There are no mouse brains in Covid-19 vaccines

9 March 2021
What was claimed

The XMRV virus is in vaccines.

Our verdict

There is no evidence that this virus is present in any vaccines or that it is linked to disease in humans.

What was claimed

Vaccines contain the SV40 virus.

Our verdict

There was some concern in the 1960s that some viruses were contaminated with SV40. No vaccine has been found to contain it since 1963, and there’s no evidence it caused cancer as some claimed.

What was claimed

There is human blood in vaccines.

Our verdict

Some vaccines use an ingredient called human serum albumin, which is derived from human blood. But the blood from donors is screened before use to prevent any contamination.

What was claimed

Vaccines cause ME/CFS.

Our verdict

Getting a vaccine may exacerbate ME/CFS symptoms in people who already have these illnesses, but there is no data from clinical trials on this.

This article was published before the MHRA announced a possible link between the AstraZeneca vaccine and a very rare type of blood clot, which can be fatal. It was correct at the time of writing. Our latest fact checks about Covid-19 and the pandemic can be found here.

A post on Facebook makes a number of claims about the ingredients in vaccines and the reactions to those ingredients. It’s not clear whether the post is talking about vaccines in general or one of the Covid-19 vaccines, so where we have information, we will discuss both.

The post lists 16 ingredients and 17 reactions.

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Neither the Pfizer/BioNTech or Oxford/AstraZeneca vaccines (which have both been approved for use in the UK) contain aluminium. Some vaccines given in the UK, for example the Meningitis B and HPV vaccines do contain tiny amounts of aluminium salts, which strengthen the immune response. There is no evidence that this aluminium builds up in the body or does any harm.


There is no mercury in either of the Covid vaccines, either in the form of ethyl mercury or thiomersal (a mercury-based chemical). Most commonly given vaccines in the UK don’t contain any form of mercury anymore, though there’s no evidence it causes harm in the amount contained in vaccines.


This refers to a cell line called MRC-5, which is a line of cells that were originally taken from an aborted foetus in 1966. These cells can be grown and further duplicated in a lab and are used for research.

AstraZeneca told Full Fact that MRC-5 cells were not used in development of its Covid-19 vaccine, though they were used to test the vaccine early on. But during manufacturing of the vaccine,a different cell strain, called HEK 293 is used, which was derived from human kidney cells of an aborted foetus. 

When the post refers to “aborted babies” being in the vaccine, that is not the case. Cells from both foetuses have been replicated over the decades, creating the HEK 293 and MRC-5 cell strains. So the actual cells used in the manufacture of this vaccine are not directly from a foetus. 

While human-derived cells are used to manufacture the vaccine, they are filtered out of the final product.

There are some vaccines used in the UK that do use these and other cell lines in production but the vaccines are purified so that no human material is likely to be in the final product.

Mouse brains

There are no mouse brains in any of the UK-approved Covid-19 vaccines. Mouse brains have been used to produce a vaccine against Japanese encephalitis. 

According to the World Health Organisation: “Most inactivated mouse brain-derived products have been discontinued. In a few countries there is continued production for domestic supply, although in general these are being phased out.”

XMRV Virus

XMRV stands for xenotropic murine leukemia virus-related virus. In the late 2000s, it was reported that this ‘new virus’ was found in some people with prostate cancer and chronic fatigue syndrome. However, many subsequent studies have not always been able to confirm this and many of the initial findings have been retracted

The initial findings led to a theory that vaccines grown in rodent cells could have been how XMRV entered humans, and therefore caused these diseases, which was never proven. Testing has not found these viruses in a number of vaccines.

It’s since been proven that XMRV is not associated with human disease for a number of reasons, including that the initial results were actually the result of tumour samples being contaminated with XMRV in the lab. 

Monkey kidneys and SV40 virus

Monkey kidneys cells are used to grow the contents of some oral polio vaccines. However, the vaccines are purified before injection so that none of the kidney cells make it to the final vaccine.

