It's been controversial for years, this theory that vaccines containing thimerosal could cause neurological and psychological problems like autism. Many studies have discounted this link, but many parents have not been convinced.
In 2001, thimerosal was removed from all routinely recommended childhood vaccines as a precautionary measure.
In this month's issue of the journal Pediatrics, two large studies from the U.K. show no significant link between thimerosal-containing vaccines and abnormal neurological development. The third study is a careful review of all research published thus far.
The Newest Evidence
In the first study in Pediatrics, researchers analyzed data on more than 14,000 children, tracking their vaccinations at 3, 4, and 6 months of age as well as their calculated mercury exposure. They compared the data with information on the children's brain and motor development from age 6 months to age 7 or 8, when many children are diagnosed with autism.
The researchers looked at various outcomes including behavioral, speech, and other development milestones.
They also took into account other factors that might have skewed their data -- like the baby's birth weight, premature birth, whether the mother smoked, if the baby was breastfed, and ethnicity.
In the end, "we could find no convincing evidence" that early childhood exposure to thimerosal had any harmful effect on the children's brain, motor skill, or behavioral development, writes lead researcher Jon Heron, PhD, a pediatric epidemiologist with the University of Bristol in England.
"This is reassuring news," writes Heron.
However, there was a slight conflict with the second study in Pediatrics.
In the second study, children getting vaccines had more behavior tics -- possibly minor, short-lived tics, writes researcher Nick Andrews, MSc, an epidemiologist with the Communicable Disease Surveillance Centre in London.
Andrews analyzed data on nearly 110,000 British children, comparing the amount of thimerosal in the diphtheria-tetanus-pertussis (DTP) and the diphtheria-tetanus (DT) vaccines with incidence of developmental disorders.
For other disorders, there was no evidence of an association with thimerosal.
In fact, premature birth was the biggest factor in developmental disorders, with 5% of preterm children and 2% of full-term children having problems. Also, boys were more likely to have these disorders, he notes.
However, a greater number of short-lived tics was found among children who received all three doses of the DPT/DT vaccine within their first year. Although an association "cannot be ruled out," it is unlikely a "true finding" since tics would be accompanied by other developmental disorders, Andrews writes.
The Big Picture
To put all the evidence into perspective, Sarah K. Parker, MD, with Children's Hospital and University of Colorado Health Sciences Center in Denver, reviewed 12 studies published between 1966 and 2004. Only four studies found a link between autism and vaccines, and those were critically flawed -- so flawed that they were rendered invalid, she writes.
The bulk of evidence "does not support" an association between thimerosal-containing vaccines and autistic disorders, writes Parker.
"Determining the cause of autism is important for future diagnosis, treatment, and prevention. However, as the evidence reviewed here suggests, these efforts may be substantially more productive if they are redirected to other hypotheses."