Ingredients in bird flu vaccine Audenz are safe in the amounts present; no evidence that they’re toxic or cause cancer

Several Facebook posts shared on 1 January 2025 claimed that the antiviral medication Audenz contains ingredients including MDCK cell protein and MDCK DNA (“canine kidney DNA”), thimerosal, polysorbate 80, and beta-propiolactone. It further claimed that some of these ingredients are toxic and carcinogenic (cancer-causing), and in the case of polysorbate 80, that it increases the risk of Crohn’s disease.

Audenz is the U.S. brand name of the vaccine used to prevent infection caused by influenza A virus H5N1, more commonly known as avian influenza or “bird flu”. It was first approved for use by the U.S. Food and Drug Administration (FDA) in 2020.

To determine the veracity of the above claims, Science Feedback reviewed the FDA’s package insert for Audenz, which includes information about the drug’s intended usage, ingredients, contraindications, and safety data. We also reviewed safe exposure limits set by health agencies for the ingredients listed to determine their potential toxicity or carcinogenicity. We discuss each ingredient in greater detail below.

Thimerosal

Thimerosal is an organic compound sometimes used as a preservative in vaccines to prevent the growth of bacteria and fungi in multi-dose vials. It was widely used in vaccines from the 1930s until the late 1990s, when the FDA decreased its use due in part to the “reformulation of vaccines and the introduction of single-dose containers”, and in part as a precautionary measure due to public concern about its safety.

The FDA’s package insert for Audenz states that “5 mL multi-dose vials contain thimerosal, a mercury derivative, added as a preservative. Each 0.5 mL dose from the multi-dose vial contains 25 mcg [micrograms] mercury.”

This amount of mercury falls well below safe exposure limits recommended by various health and safety agencies for infants under six months of age based on a low measure of average body weight (fifth percentile), as outlined in Figure 1 below:

Figure 1 – Safety exposure limits for thimerosal, as recommended by the U.S. Environmental Protection Agency (EPA), the U.S. Agency for Toxic Substances and Disease Registry (ATSDR), and the U.S. Food and Drug Administration (FDA). ug = micrograms, mg = milligrams. Source: Children’s Hospital of Philadelphia.

For context, the World Health Organization (WHO)’s measure of the fifth percentile of body weight is between 6 and 6.6 kg at six months of age. Thus, even the most conservative estimate of the safe exposure limit of 65 mcg by the EPA is well above the amount of mercury present in a single dose of Audenz. The 25 mcg of mercury in a 0.5mL dose of Audenz would therefore fall within safe limits for adults and children above six months given their substantially larger body weights.

For further context, misinformation regarding thimerosal’s alleged harm has circulated since the late 1990s in large part due to a 1998 study by Andrew Wakefield, which falsely claimed that vaccines containing thimerosal increased risk of autism. The journal that published the paper has since retracted the study and numerous scientific studies have found no credible evidence supporting an association between thimerosal and autism^[1,2,3].

It’s also important to note that there are differences between ethylmercury (the type of mercury found in thimerosal), and methylmercury, given that the harms of methylmercury are sometimes inaccurately associated with ethylmercury. Methylmercury is more toxic and accumulates more easily in living organisms compared to ethylmercury. The FDA also shared that they’re “totally different materials”:

“Methylmercury is formed in the environment when mercury metal is present. If this material is found in the body, it is usually the result of eating some types of fish or other food. High amounts of methylmercury can harm the nervous system. This has been found in studies of some populations that have long-term exposure to methylmercury in foods at levels that are far higher than the U.S. population. […] Ethylmercury is formed when the body breaks down thimerosal. Low-level ethylmercury exposures from vaccines are very different from long-term methylmercury exposures because ethylmercury is broken down by the body differently and clears out of the blood more quickly.”

Polysorbate 80

Polysorbate 80 is a synthetic compound used as an emulsifier and stabilizer in various products, including vaccines, pharmaceuticals, food, and cosmetics. It helps to keep ingredients evenly mixed and improves the solubility and stability of certain substances.

According to its FDA package insert, Audenz contains 1.175 mg polysorbate 80 as part of the MF59C.1 adjuvant, and may include a residual amount of ≤ 0.375 mg polysorbate 80, for a total of 1.55 mg at most.

In a 2023 report, the European Medicines Agency (EMA) recommended a safety limit of 3 mg of polysorbate 80 per kilogram of body weight per day based on an evaluation of preclinical and clinical data regarding potential health risks associated with polysorbate 80. Their report noted that the “polysorbate dose of 1.175 mg/vaccine dose is equivalent to about 0.02 mg/kg for a 60 kg person”, well below the recommended limit of 3mg/kg/day.

