We were initially highly skeptical that differences in the concentrations of
thimerosal in
vaccines would have any effect on the incidence rate of
neurodevelopmental disorders after childhood immunization. This study presents the first epidemiologic evidence, based upon
tens of millions of doses of
vaccine administered in the United States, that associates increasing
thimerosal from
vaccines with
neurodevelopmental disorders. Specifically, an analysis of the
Vaccine Adverse Events Reporting System (VAERS) database showed statistical increases in the incidence rate of
autism (relative risk [RR] = 6.0),
mental retardation (RR = 6.1), and
speech disorders (RR = 2.2) after
thimerosal-containing
diphtheria,
tetanus, and acellular
pertussis (
DTaP) vaccines in comparison with
thimerosal-free
DTaP vaccines. The male/female ratio indicated that
autism (17) and
speech disorders (2.3) were reported more in males than females after
thimerosal-containing
DTaP vaccines, whereas
mental retardation (1.2) was more evenly reported among male and female
vaccine recipients. Controls were employed to determine if biases were present in the data, but none were found. It was determined that overall adverse reactions were reported in similar-aged populations after
thimerosal-containing DTaP (2.4 +/- 3.2 years old) and
thimerosal-free DTaP (2.1 +/- 2.8 years old) vaccinations. Acute control adverse reactions such as deaths (RR = 1.0),
vasculitis (RR = 1.2),
seizures (RR = 1.6), ED visits (RR = 1.4), total adverse reactions (RR = 1.4), and
gastroenteritis (RR = 1.1) were reported similarly after
thimerosal-containing and
thimerosal-free
DTaP vaccines. An association between
neurodevelopmental disorders and
thimerosal-containing
DTaP vaccines was found, but additional studies should be conducted to confirm and extend this study.