A new US study suggests that no consistent links were found between pre- and post-natal early childhood exposure to thimerosal-containing vaccines and neuropsychological development. While the study has generally been welcomed as reassuring for parents, there is a suggestion the conclusion is not quite so clear cut.
The study was conducted by researchers from the US Centers for Disease Control and Prevention (CDC) and is published in the 27th September issue of the New England Journal of Medicine..
Thimerosal (also called thiomersal) is a mercury based vaccine preservative developed around 1930, but in the US its use has declined in recent years due partly to changes in the way vaccines are preserved and because of precautionary federal ruling.
Some have suggested its inclusion in vaccines given to babies could lead to mercury toxicity and impair neurologic development.
This latest CDC study was based on 1,047 children aged 7 to 10 years, with no history of neurologic conditions, from four HMOs (health maintenance organizations) taking part in the CDC’s Vaccine Safety Datalink. Children with low birth weight, meningitis, encephalitis, and other serious neurological conditions were excluded.
The researchers looked at the children’s vaccination and medical records, recorded interviews with mothers, and calculated exposure to vaccines using thimerosal at three points: before birth, during the first 28 days after birth, and during the first 7 months of life.
Estimates of mercury exposure included mothers’ prenatal fish consumption, and pre- and post-natal exposure to immunoglobulin and vaccines.
They assessed over 42 measures of general and specific motor and developmental neuropsychology, including: visuospatial, speech and language, verbal memory, attention, fine motor coordination, executive-functioning, behavior regulation, and tics.
The results showed that the median cumulative exposure to mercury from thimerosal-containing vaccines from birth to 7 months ranged from 0 to 188 micrograms (median value 112.5).
There was no overall consistent link between neuropsychological function and mercury exposure, but higher levels of mercury exposure were linked to minor differences in outcomes both positive and negative. These higher level mercury effects were different for each of the three exposure time points and between male and female babies.
Writing in various commentaries in Journal Watch, also published by the New England Journal of Medicine, experts appear to have mixed views on the study’s results and conclusions.
For instance, one expert (JWatch Infectious Diseases, Richard T Ellison III), said that although the study did not exclude the possibility that there might be some subtle neuropsychological effects from thimerosal, the pattern is likely to be due to chance. Others called the lack of a consistent pattern “reassuring”, or that there were “no clear adverse outcomes”.
But another more critical expert (JWatch Psychiatry, Barbara Geller) wrote that while the researchers analyzed the data expertly to show there were no “meaningful clinical problems” linked to thimerosal, their conclusions should be questioned because the study has serious limitations. For example only 30 per cent of cases were studied, and how the excluded cases may have differed was not discussed.
Other limitations highlighted were the fact the data was collected retrospectively, the cases that were excluded (for instance babies of low birthweight) may have been the ones most vulnerable to a neuroactive substance, and the fact these potential weaknesses were not discussed either. This expert suggested parents need to be told about the “complexities of studying this problem”.
Thimerosal is an antifungal and antiseptic based on ethyl mercury and contains nearly 50 per cent mercury by weight.
In 1999 the American Academy of Pediatrics asked that thimerosal be removed from all pediatric vaccines. Two years later the Institute of Medicine said there was not enough evidence to show whether autism and thimerosal are linked, but it was possible in theory, and in 2004, after new epidemiological studies were concluded, they rejected the idea of a link.
In 2002 the CDC and FDA said that as a precaution, thimerosal was to be discontinued in some paediatric vaccines, although there was no proof that it was dangerous in low doses.
More recently, in July 2006, the World Health Organization (WHO) Global Advisory Committee on Vaccine Safety concluded there was “no evidence of toxicity in infants, children or adults exposed to thimerosal (containing ethyl mercury) in vaccines”.
The CDC is conducting a separate study on thimerosal and autism, a subject of much heated debate. Thousands of families in the US and other countries have alleged that childhood vaccines, many containing thimerosal, are linked to autism.
“Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years.”
Thompson, William W., Price, Cristofer, Goodson, Barbara, Shay, David K., Benson, Patti, Hinrichsen, Virginia L., Lewis, Edwin, Eriksen, Eileen, Ray, Paula, Marcy, S. Michael, Dunn, John, Jackson, Lisa A., Lieu, Tracy A., Black, Steve, Stewart, Gerrie, Weintraub, Eric S., Davis, Robert L., DeStefano, Frank, the Vaccine Safety Datalink Team.
N Engl J Med 2007 357: 1281-1292.
Volume 357, pages 1281-1292; September 27th, 2007; Number 13.
Written by: Catharine Paddock