SciCheck Digest

Influenza vaccines contain small amounts of various ingredients that allow them to work and keep them safe and long-lasting. A misleading meme suggestively lists more than two dozen substances it claims are in flu vaccines. But most are not present — and the ones that are aren’t dangerous.

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All vaccines contain tiny amounts of various ingredients, including the active ingredient, or antigen, which triggers the body to mount an immune response and produce antibodies. Some vaccines may also include stabilizers, adjuvants, preservatives and minuscule amounts of other ingredients used during their production. All of these components are safe in the concentrations used in vaccines, and many are either naturally present in our bodies or found in water, food or other products we ingest or use regularly. Manufacturers list the components of each vaccine on publicly available package inserts. 

Seasonal influenza, or flu, vaccines have a “good safety record,” according to the Centers for Disease Control and Prevention, and the benefits outweigh the risks. While the effectiveness of the shots varies each year, getting vaccinated is the best way to protect yourself and others from the flu.

There are different types of flu vaccines, with different production processes, doses and ways of administration, as we will explain. But all of them contain components of influenza viruses, which change every year to try to match the viruses that will be circulating during each season. All the 2023-2024 flu vaccines are quadrivalent, meaning they contain components of, and protect against, four influenza viruses, two influenza A viruses and two influenza B viruses. Just as with all vaccines, to be effective and to remain potent and free of contamination, some flu vaccines contain adjuvants, stabilizers, preservatives or other residual byproducts. 

But a viral meme that reads “FLU SHOT INGREDIENTS” lists many substances that are not present in any current flu vaccines, including “antifreeze,” “cow muscle tissue” and “monkey kidney cells.” When checking for the ingredients in databases compiled by the Johns Hopkins Bloomberg School of Public Health Institute for Vaccine Safety, we found that only a fraction of the 27 listed in the meme are present, in very small quantities, in at least one influenza vaccine.

An earlier version of the meme, which circulated and was fact-checked by others in 2020, said the flu shot includes “some of these” ingredients. The words “some of these” were deleted in this version. The image provides no source for the information.    

While the post doesn’t explicitly say the listed ingredients are dangerous, the implication is clear. “Demons and witches brew,” wrote an Instagram user when sharing it.

Most of the listed ingredients — while not necessarily in flu vaccines — are present in other vaccines, or are part of the manufacturing process. But these have not been shown to be harmful.

“There is no vaccine that includes everything on this list,” Dr. Kawsar R. Talaat, associate professor of international health at the Johns Hopkins Bloomberg School of Public Health, told us in a phone interview. “I think that they have gathered these ingredients and use different names for them in order to scare people as much as possible.”

A Misleading List

Two of the “ingredients” listed in the meme are “Human Embryonic (aborted baby) Lung Culture” and “Aborted Human Diploid Cell Cultures (WI-38 & MRC-5).” But as we’ve written, and as the Children’s Hospital of Philadelphia explains, vaccines do not contain fetal cells or tissues. Some vaccines use fetal cell lines to grow vaccine viruses. MRC-5 and WI-38 are two cell lines taken from lung tissue of two embryos aborted in the 1960s for the preparation of some vaccines. But none of the influenza vaccines available in the U.S. uses fetal cells for production. The viruses or viral proteins used to make flu vaccines are produced in chicken eggs or in non-human cell cultures. We’ll explain more of this process later.

Similarly, while tiny amounts of aluminum are present in other vaccines as an adjuvant to boost the immune response — and has been safely used for decades — it’s not in any flu vaccines.

Flu vaccines do not contain antifreeze either, as the meme incorrectly claims. The active ingredient of the antifreeze and coolant used in cars or vehicles is ethylene glycol, which is toxic, and isn’t used in any vaccine. Older formulations of the influenza vaccines used tiny amounts of polyethylene glycol, a product used in some cosmetics, skin conditioners and in laxative medicines used for constipation. Polyethylene glycol is nontoxic and very different from ethylene glycol. There is no polyethylene glycol in the current flu vaccines.

Anyone can check the Johns Hopkins database or consult the package inserts to see that none of the current flu vaccines contains ethanol, aluminum dye or aluminum, any type of acetone, “monkey kidney cells” or Vero cells, “fetal cow serum” or bovine serum, barium, E. coli, “cow muscle tissue,” “DNA from pig circoviruses (PCV)”, “embryonic guinea pig cells,” human albumin (a protein naturally made in the liver), “Mueller’s media (containing cow extracts),” or dextrose (a type of sugar), as the meme incorrectly claims. 

Some substances listed in the meme are present in some flu vaccines or other vaccines. But the quantities used are very small and safe.

“Because chemical names can be unfamiliar and relative quantities of exposure are often central to their safety (i.e., ‘the dose makes the poison’), it is easy for people to become confused, concerned, or downright scared when they see a list like this,” Charlotte A. Moser, co-director of CHOP’s Vaccine Education Center, told us in an email.  

Tiny amounts of certain detergents, for example, may be present in some flu vaccines, but these are better thought of as surfactants that keep the ingredients blended together.

“Detergent can also be surfactant, if you want to use that term, which are things that help keep things into solutions,” Talaat told us. “So the bread from the grocery store, Oreos, Twinkies, they all have surfactant or detergent to make it homogenous.”

“Most of the things that we use in the flu vaccine are things that are very common in other things that we encounter on a … daily basis,” Talaat added. They may sound scary when you put them in a list like this, she said, but they’re not. “The important thing is that flu vaccines, like the other vaccines that are available have been tested extensively, have been used in billions and billions of people over many years and are incredibly safe and protect us from the most severe influenza disease.”

