The question of mercury in flu vaccines is common, and it is important to understand the specific type of mercury involved. A mercury-containing organic compound called Thimerosal has been historically used as a preservative in some vaccines. It remains present in certain multi-dose vials of flu vaccine, though many common flu vaccines today are available without it. The mercury in Thimerosal, known as ethylmercury, is distinct from methylmercury, the form commonly associated with environmental concerns and found in certain foods.
Understanding Thimerosal
Thimerosal, an ethylmercury-containing compound, has served as a preservative in some vaccines since the 1930s. Its purpose in multi-dose vaccine vials is to prevent the growth of harmful bacteria and fungi. When a vial is accessed multiple times to draw individual doses, there is a possibility of introducing microbes, and Thimerosal helps maintain the vaccine’s sterility and safety.
Ethylmercury, the form of mercury in Thimerosal, differs from methylmercury. Methylmercury is found in certain fish and can accumulate in the body, potentially causing toxicity. In contrast, ethylmercury is processed and eliminated from the body at a much faster rate. This rapid clearance means ethylmercury is less likely to build up and cause harm compared to methylmercury.
Flu Vaccine Formulations Today
Flu vaccine formulations have evolved, with many options now available that do not contain Thimerosal. Most single-dose flu vaccine syringes and nasal spray vaccines are manufactured without this preservative. These single-dose presentations inherently reduce the risk of contamination, eliminating the need for a preservative.
Despite the availability of Thimerosal-free options, Thimerosal is still included in some multi-dose vials of flu vaccine. Its use in these vials helps prevent contamination each time a dose is withdrawn. Multi-dose vials can also offer a cost-effective method for widespread immunization campaigns. A large percentage, approximately 94% or more, of the U.S. flu vaccine supply is now Thimerosal-free.
Contextualizing Mercury Exposure
Understanding the distinction between different forms of mercury is important when considering vaccine ingredients. Ethylmercury, derived from Thimerosal, is metabolized and cleared from the body more quickly than methylmercury.
The trace amounts of ethylmercury in some vaccines are minimal when compared to daily environmental exposures to methylmercury. For instance, a typical dose of a Thimerosal-containing flu vaccine, which contains about 25 micrograms of mercury, is roughly equivalent to the amount found in a 3-ounce can of tuna fish. This perspective highlights that vaccine-related mercury exposure is both of a less harmful form and in very small quantities compared to common dietary sources.
Ensuring Vaccine Safety
All vaccines, including flu vaccines, undergo rigorous safety testing and continuous monitoring. In the United States, the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) evaluate and oversee vaccine safety. This oversight includes assessment of all vaccine ingredients, such as Thimerosal.
Extensive research has consistently found no evidence of harm from low doses of Thimerosal in vaccines, beyond minor reactions like redness or swelling at the injection site. Numerous studies have shown no link between Thimerosal in vaccines and neurodevelopmental disorders or other serious health problems.