Thimerosal has been added to vaccines and other biologics since the 1930s with no known adverse effects. However, this preservative contains trace amounts of mercury, and the increasing number of recommended childhood vaccinations has raised concerns about cumulative mercury exposure.
Apprehension surrounding mercury in vaccinations has heightened in light of rising rates of autism in children. While medical evidence does not support an association between thimerosal and autism, many parents remain anxious about vaccinating their children.
Some epidemiologists point out that more cases of autism are now being documented due to increased physician awareness and more sensitive diagnostic criteria—and because some disorders that were formerly classified in different categories are now being called autism—and that changes in data collection have created an exaggerated rise in autism’s incidence. This argument has not assuaged the fears of thimerosal’s opponents, however.
Therefore, despite good evidence of thimerosal’s safety, in 1999 the American Academy of Pediatrics and the US Public Health Service recommended that childhood immunizations be manufactured without the preservative. Although some formulations of influenza vaccines still contain it, the last thimerosal-containing childhood vaccines expired in 2003. (Centers for Disease Control and Prevention. Notice to readers: thimerosal in vaccines: a joint statement of the American Academy of Pediatrics and the Public Health Service. Morb Mortal Wkly Rep 1999;48(26):563-565)
The Hannah Poling Case
Unfortunately, the thimerosal issue wasn’t clarified when, in the fall of 2007, the US government ruled that the receipt of several vaccinations was at least partly responsible for causing autism in a child who suffered from a rare mitochondrial disorder. Physicians and parents were further confounded by the highly-charged discussion surrounding this case, the details of which were announced in March 2008:
- Hannah Poling (now nine years old) was a healthy 19-month old until she received five vaccines (DTaP, Hib, MMR, varicella, and IPV); two days later she became irritable, lethargic, and feverish.
- Several months later, after she failed to meet developmental milestones, Hannah was diagnosed with encephalopathy caused by a mitochondrial enzyme deficit.
- Although children with mitochondrial enzyme deficiencies often develop neurologic deficits during the first two years of life, Hannah’s parents (her father is a neurologist) contended the immunizations CAUSED the mitochondrial syndrome.
- Hannah’s father stated that "the results in this case may well signify a landmark decision with children developing autism following vaccinations" (CNN. American Morning March 6, 2008, television broadcast); Dr. Julie Gerberding, director of the Centers for Disease Control and Prevention, responded with the following: "Let me be very clear that the government has made absolutely no statement… indicating that vaccines are a cause of autism."
- The CDC maintains that Hannah Poling simply exhibits autistic features consistent with her encephalopathy. Many syndromes (e.g., Rett's syndrome, tuberous sclerosis, fragile X syndrome, and Down's syndrome) also include autistic features, but they are not considered classic autism. (Offit P. Vaccines and autism revisited—the Hannah Poling case. NEJM 2008;358(20):2089-2091)
Where We Stand Now
Several conditions will probably need to be met before the thimerosal question can be answered satisfactorily:
- Some “washout” period will be necessary to see if changes in diagnostic methodology are responsible for at least part of the recent rise in the incidence of autism.
- Similarly, if thimerosal contributes to autism, the incidence and prevalence of this disorder should peak and decline, since thimerosal is no longer found in childhood vaccinations.
- The specific causes of autism—which is a multi-factorial syndrome whose diagnosis is based on clinical observation—must be clarified.
Finally, criteria for compensation through the Vaccine Injury Compensation Program should be more strictly defined in order to identify those specific syndromes that can be caused by vaccinations.
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