There was concern that one polio vaccine administered in the 1960s was contaminated with a virus called SV40 which was present in these kidney cells, and that SV40 causes cancer. However, no SV40 has been found in vaccines since 1963, and people given the vaccine before 1963 were not more likely to get cancer. Therefore, there is no evidence that SV40 in vaccines caused cancer.


There may be trace amounts of formaldehyde in some vaccines, including the Hepatitis B vaccine, where it’s used during production to kill the virus. However, the amount that could theoretically end up in a vaccine is not high enough to be dangerous.

The Vaccine Knowledge Project at the University of Oxford says the human body produces and uses formaldehyde. It also adds that the amount of natural formaldehyde in a two month-old’s blood is around ten times greater than the amount found in any vaccine. 


The Oxford-AstraZeneca vaccine does contain genetically modified organisms. The vaccine is a genetically modified harmless chimpanzee virus, which causes the common cold in chimpanzees. The virus has been genetically modified to contain the genetic sequence of a surface spike protein on the Covid-19-causing coronavirus, and also to prevent it replicating  in the human body. 

When the vaccine enters cells inside the body, it uses this genetic sequence to produce the surface spike protein which allows the immune system to recognise and attack the coronavirus if it later infects the person.

Polysorbate 80

The Oxford-AstraZeneca vaccine does contain the emulsifier polysorbate 80. It is there to hold the ingredients together and is only present in tiny amounts. 

Polysorbate 80 is a common food additive, and vaccines contain far less of it than any food does.

Human blood

There is no human blood in the Covid-19 vaccines being used in the UK.

Some vaccines use an ingredient called human serum albumin, which is derived from human blood. But the blood from donors is screened before use and manufactured in such a way to prevent any contamination. No viral diseases have ever been associated with the use of human serum albumin.


Monosodium glutamate, or MSG, is a flavouring that is safe to use in food. It’s used in small amounts in some vaccines to stabilise them, and protect against things like temperature change. 

There’s no MSG in the Covid-19 vaccines used in the UK.


There is mRNA (messenger RNA) in the Pfizer-BioNTech and Moderna Covid-19 vaccines. 

These vaccines both work by delivering some genetic code (mRNA) of a specific protein on the virus surface to human cells. The body can then make this protein, so the immune system can recognise it later if infected and launch a successful immune response.

This introduction of mRNA into human cells does not change the DNA of the human cells and if these cells replicate, the mRNA would not be incorporated into the new cells’ genetic information.


It’s not clear what exactly the post is referring to here—nanotechnology refers to the science of things between 1 and 100 nanometres in size. Nanoparticles of this size are often used in medicine

Lipid nanoparticles are used in both the Pfizer and Moderna Covid-19 vaccines (which have both been approved for use in the UK) to safely deliver the mRNA to cells. We’ve seen claims that these nanoparticles can have negative effects on the reproductive system, but that’s not the case.


Most of these diseases or reactions listed in the Facebook post have no links to any of the Covid-19 vaccines. No vaccine is totally without risk, but if someone does have a side effect from a Covid-19 vaccine, it’s likely to be tenderness at the injection site, a headache or generally mild flu-like symptoms. If you do have more severe symptoms, you are concerned or your symptoms get worse, you should call NHS 111.


There’s no evidence that vaccines cause Alzheimers. There were claims that aluminium, sometimes present in tiny amounts in vaccines, could be linked to the disease. But there’s no evidence that this is the case.

As we’ve said, there are no aluminium salts in either the Pfizer or AstraZeneca vaccines.


There is categorically no evidence that any vaccines cause autism. The research that initially suggested a link has now been discredited. Multiple analyses looking at over a million children have found no connection between the two.

Cancer, leukemia or tumours

As we’ve written above, claims about increased cancer from vaccines because of SV40 or XMRV were retracted or later disproved. There’s no credible evidence to suggest that any vaccines cause cancer.

Chronic fatigue syndrome

As above, studies connecting vaccines and chronic fatigue syndrome (otherwise known as myalgic encephalomyelitis or ME) have since been retracted.

