q000009:adverse effects*; D016016:Proportional Hazards Models; D012189:Retrospective Studies; D013064:Speech Disorders / Q000139:chemically induced; D016019:Survival Analysis; D013849:Thimerosal / Q000008:administration & dosage / Q000009:adverse effects*; D020323:Tics / Q000139:chemically induced
D022422:Diphtheria-Tetanus Vaccine; D015721:Diphtheria-Tetanus-Pertussis Vaccine; D011310:Preservatives, Pharmaceutical; D013849:Thimerosal