FULL CLAIM: Dogs are an ingredient of flu shots
REVIEW
A Facebook post from November 2024 claimed that dogs are an ingredient in the influenza vaccine, commonly known as the flu shot. The post also implied that this could pose a health risk, questioning the potential consequences of “mass innoculating [sic] a population with dog DNA.”
This claim bears similarities to previous assertions about vaccines containing human fetal tissues or DNA, which were later debunked by Science Feedback. By suggesting that vaccines are made with controversial or unnatural ingredients, these claims may be an attempt to deter people from vaccination. In this article, we’ll show why the claim about dogs is inaccurate and reflects a misunderstanding of vaccine manufacturing processes.
Most of the influenza vaccines distributed in the U.S. are inactivated vaccines. This means that they contain whole viruses that have been chemically “killed” but retain the ability to train our immune system and fight off the flu. Viruses can only replicate inside of cells. Therefore, the flu viruses used to make the vaccines first need to be mass-produced in cells.
Usually, manufacturers use fertilized chicken eggs to produce the viruses before inactivating them. This procedure, which has been used for decades, offers an effective way to replicate the viruses. However, this approach has limitations. Contrary to general belief, the risk of egg allergies isn’t one of them.
The first limitation is that egg-based production requires a large number of eggs and thus cannot be easily scaled up when the demand for vaccines suddenly increases.
Second, the virus might mutate while replicating in the egg. These genetic mutations improve the virus’ adaptation to the egg environment, but can make it less similar to the flu virus circulating in the human population. As a result, the vaccines may lose some effectiveness in protecting humans against the flu[1].
To circumvent the limitations of egg-based vaccines, manufacturers developed cell-based flu vaccines, where the viruses are replicated in cell lines—cells grown in a Petri dish— instead of eggs. The cell-based flu vaccine is commercialized under the name Flucelvax in the U.S. and Optaflu in other countries.
To make this vaccine, the flu virus is replicated in a cell line obtained from a dog, hence the post’s claim that dogs are used in flu vaccines. However, this doesn’t mean that dogs or dog cells are ingredients in the vaccine.
First, the cell line used, called MDCK (Madin Darby Canine Kidney), was harvested from a single Cocker Spaniel in 1958 and has been grown in laboratories since then. Therefore, the production of cell-based vaccines doesn’t require further, repeated harvesting of dog cells or tissues.
Second, these cells are only used to produce the virus and are removed during the manufacturing process, thus they don’t enter the final product. It’s similar to the fact that we use soil to grow maize, yet we don’t eat that soil when consuming corn cobs.
A document from the pharmaceutical company Merck illustrates the purification process of cell-based vaccines. After producing the virus, the cells are filtered out by centrifugation and filtration. Subsequently, steps of ultrafiltration/diafiltration and enzyme-based DNA degradation help remove the remaining cellular debris and DNA.
Therefore, the final vaccine product doesn’t contain MDCK cells. The vaccine package insert of Flucelvax/Optaflu specifies that the final vials only contain residual amounts proteins and DNA fragments from MDCK cells. A study that analyzed the manufacturing process of that vaccine reported an amount of MDCK DNA lower than 10 picograms per dose (one picogram is one hundred-billionth the size of a gram). For context, a 500g beef steak contains about one gram of DNA. ,The study also found that the risk of encountering an MDCK cell in a vaccine dose was 1 in 1034.
In summary, cell-based vaccines use cells of human or animal origin that were isolated decades ago. The vaccine manufacturing process contains several steps to remove cells, cell debris, and cell DNA. Thus, cell-based vaccines like the flu vaccine Flucelvax/Optaflu don’t contain human or animal tissues.
CORRECTION (20 Dec. 2024):
The term “cell debris” was removed from the penultimate paragraph, to acknowledge the fact that “cell debris” isn’t specifically defined and thus traces amounts of residual MDCK DNA or protein could be interpreted as cells debris.
REFERENCES
- 1 – Rockman et al. (2022) Cell-Based Manufacturing Technology Increases Antigenic Match of Influenza Vaccine and Results in Improved Effectiveness. Vaccines.
