The presence of mercury in the seasonal influenza vaccine, or flu shot, is a public concern stemming from the compound Thimerosal. Thimerosal contains mercury and has been used as a preservative in some vaccine formulations for decades. This article provides factual information regarding Thimerosal’s presence, the amount involved, and the chemical context necessary to understand its safety profile.
The Role of Thimerosal as a Vaccine Preservative
Thimerosal is an organomercurial compound used since the 1930s as a preservative. Its primary purpose in vaccines is to prevent the growth of harmful bacteria and fungi that could contaminate the vaccine. This contamination is a specific risk for multi-dose vials, which are accessed multiple times to draw out individual doses.
Thimerosal acts as a safeguard against microbial growth after the vial is opened, ensuring the sterility of the remaining doses over time. This function is important for large-scale immunization efforts. Improvements in manufacturing have significantly reduced the need for preservatives, leading to a decline in Thimerosal use across many vaccine types.
Quantifying Mercury Content in Flu Vaccines
The amount of mercury in a flu shot depends on the type of vaccine presentation used. A standard dose from a multi-dose vial, which is preserved with Thimerosal, contains approximately 25 micrograms of mercury. This figure represents the mercury component of Thimerosal.
Single-dose syringes, which account for the majority of the current U.S. flu vaccine supply, generally do not contain Thimerosal as a preservative. These presentations may contain only trace amounts of mercury, typically less than one microgram, due to residuals from the manufacturing process. The 25-microgram amount found in a multi-dose vial is comparable to the methylmercury found in a three-ounce serving of certain types of tuna fish.
Ethylmercury Versus Environmental Methylmercury
The chemical distinction between the mercury in vaccines and environmental mercury is central to understanding the safety profile. Thimerosal breaks down into ethylmercury, the type of mercury present after vaccination. This is chemically different from methylmercury, the organic compound found in the environment and accumulated in fish.
Methylmercury is associated with neurotoxicity and accumulates in the human body, possessing a half-life of about 70 days. Conversely, ethylmercury is metabolized and excreted much more quickly. Studies show that ethylmercury clears from the blood with a short half-life, estimated to be between three and seven days in adults.
This rapid clearance makes ethylmercury less likely to accumulate to harmful levels compared to environmental methylmercury. Their different toxicokinetic profiles mean that safety assessments based on methylmercury are not applicable to the ethylmercury in vaccines. Public health organizations have consistently determined that the small amount of ethylmercury in some vaccines poses no health risk.
Are There Thimerosal-Free Flu Vaccine Options?
Several Thimerosal-free options are widely available for individuals who prefer to avoid the preservative entirely. The vast majority of flu vaccines administered in the U.S. are already Thimerosal-free, supplied in single-dose vials or pre-filled syringes. These containers do not require a preservative because they are only accessed once.
The nasal spray flu vaccine, a live attenuated influenza vaccine, is also Thimerosal-free for eligible individuals between the ages of two and 49 years. Additionally, nearly all vaccines routinely recommended for children six years of age and younger are available in Thimerosal-free formulations.