The CDC’s Advisory Committee on Immunization Practices is set to vote later this week on the issue of thimerosal in flu vaccines. What exactly is thimerosal and is it actually harmful for people that take vaccines containing the substance?
Thimerosal is a mercury-based organic preservative that historically was put in several vaccines in low quantities in order to prevent the growth of harmful bacteria and fungi, particularly when multi-dose vials were used. Vaccines can become accidentally contaminated, as may occur with multiple needle punctures with multi-dose vials.
Before the late 1990’s, infants were recommended to receive three vaccines that contained thimerosal- hepatitis B, Haemophilus influenzae type b and diphtheria-tetanus-acellular pertussis. There are no known health risks associated with thimerosal at the concentrations used in vaccines, according to the FDA.
Secretary of Health and Human Services Robert F. Kennedy Jr. disagrees. In his 2014 book, he states (as reported in Politico), “there is a virtually unanimous scientific consensus among the hundreds of research scientists who have published peer-reviewed articles in the field that Thimerosal is immensely toxic to brain tissue.”
Thimerosal, as a preservative gets metabolized as ethylmercury, which is distinct from the more toxic methylmercury. In large doses, both compounds can be toxic to the brain and kidney, resulting in tremors, memory loss, mood swings, depression, protein in the urine and kidney damage. However, ethylmercury poses a significantly decreased risk for humans because it has a much shorter half-life of less than week compared to methylmercury, which has a half-life of 1.5 months according to the World Health Organization. Ethylmercury is removed from the body fast and actively excreted into the gut, as oppose to methylmercury that can accumulate in the body and result in potential toxic effects.
In 1999, because of scientific uncertainty at the time and theoretical concerns about thimerosal, the preservative was removed from nearly all childhood vaccines as a precautionary measure per the FDA, not because it was shown to cause any harm. The only childhood vaccine that still contains thimerosal in some formulations is the flu vaccine.
Part of the reason thimerosal was removed from childhood vaccines was because an infant in the early 1990s receiving childhood vaccines could be exposed to a cumulative dose of mercury as high as 187.5 micrograms by the age of 6 months, which exceeded the guidelines put forth by the EPA, but not the WHO. Even at a dose of 187.5 micrograms of ethylmercury, no studies have shown adverse health effects or harm to humans, other than local skin reactions at the injection site of the vaccine, as shown in a study published in the journal Pediatrics.
Despite what Secretary of HHS Kennedy has suggested, thimerosal has not been shown to cause autism. In fact both the FDA and multiple peer-reviewed scientific studies have asserted and shown no link between thimerosal use in vaccines and autism.
The use of thimerosal in U.S. FDA licensed vaccines has significantly declined because of reformulations and the availability of vaccines in single-dose containers. Currently, a couple of flu vaccines are formulated to contain thimerosal, although the majority of flu vaccines that currently exist do not contain thimerosal.
The ACIP will convene later this week to hold a vote on the status of thimerosal on vaccines. Even if completely removed from the flu vaccine, the vaccine will not “become” safer, as decades of research has already shown thimerosal to be safe and effective as a preservative.
Experts warn that holding the vote could cast doubt on vaccine uptake. Dr. Jeremy Faust, Editor-in-Chief of MedPage Today, writes, “Elevating this debunked myth to national policy lends credence to misinformation, and sets the stage for other actions that may undermine vaccine confidence in the United States.”
