This is the first of two posts about thimerosal-containing vaccines and autism. The first post is about the scientific evidence for a connection between thimerosal-containing vaccines and autism. The second post will discuss whether pharmaceutical companies and federal regulators have responded appropriately to thimerosal concerns.
Until recently, I was agnostic about the safety of thimerosal. Then Orac stepped into the fray, repeatedly. I was thus inspired to investigate further.
As far as I can tell, the thimerosal/autism connection is totally unsupported by evidence. Autism diagnoses have been rising steadily over the past few decades. However, we shouldn't infer that autism incidence must therefore be increasing. Autism awareness has also risen steadily over the years, thanks to the hard work of activists, researchers, and clinicians. The diagnostic category has also broadened significantly since the 1940's to include an entire spectrum of developmental disorders.
Thimerosal made its debut as a vaccine preservative in the 1930s. The term "autism" had been used since 1911 to describe the symptom of extreme pathological introversion. However, autism wasn't recognized as a distinctive developmental disorder until 1943.
According to the World Health Organization's statement on thimerosal, concern over the safety of thimerosal was initially sparked by a confusion. After 70 seemingly uneventful years of widespread use, thimerosal came under scrutiny from the FDA in 1999. During a review of vaccination recommendations, some experts became concerned that the cumulative mercury exposure in the standard vaccine schedule exceeded U.S. exposure limits for mercury. As it turned out, the exposure limits were for set methyl mercury, whereas thimerosal is a derivative of ethyl mercury. The FDA had been treating ethyl and methyl mercury as equivalent, but we now know that two compounds have significantly different toxicological properties:
Expert consultation and data presented to the Global Advisory Committee on Vaccine Safety (GACVS) on 20-21 June 2002 indicate that the pharmacokinetic profile of ethyl mercury is substantially different from that of methyl mercury. The half-life of ethyl mercury is short (less than one week) compared to methyl mercury (1.5 months) making exposure to ethyl mercury in blood comparatively brief. Further, ethyl mercury is actively excreted via the gut unlike methyl mercury that accumulates in the body. Two independently-conducted epidemiological studies have been completed in the United Kingdom. These studies further support the safety of thiomersal-containing vaccines in infants in the amounts used in existing vaccines.
The FDA Center for Biologicals Evaluation and Research (CBER) offers a very comprehensive resource on thimerosal and vaccines.
In 2003, Geier and Geier claimed to have found a statistically significant association between thimerosal-containing vaccines and autism based on an analysis of data from the CDC's Vaccine Adverse Event Reporting System (VAERS). The American Academy of Pediatrics severely criticized the study for shoddy methodology and factual errors. According to the AAP:
The most important weakness of the article is the reliance on VAERS data to draw conclusions about adverse event associations or causality. VAERS is a passive surveillance system for reporting possible vaccine adverse events that depends on health care professionals, patients, and others to file reports. Health effects reported to VAERS as being associated with vaccines may represent true adverse events, coincidental occurrences, or mistakes in filing. Inherent limits of VAERS include incomplete reporting, lack of verification of diagnoses, and lack of data on people who were immunized and did not report problems.
Perhaps the best evidence against the thimerosal/autism hypothesis is the fact that banning thimerosal doesn't reduce autism rates. If thimerosal caused autism, you would expect autism rates to fall after the offending vaccines were stricken from the immunization schedule. Several nations including Denmark and Canada have already banned thimerosal-containing vaccines, but no declines in autism have been observed so far. In the United States, the FDA has worked with vaccine manufacturers to decrease thimerosal exposure in the standard infant immunization schedule by 95%. So far, decreased exposure hasn't translated into decreased autism incidence.
In my next post, I'll talk about the allegations of collusion, influence peddling, and data suppression. So far, I see no evidence of a conspiracy or a coverup. However, the evidence points to the usual combination public relations and influence peddling.
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