Thimerosal as a Preservative in Vaccines

AU National Review

Preservatives are used in vaccines to prevent or kill any growth of bacteria, fungi, or other microorganisms that might have contaminated the vaccine vial. Microorganisms are able to get into the vials that are punctured more than once with a needle. Multiple punctures only occur in vaccine vials that are labeled as multi-dose vials and are intended to be used with different needles more than one time.

Due to tragic events in vaccine history, such as the infection of children with deadly bacteria after injection with multi-dose vaccines, the United States Code of Federal Regulations (the CFR) required preservatives be added to multi-dose vials of vaccines. Because of many incidences of infections due to vaccinations, preservatives have been required in multi-dose vaccines since the 1930’s.

Since then, thimerosal has been the most widely used preservative in multi-dose vaccines. It has a long record of safety and has been proven effective at preventing bacterial and fungal contamination of vaccines. However, over the past several years, thimerosal has become a target for controversy due to its mercury concentration and concern for safety.

Let’s talk thimerosal:

Yes, thimerosal contains mercury. It is about 50% mercury by weight. A vaccine composed of 0.01% thimerosal as a preservative contains approximately 25 micrograms of mercury per 0.5 mL dose.1

Mercury is an element that is found as inorganic mercury salt in the water, soil, plants, and animals. It occurs naturally in the Earth’s crust, air, soil, and water. Since the beginning of time, occurrences such as volcanic eruptions, rock weathering, and coal burning have all contributed to release of mercury into the environment. The mercury in the environment then gets changed by certain types of bacteria into a compound called methyl mercury. Methyl mercury accumulates in the aquatic food chain an organic form and can land in larger quantities on your plate in seafood. These organic forms of mercury are more easily absorbed and harder to eliminate from the body when ingested.1

At high levels, methyl mercury can be toxic to people. Methyl mercury is a neurotoxin and not the form of mercury found in thimerosal. The consumption of this form of mercury, such as through eating contaminated fish or exposure to methyl mercury fungicide, for example, have been associated with physical side effects. Many studies have been conducted on methyl mercury, of which the mercury health guidelines have been established from.

But not all forms of mercury are the same.

Thimerosal is a derivative of ethyl mercury. This tiny change in wordage relates to a hugely different toxicological profile. Studies comparing methyl and ethyl mercury suggest that these two types of mercury are processed differently in the body.Ethyl mercury has been shown to break down by the body and get excreted much more rapidly than methyl mercury, and therefore is much less likely to accumulate in the body and cause harm.

Thimerosal Toxicity:

Allergic reactions to thimerosal have been described in the clinical setting.1 Most of the reactions seen have been delayed hypersensitivity reactions such as redness and swelling at the injection site.4,5 The reactions tend to be mild and last only up to a few days. However, when an allergic reaction is seen in the clinical setting, it’s nearly impossible to tell whether the reaction was due to the thimerosal or to another component of the vaccine.1

One study published in 1931 involved the injection of a 1% thimerosal solution into subjects, with no reported side effects except for 2 cases of skin sloughing.6 As stated before, the thimerosal component in vaccines currently is 0.01%, ten-fold lower than the concentration used in this study.

Another study conducted in 1985 compared the toxicity of ethyl and methyl mercury in adult rats. After feeding the rats the mercury concentrations daily, the study found that the ethyl mercury, the mercury found in thimerosal, to be less neurotoxic than the methyl mercury.Still, guidelines for mercury safety are based off of methyl mercury.

Finally, a study conducted in 2002 may be of the most importance. Due to the concern that infants may lack the ability to elimate the mercury in vaccines, a study conducted at the University of Rochester and National Naval Medical Center in Bethesda, MD looked at the levels of mercury in the blood and other samples from infants who had received routine vaccinations containing thimerosal. They found that in every infant included in the stud,y the levels of mercury in the blood samples didn’t exceed the methyl mercury safety guidelines.8 Also, they found that mercury was cleared from the infants’ blood faster than predicted for methyl mercury, and that most of the mercury was excreted in the stool fairly quickly.8

The study suggests that thimerosal is removed much more rapidly from the body than methyl mercury. Currently, follow-up studies are being conducted, paid for by the National Institute of Allergy and Infectious Diseases (NIAID).

Thimerosal and neurodevelopmental disorders, such as autism:

Many studies have shown that there is no association between thimerosal exposure and autism.

