The list of ingredients for most vaccines is proprietary information – this means vaccine companies are not required to disclose their ingredients to the consumer. Therefore, even if the consumer, or the consumer’s pet, has a reaction to a vaccine, the vaccine the company is not required to divulge the ingredients of the vaccine.

One purpose of wanting to know the full ingredient listing is to identify the allergenic cause for that particular pet. As there are multiple brands of vaccines on the market, it might be possible to select a different vaccine, with different ingredients, and thereby avoid an allergic reaction in the animal receiving the vaccine, if the information were available.

Most often, it is the rabies vaccine that is associated with reactions in dogs and cats. A typical reaction is swelling at the site of the vaccination. One concern for pet owners is that repeated vaccinations will worsen the allergic response, potentially leading to dangerous anaphylactic reactions. An anaphylactic reaction could actually lead to death of the vaccine allergic pet.

In the United States, there are four commonly administered rabies vaccines for dogs and cats.

Manufacturer Brand Name Ingredients
Boehinger Ingelheim Rabvac Rabies virus (killed), proprietary adjuvant
Zoetis Defensor Rabies virus (killed), Aluminum hydroxide, Gentamicin, Merthiolate (as mercury)
Merial Imrab Rabies virus (inactivated), +/- Thimerosal
Merck Nobivac Rabies virus (inactivated), Aluminum phosphate, Thiomersal

Merial makes another rabies vaccine specifically for cats, Purevax. Unlike other vaccines, its ingredients are specifically listed: sterile water, recombinant rabies vaccine in a Canarypox vector.

The majority of rabies vaccines contain mercury additives. These additives go by different names –Thimerosal, Thiomersal, Merthiolate, and others. Merthiolate tincture, used by our parents (or grandparents), was used as a topical disinfectant or antibacterial for wounds, scrapes, and cuts. In the late 1990s, Merthiolate and Mercurochrome were removed from the market in the US because they contained mercury. Mercury can be dangerous – it is a poison.

However, since 1927, Merthiolate, more commonly called Thimerosal, has been used as a preservative and/or antiseptic in vaccines. Through an interesting paradox, topically applied healthcare products containing mercury are considered dangerous; whereas, vaccines containing injectable mercury are considered safe.

The relationship between pet health, the use of vaccines, and Thimerosal is complicated. Scott-Moncrief et al. published a study documenting that use of vaccines increased dogs’ risk of anti-thyroid antibodies; dogs receiving rabies vaccine were more affected than those receiving only distemper combination-type vaccines.[1] In another study, Bellabarba et al. found T4, circulating thyroid hormone, was unable to bind to serum protein; serum protein binding transports T4 to every cell in the body for use at the cellular level.[2] Hypothyroidism it the most common endocrine disease of dogs.

Hypothyroidism is one disease associated with vaccinosis in pets. (Dr Becker has a great discussion of vaccinosis here.)

To reduce the effects of vaccinosis, researchers involved with the Rabies Challenge Fund have thus far demonstrated a minimum of 5 year duration of immunity to rabies after vaccination.

Until federal law accepts longer duration of immunity for rabies vaccines, pet owners and veterinarians should use mercury-free vaccines.

After all this discussion, we are left with the question of what is in a vaccine. While we don’t know specific ingredients in an individual vaccine, as that information is proprietary, we do know some vaccines contain some, or all, of the following:

  • “[V]ery small amounts of the culture material used to grow the virus or bacteria used in the vaccine, such as chicken egg protein…
  • Aluminum gels or salts of aluminum which are added as adjuvants to help the vaccine stimulate a better response. Adjuvants help promote an earlier, more potent response, and more persistent immune response to the vaccine.
  • Antibiotics which are added to some vaccines to prevent the growth of germs (bacteria) during production and storage of the vaccine. No vaccine produced in the United States contains penicillin.
  • Egg protein is found in influenza and yellow fever vaccines, which are prepared using chicken eggs. Ordinarily, persons who are able to eat eggs or egg products safely can receive these vaccines.
  • Formaldehyde is used to inactivate bacterial products for toxoid vaccines, (these are vaccines that use an inactive bacterial toxin to produce immunity.) It is also used to kill unwanted viruses and bacteria that might contaminate the vaccine during production. Most formaldehyde is removed from the vaccine before it is packaged.
  • Monosodium glutamate (MSG) and 2-phenoxy-ethanol which are used as stabilizers in a few vaccines to help the vaccine remain unchanged when the vaccine is exposed to heat, light, acidity, or humidity.
  • Thimerosal is a mercury-containing preservative that is added to vials of vaccine that contain more than one dose to prevent contamination and growth of potentially harmful bacteria. (”

Mercury Fun Facts:

  • Mercury was used in the US hattery industry until 1941; mercury caused Mad Hatter’s disease.
  • Mercury was used in oral thermometers but has since been replaced with an (almost) inert substance for our safety.
  • Mercury continues to be a component of silver amalgam dental fillings.
  • Hooker et al. documents malfeasance in CDC reports of Thimerosal safety.[3] The implication in humans is that the exponential increase in autism rates is linked to MMR (Measles, mumps, rubella) vaccine and potentially the Thimerosal adjuvant contained in said vaccine. (For more on this see
  1. Scott-Moncrieff, J.C., et al., Evaluation of antithyroglobulin antibodies after routine vaccination in pet and research dogs. Journal of the American Veterinary Medical Association, 2002. 221(4): p. 515-521.
  2. Bellabarba, D. and R. Tremblay, Effect of Sodium Ethylmercurithiosalicylate (Thimerosal) on Serum Binding of Thyroid Hormones. Canadian Journal of Physiology and Pharmacology, 1973. 51(2): p. 156-159.
  3. Hooker, B., et al., Methodological Issues and Evidence of Malfeasance in Research Purporting to Show Thimerosal in Vaccines Is Safe. BioMed Research International, 2014. 2014: p. 8.

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