There’s a possibility that people who already have ME or chronic fatigue syndrome (CFS)  could experience a relapse after getting the Covid-19 vaccination, as reported anecdotally by the ME Association. 

It says: “As with any other vaccine, it is possible that the COVID-19 vaccines will also cause a temporary exacerbation of flu-like ME/CFS symptoms.

“And there is also a possibility that they could trigger a more significant exacerbation of ME/CFS symptoms. At present, we have no data from the clinical trials as to what happens to people with ME/CFS with any of these vaccines”.

Allergies, Anaphylaxis

According to the Vaccine Knowledge Project: “As with any medicine or food, there is a very small chance of a severe allergic reaction (anaphylaxis) to the ingredients contained in vaccines. 

“Anaphylaxis is different from less severe allergic reactions because it causes life-threatening breathing and/or circulation problems. It is always extremely serious but can be treated with adrenaline. Health care workers who give vaccines know how to do this.”

The Vaccine Knowledge Project says that, in the UK between 1997 and 2003, there were a total of 130 reports of anaphylaxis following all immunisations. During the same period, 117 million doses of vaccines were administered. This means that the overall rate of anaphylaxis is around 1 in 900,000.

It’s true that after the Pfizer-BioNTech vaccine was rolled out in the UK, two people had anaphylaxis in reaction to an ingredient in the vaccine, though both already had severe allergies. Because of this, the Covid-19 vaccines are not given to people who have previously had allergic reactions to either their first dose, or an ingredient in the vaccine.

Autoimmune disease, diabetes and arthritis

Autoimmune diseases include things like multiple sclerosis, rheumatoid arthritis, and some other types of arthritis, which can sometimes be triggered by infections, where the immune system attacks the body. No vaccine has been consistently shown to cause autoimmune diseases.

There is no credible evidence that vaccines cause diabetes. Rumours have persisted that diabetes was linked to the MMR vaccine used in the US, because it is mentioned as a possible adverse reaction on the accompanying leaflet for doctors.

This doesn’t mean this vaccine has ever caused diabetes, just that a case of diabetes was reported after someone was vaccinated. There is no evidence linking the two.

We’ve written more about this here.

Encephalitis or Myocarditis

These describe inflammation of the brain and heart respectively, caused by an infection or a problem with the immune system.

Encephalitis has very occasionally been observed as an adverse reaction following a vaccine. This means it occured after someone received the vaccine, but does not always mean it was related to getting the vaccine. 

The Encephalitis Society says “While vaccines are much safer than actually having the disease, they, like all medicines can carry a small risk of adverse reaction. 

“The majority of adverse reactions are very mild, but can, very rarely include post-vaccination Encephalitis. The risk of developing vaccine-related Encephalitis is extremely small in comparison to the health risks associated with the diseases that vaccines prevent.”

The MHRA has monitored a number of cases of both myocarditis and pericarditis following vaccination. At the time that this article was updated (August 2021), it noted "a recent increase in reporting of these events in particular with the Pfizer/BioNTech and Moderna vaccines, with a consistent pattern of cases occurring more frequently in young males and shortly after the second dose of the vaccines. 

“These reports are extremely rare, and the events are typically mild with individuals usually recovering within a short time with standard treatment and rest."

Death or holocaust

Any approved vaccine has gone through numerous animal and human studies to make sure it is both safe and effective. The Pfizer, AstraZeneca and Moderna vaccines that have been given temporary authorisation in the UK have gone through many stages of testing in animals and people that have proved this.

The results of these studies then have to be peer reviewed, and approved by government agencies. If people died straight after being given the vaccine, or started suffering any kind of disease, this would be quickly picked up by researchers, and use of the vaccine would be paused or the study would be stopped straight away. This is what the Medicine and Healthcare products Regulatory Agency’s Yellow Card Scheme is for. 

Anyone can report a suspected side effect for any medicine, including vaccines, and this information is used to make sure that the safety of vaccines is monitored even after its roll-out.

Update 17 August 2021

This article was updated to reflect new information about vaccine safety, which appeared after it was published.

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