The Facebook posts also claimed that polysorbate 80 is carcinogenic and increases the risk of Crohn’s disease. However, polysorbate 80 isn’t listed on the International Agency for Cancer Research (IARC)’s list of agents classified as carcinogenic to humans.

There is limited evidence that suggests polysorbate 80, when used as a food additive, may cause inflammation or bacterial growth associated with Crohn’s disease^[4,5]. However, we found no evidence confirming causal association between polysorbate 80 and increased risk of Crohn’s disease.^#

Beta-propiolactone (BPL)

Beta-propiolactone (β-Propiolactone, or BPL) is a chemical compound used as a disinfectant and sterilizing agent in vaccine production. It’s primarily employed to inactivate viruses or bacteria in vaccines, ensuring that they’re safe for use without losing their ability to trigger an immune response.

The Facebook posts’ claim that BPL is carcinogenic was partly accurate. The International Agency for Research on Cancer (IARC) classifies BPL as a Group 2B possible carcinogen, meaning there is limited evidence for its carcinogenicity in humans.

However, Audenz contains ≤ 0.1 mcg of BPL, used to inactivate virus cells. It’s worth noting that because BPL can be highly toxic, its safety limits depend on the context in which it’s used. The Occupational Safety and Health Administration, a division of the U.S. Department of Labor, published a recommended BPL limit of 0.5 parts per million (ppm), or 500 mcg. The amount of BPL in Audenz falls below this limit.

MDCK cell protein and DNA (“canine kidney DNA”)

MDCK (Madin-Darby Canine Kidney) cells are a cell line originally derived from the kidney of a female Cocker Spaniel in the 1950s. These cells have the unique ability to grow and replicate in laboratory conditions, making them a useful tool for scientific research and biotechnology, especially in the production of vaccines. MDCK cells can cultivate viruses in large quantities, which helps to ensure high yields of viral material for vaccine production.

As Science Feedback explained in a previous claim review, these MDCK cells are removed during the vaccine manufacturing process, and thus aren’t present in the final vaccine product, with the occasional exception of residual proteins. In Audenz, the possibility of these residual proteins was noted by its FDA package insert, which said that each dose of vaccine may include less than 3.15 micrograms of MDCK cell protein and ≤ 10 nanograms of MDCK cell DNA.

The Children’s Hospital of Philadelphia explained why DNA from growth in cells isn’t harmful:

“Because DNA is not stable when exposed to certain chemicals, much of it is destroyed in the process of making the vaccine. Therefore, the amount of DNA in the final vaccine preparation is minimal (billionths or trillionths of a gram) and highly fragmented. Because the DNA is fragmented, it cannot possibly create a whole protein that could be harmful. […] DNA from the vaccine is not able to incorporate itself into cellular DNA. In fact, if this could be accomplished, gene therapy would be much easier than it has been.”

Conclusion

The claims made in Facebook posts about the ingredients in Audenz, including MDCK cell protein, MDCK DNA, thimerosal, polysorbate 80, and beta-propiolactone, misrepresent the actual safety profile of the vaccine. The amounts of these substances in Audenz are well below established safety limits, and there is no credible evidence linking them to harmful health effects such as carcinogenicity or an increased risk of Crohn’s disease at the amounts present.

 

# PubMed search: (polysorbate 80[Title/Abstract]) AND (Crohn’s disease[Title/Abstract])

 

REFERENCES

• 1 – Taylor et al. (2014) Vaccines are not associated with autism: An evidence-based meta-analysis of case-control and cohort studies. Vaccine.
• 2 – Madsen et al. (2002) A Population-Based Study of Measles, Mumps, and Rubella Vaccination and Autism. The New England Journal of Medicine.
• 3 – Uno et al. (2012) The combined measles, mumps, and rubella vaccines and the total number of vaccines are not associated with development of autism spectrum disorder: The first case–control study in Asia. Vaccine.
• 4 – Katsoudas et al. (2024) Dietary Emulsifier Exposure in People With Inflammatory Bowel Disease Compared With Healthy Controls: Is There a Cause for Concern?. Inflammatory Bowel Diseases.
• 5 – Sandall et al. (2020) Emulsifiers Impact Colonic Length in Mice and Emulsifier Restriction is Feasible in People with Crohn’s Disease. Nutrients.