Thimerosal exposure in
infants and developmental disorders: a retrospective cohort study in the
United kingdom does not support a causal
association.
Andrews N, Miller E, Grant A, Stowe J,
Osborne V, Taylor B.
Statistics Unit, Health Protection
Agency, Communicable Disease Surveillance Centre, London, United
Kingdom.
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.