A new study reveals that thimerosal is cleared from children's bodies at least ten times faster than was previously thought.
The mercury containing preservative thimerosal was removed from childhood vaccines after fears that it played a role in the development of autism. Autism prevalence studies since have shown mixed results with some reporting a decrease in new autism cases after the removal of thimerosal and others showing no change from the previous upward trend.
Now a new study has looked at what happens to thimerosal in the bodies of infants and found that levels in the blood do not get as high as expected and the mercury is also cleared much faster than previously thought.
The form of mercury contained in thimerosal is ethyl mercury. Those who have said thimerosal is unlikely to contribute to autism have always said that this form of mercury is much less of a danger than another form, methyl mercury, which is the form found in contaminated seafood for example.
The study conducted by Dr. Michael E. Pichichero and colleagues of the University of Rochester in New York is published in the journal Pediatrics. The researchers examined 72 newborn babies, 72 2-month old infants and 72 6 month-olds at R. Gutierrez Children's Hospital in Buenos Aires. Thimerosal is still used in vaccines in Argentina.
They found that levels of mercury found in the blood rose dramatically shortly after vaccination then dropped quite rapidly with a half-life of only 3.7 days. This compares to methyl mercury which has a half-life of 44 days.
Previous studies using methyl mercury rather than ethyl mercury (thimerosal) have also demonstrated much larger rises in blood mercury levels than seen in this latest study.
Dr. Pichichero and colleagues also found that levels of mercury in the blood remained constant from birth to 6 months of age suggesting that the toxic metal doesn't accumulate, at least not quickly, and is constantly being removed from the body.
It was also found that most of the mercury was eliminated from the body through bowel movements rather than through the urine. This is reassuring as it means mercury is not coming into contact with the kidenys in large amounts.
The results from the study suggest that the ethyl mercury from thimerosal in vaccinations doesn't hang around in the body long enough for it to accumulate in the brains and other tissues to cause damage. This is just an assumption however since the researchers couldn't examine the brain and other organs for levels of mercury.
Isaac Pessah of the UC Davis MIND Institute, talking to the Los Angeles Times, also pointed out that the study had only looked at the affect of thimerosal containing vaccines in healthy children. He said they didn't address "the key issue of whether a subset of kids with metabolic disorders would handle it differently."
Indeed with an increasing feeling that autism is the result of genetic susceptibility interacting with environmental factors, the question that needs answering is whether mercury levels are higher and accumulate faster in those infants that go on to develop autism. Previous studies have shown high levels of mercury in autistic children as well as abnormalities in the enzymes that detoxify mercury and other heavy metals.
Headline makers have been quick to interpret this study as conclusive proof that vaccinations in general play no role in the causation of autism. While it does indicate that vaccinations may not be a major source of mercury this does not mean vaccinations don't have a role to play, nor that mercury doesn't since there are many other sources, particularly of the more dangerous methyl variety.
In terms of vaccinations, mercury content is not the only factor that could potentially link them to autism. Research from the UC Davis MIND Institute and other institutions have found strong evidence of immune system abnormalities in autistic children. The findings include hyperactivity of natural killer cells and B-cells (antibody producing cells) in those with autism (see news story). Researchers speculated that these results suggest that a stimulus to the immune system early in life may contribute to overactivation of the immune system and the development of autism.
Could vaccinations be such a trigger? Only further research can answer that question.