(For a list of studies on thimerosal and autism, check out the CHOP website, here.)

In 2001, the Institute of Medicine (IOM) convened a committee to discuss and review vaccine safety-related issues. One review focused on the potential relationship between thimerosal in vaccines and neurodevelopmental disorders. They concluded that the evidence that thimerosal caused autism, attention deficit hyperactivity disorder (ADHD), and speech or language delay was not great enough to link the two.9

In 2004 the IOM issued its final report on thimerosal and autism. The committee concluded that there is no causal relationship between thimerosal containing-vaccines and autism.1 Also, they stated that the benefits of vaccination are tried and true and that “the hypothesis of susceptible populations is presently speculative, and that widespread rejection of vaccines would lead to increases in incidences of serious infectious diseases like measles, whooping cough and Hib bacterial meningitis”.1

However, the IOM committee decided that the removal of thimerosal from vaccines was “a prudent measure in support of the public health goal to reduce mercury exposure of infants and children as much as possible”.1

So, is thimerosal safe?

The three U.S. health agencies, The Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA), and the National Institutes of Health (NIH), have looked at all of the current research on thimerosal and have found it to be safe to use in vaccines.3

Also, the organizations, The National Academy of Sciences’ Institute of Medicine, the Advisory Committee on Immunization Practices (ACIP), and the American Academy of Pediatrics (AAP) have found thimerosal to be safe after reviewing the research.3

What vaccines still contain thimerosal?

Some of the influenza vaccines are produced in multi-dose vials, and therefore contain thimerosal.  The single-dose vials and the live attenuated flumist nasal spray do not contain thimerosal.

Several other vaccines do contain thimerosal. For a complete list of the vaccines containing thimerosal, check out the FDA’s webpage, here.

The removal of thimerosal from vaccines:

Since 2001, there have been no new vaccines containing thimerosal licensed by the FDA for use in children younger than six years of age.  It’s because of the increasing concerns by consumers that the Food and Drug Administration has begun to work with  vaccine manufacturers to eliminate or at least reduce the amount of thimerosal from  vaccines.1

All hepatitis B vaccines are available as thimerosal-free or trace-thimerosal-containing formulations currently in the US.

All immune globulin preparations, such as Rho(D) (given to pregnant women who are Rh-), are thimerosal-free.

In March 2001, the FDA approved a thimerosal-free DTaP vaccine.

In September 2001, there became a thimerosal-free version of the influenza vaccine.

Two Tetanus and diphtheria vaccines (Td), which are used in children greater than seven years old, are now available as thimerosal-free, or as trace-thimerosal-containing formulations.

And currently thimerosal has been removed from or reduced to merely trace amounts in all vaccines on the recommended vaccine schedule for children under the age of six. The only exception to this is the inactivated influenza vaccine, of which there is a preservative-free version available that only contains trace amounts of thimerosal. The preservative-free flu vaccine is however, in limited supply and only available for use in infants, children, and pregnant women.

Discussions are still continuing about removing thimerosal from ALL vaccines…


*Note: Vaccines that are considered trace-thimerosal-containing formulations have 1 microgram or less of mercury per dose. Full thimerosal vaccines have around 25 micrograms per dose, with the exception of inactivated influenza vaccine. 



  1. U.S. Food and Drug Administration (FDA). Department of Health and Human Services.
  2. Vaccine Education Center at The Children’s Hospital of Philadelphia (CHOP).
  3. The Centers for Disease Control and Prevention (CDC).
  4. Cox NH, Forsyth A. Thimerosal allergy and vaccination reactions. Contact Dermatitis 1988;18:229-233.
  5. Grabenstein JD. Immunologic necessities: diluents, adjuvants, and excipients. Hosp Pharm 1996; 31:1387-1401.
  6. Powell HM, Jamieson WA. Merthiolate as a Germicide. Am J Hyg 1931;13:296-310.
  7. Magos L, Brown AW, Sparrow S, Bailey E, Snowden RT, Skipp WR. The comparative toxicology of ethyl- and methylmercury. Arch Toxicol 1985,57:260-267.
  8. Pichichero ME et al. Mercury levels in premature and low birth weight newborn infants after receipt of thimerosal-containing vaccines. J Pediatr. 2009 Oct;155(4):495-9. Epub 2009 Jun 26.
  9. IOM (Institute of Medicine). Thimerosal-containing vaccines and neurodevelopmental disorders. Washington DC: National Academy Press; 2001.