Mercury containing vaccines and developmental problems in kids
A new study published in the New England Journal of Medicine looked at over 1000 children to see if early exposure to a mercury containing preservative used in vaccines affected children aged 7 - 10 years of age. This large study concluded that there was no correlation between thimerosal (mercury containing preservative) and the neuropsychological outcomes at 7 to 10 years.
The short version
Thimerosal, a mercury containing preservative previously used in vaccines raised concerns about developmental issues in kids. This new large study did not find significant problems associated with high thimerosal exposure. The investigators did not study autism. Another CDC study is looking at the concern that vaccination might be linked to autism. The scientific data to date does not support any such link.
Thimerosal as a preservative
Thimerosal has been used as a preservative in vaccines since the 1930s. It is 49.6% mercury by weight and is metabolized into ethyl mercury and thiosalicylate. In 1999, the Food and Drug Administration (FDA) estimated that infants who were immunized according to the recommended schedule could receive amounts of mercury exceeding the limits set by the Environmental Protection Agency for exposure to methyl mercury. As a precautionary measure, the Public Health Service and the American Academy of Pediatrics urged vaccine manufacturers to remove thimerosal from all infant vaccines as soon as was practical and recommended that studies be carried out to understand better the risks associated with mercury exposure from thimerosal-containing vaccines.
The early exposure of infants to mercury raised concerns about its effect on the development of the nervous system in children. Concern was also raised about a possible link to autism. This study did not look at autism but the scientific literature to date shows no link between autism and vaccination. A CDC sponsored study looking at vaccination and autism in 250 children is expected to be published in the next year or so.
Changes in child functioning seen in this study
Out of 378 statistical studies of the data collected, 19 positive or negative results were expected by random chance. Exactly 19 trends were seen that were positive or negative. 12 endpoints showing increased functioning were associated with increased exposure to mercury and 7 were associated with negative functioning. Overall 98-99% of all the statistics did not associate with mercury exposure.
Tics and mercury exposure
Tics are involuntary movements of muscles. Popularly these are thought of as twitching of a muscle but clinically they can be motor (twitching) or speech tics (phonic). They can be transient, disappearing after short amount of time or long term. Short term tics are not medically important and not associated with any medical problems. An increase in tics was seen associated with increased mercury exposure but this could be random chance. It was one of the 19 changes seen out of 378.
However an increase in tics was seen in previous studies and so the authors suggest this might be looked at more closely. Because of the short period over which these children were assessed, it is not known if these were transient or permanent tics. On the positive side, parents reporting tics in their children did not associate with mercury exposure and they would be in a position to assess their children over longer periods of time. Further studies are planned to follow up on this observation.
Almost all vaccines do not contain thimerosal in 2007
Other than the flu vaccine, no vaccines contain thimerosal today. The flu vaccine comes in low thimerosal and thimerosal free forms. Vaccines themselves save many lives and prevent many life altering illnesses in our children. There is a very high immunization rate in many developed countries. In the U.S.A. 99.7% of all infants are vaccinated. In fact this posed a problem for the researchers in trying to find a control group that was not vaccinated.
Why not compare to a non-vaccinated population?
As mentioned above, there are very few children who are not vaccinated in the U.S. where this study was carried out. One could look at historical controls, those who grew up in an age with low vaccination but this is not a good scientific control. This is because many changes have taken place. In fact, Dr. Anne Schuchat explains"So imagine if there was some kind of problem with cell phones or some kind of problem with computers, things that have changed quite a bit over time, there could be big problems with looking – enrolling children from an earlier time period and comparing them with children from a later time period."
Other suggestions have been to compare to a current population who may have a lower vaccination rate such as the Amish community. Again this is not a good scientific control as this population has many other differences to the general population that would interfere with interpreting the results.
Overall this study seems to reassure that there is no significant link to developmental disorders in vaccinated children. The data has now been made available to other scientist to run their own statistical analyses. Currently all scientific studies show no link between vaccination and developmental disorders or autism