What’s in the Flu Vaccines

As we said, the major components in the flu vaccines are the flu viruses or viral proteins used to prompt our bodies to make an immune response. 

Most flu vaccines contain whole inactivated viruses. One, the FluMist nasal spray vaccine, uses weakened viruses. Some others use surface proteins from the viruses. None of them can cause the flu, since the viruses are either dead or too weak to replicate effectively to make us sick. 

Vaccine manufacturers receive candidate vaccine viruses selected for a particular season and use different technologies to grow those viruses and make their vaccines. 

Most flu vaccines are made by inoculating and growing viruses in chicken eggs. The fluid containing the viruses is then harvested from the eggs, and the viruses are purified and killed or weakened. Egg-based flu vaccines (Afluria, Fluad, Fluarix, FluLaval, Fluzone and FluMist) may contain traces of egg proteins, but people with an egg allergy can still get any of these vaccines, based on recommendations from the CDC’s Advisory Committee on Immunization Practices. Previously, the ACIP recommended people with severe egg allergy get their vaccine in a medical office, but starting this season, those measures are no longer recommended. 

One of the flu vaccines (Flucelvax) is made by growing influenza viruses in cell culture, specifically Madin-Darby canine kidney, or MDCK, cells. The cells used to make this cell line were isolated in 1958 from the kidney of a cocker spaniel. To make the vaccine, manufacturers grow and collect the viruses, inactivate them, and then purify the antigens. Since the 2021-2022 season, this vaccine is egg-free. But it may contain very tiny amounts of MDCK cell DNA or protein.

There is also a recombinant flu vaccine (Flublok) made up of just influenza surface proteins. The vaccine uses recombinant DNA technology to add the genes for each flu strain’s surface proteins to a baculovirus, a virus that infects insects. The “recombinant” baculovirus is then put into a cell line — one derived from cells of the fall armyworm, an insect related to moths, caterpillars and butterflies — to produce the proteins. The proteins are then extracted and purified. This flu vaccine is egg-free, but it may contain residual amounts of fall armyworm and baculovirus DNA and proteins from the manufacturing process.

As we mentioned, the vaccines also may use stabilizers, preservatives or adjuvants so that they remain safe and effective. 

Stabilizers are used to keep the antigen stable during shipping and storage. One of the stabilizers used in the nasal spray vaccine is porcine, or pig, gelatin. It’s worth noting that we consume gelatin made from cow and pigs in numerous food products, from gummy candies or marshmallows to wine or mints. Although the vaccine only contains 2 milligrams per dose (for reference, a teaspoon is equivalent to 5,000 mg), people with a severe allergic reaction to gelatin should get a different flu vaccine, as recommended by the CDC. As CHOP’s Vaccine Education Center explains, people who have severe allergic reactions to foods containing gelatin should avoid vaccines that contain gelatin.  

Some vaccines also contain preservatives, added to prevent contamination. “Whereas stabilizers keep what’s supposed to be in the vial stable, preservatives keep what shouldn’t be in the vial out,” the CHOP website explains. This is important when a vial contains multiple doses because bacteria or other microbes could enter the vial and contaminate the contents.

Three of the flu vaccines (Afluria, Fluzone and Flucelvax) use the preservative thimerosal, which contains ethylmercury, in multidose vials. Ethylmercury is a kind of mercury that is safer than methylmercury, which can be present in some fish and animals and can be toxic at high levels. Ethylmercury is less dangerous because it’s broken down and excreted faster from the body than methylmercury. According to the Food and Drug Administration, the amount of mercury contained in a vaccine is roughly the same amount contained in a 3 ounce can of tuna fish. The small amounts of thimerosal in vaccines have been shown not to be harmful. There is no link between thimerosal in vaccines and autism, as we’ve explained.

Vaccines may also contain some leftover products from the manufacturing process. As explained by CHOP’s Vaccine Education Center, since the ingredients of the vaccine come from biological systems, it’s sometimes impossible to completely remove substances used during the process. Although most of the time the amounts that remain in the vials are insignificant, the substances still need to be listed. 

Formaldehyde, which is mentioned in the meme, is one of them. This organic compound is used in vaccines to inactivate viruses, so they don’t replicate and make us sick. Formaldehyde is also produced naturally in our bodies, where it has a role in producing amino acids, and is also present in the environment and in food we eat. According to the FDA, the amount of formaldehyde “present in some vaccines is so small compared to the concentration that occurs naturally in the body that it does not pose a safety concern.”

The agency says that, for example, in a newborn baby of 6 to 8 pounds, the amount of formaldehyde in their bodies is 50 to 70 times higher than the maximum amount they could receive from a singe dose of a vaccine or from vaccines administered over time. Repeated exposure to formaldehyde, mostly through breathing air containing the compound, is associated with cancer, but there is no evidence linking cancer to the small amounts of formaldehyde in some vaccines. 

Some flu vaccines (Afluria, Flublok and Fluzone) may include chloride, or salt. Salts, often based on sodium and potassium, keep the pH balance and keep the active ingredients suspended, according to the University of Oxford’s Vaccine Knowledge Project. “These are common and harmless,” the organization says. 

People with concerns about specific ingredients can ask their doctors and choose which of the nine flu vaccines offered this season is the best for them. The CDC says everyone 6 months and older should get a flu vaccine every season, with specific recommendations for each age group.

Editor’s note: SciCheck’s articles providing accurate health information and correcting health misinformation are made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over FactCheck.org’s editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation.

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