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Small amounts of DNA from the manufacturing process may remain in the mRNA COVID-19 vaccines. Purification and quality control steps ensure any leftover DNA is present within regulatory limits. There isn’t reason to think that this residual DNA would alter a person’s DNA or cause cancer, contrary to claims made online.
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The COVID-19 vaccines made by Pfizer/BioNTech and Moderna are produced with help from DNA templates, which include instructions for making the mRNA that encodes the spike protein. Manufacturers take steps to purify the final vaccine components, cutting up and removing the DNA, although there could be a very small amount of DNA left.
Past research and mechanistic logic indicate that any DNA remaining after these purification and quality control steps is likely inconsequential. However, in recent months unsubstantiated theories have spread online that DNA remaining in mRNA vaccines could integrate into a person’s own DNA and cause cancer, or even that the vaccines are already causing cancer.
A spokesperson from the U.S. Food and Drug Administration told us in an email that “no safety concerns related to residual DNA have been identified.” The spokesperson added that “with regard to the mRNA vaccines, while concerns have been raised previously as theoretical issues, available scientific evidence supports the conclusion that the minute amounts of residual DNA do not cause cancer or changes to a person’s genetic code.”
A spokesperson for the European Medicines Agency — which helps regulate medical products in the European Union — told us via email that the agency “can confirm that we have not seen any reliable evidence of residual DNA exceeding approved/safe levels” for the Pfizer/BioNTech or Moderna COVID-19 vaccines. Nor is the EMA “aware of scientific evidence showing that the very small amounts of residual DNA that may be present in vaccine batches could integrate into the DNA of vaccinated individuals,” the spokesperson continued.
Various experts also told us that it is unlikely that residual DNA in the vaccines could integrate into DNA or cause cancer, even in theory. And as we have previously written, there isn’t evidence to date that the vaccines cause cancer or have led to an increase in cancer.
Marc Veldhoen, an immunologist at the Instituto de Medicina Molecular João Lobo Antunes in Portugal, told us via email that residual DNA would be expected, but he refuted the idea that it could cause cancer. “Yes, there would be some fragments, but within the limit this is allowed and without any clinical consequence,” he said.
This family of claims was originally inspired by a preprint posted in April, which said there was “DNA contamination that exceeds” the EMA and FDA regulatory limits in Moderna and Pfizer/BioNTech vaccine vials sent anonymously to the authors in the mail without cold packs. This led to other reports of DNA in mRNA vaccine vials, including a second preprint that analyzed largely expired vaccine vials obtained at pharmacies in Canada. None of this work has been published in peer-reviewed journals, and many elements of it have been criticized.
We reached out to Kevin McKernan, an author on both preprints, to better understand his views. Rather than replying to our email, he posted a screenshot of it on X, formerly known as Twitter, and included responses there. McKernan, who has an undergraduate degree in biology, is the founder of Medicinal Genomics, a company that markets test kits and genomics-related services to the cannabis, hemp and mushroom industries.
Some of the alleged concern has focused on the possibility, raised in the original preprint, that some of the residual DNA in the Pfizer/BioNTech vaccine is from a monkey virus called SV40. The EMA confirmed to us that the plasmid, or DNA template, used to make the Pfizer/BioNTech vaccine contains some short sections of DNA from this virus. A Pfizer spokesperson also told us via email that “specific, non-infectious parts of the SV40 sequence, which are commonly used in the pharmaceutical industry are present in starting material used by Pfizer and BioNTech.”
But none of the sequences identified in the preprint are known to cause cancer, contrary to recent social media posts that say “SV40, a cancer causing sequence” was “put in the Covid Vaccine.”
Experts say there isn’t reason to think that any small pieces of leftover DNA, including SV40 DNA, in the vaccines would be harmful.
“It is very unlikely that any residual DNA would integrate into a person’s genome and if it did it would be even much less likely to cause cancer,” Barry Milavetz, a molecular biologist who studies SV40 at the University of North Dakota, told us in an email.
South Carolina Senate Committee Meeting Amplifies DNA Claims
Reports of residual DNA in the mRNA COVID-19 vaccines and its purported dangers spread further after a Sept. 12 South Carolina Senate committee listening session. One speaker, molecular biologist and cancer geneticist Phillip Buckhaults from the University of South Carolina, shared his own findings that DNA pieces were present in leftover vaccine in the bottom of used Pfizer/BioNTech vials.
In his presentation, which was shared widely online, he said that DNA “can and likely will” integrate into the genomes of people’s cells, and he shared concerns about various potential health impacts, including cancer. As we’ve said, other experts and regulatory agencies disagree that residual DNA is likely to integrate into a person’s own DNA.
“It was surprising to me to see any DNA in this product, and I am a bit concerned about the theoretical possibility of genome modification,” Buckhaults told us in an email. “I want the scientific community to help find out if this is a real hazard or not a problem.”
He also said that he did not intend for his comments “to be widely circulated in the public and compromising people’s confidence in vaccines.”
Another widely posted clip from the listening session was of Janci Lindsay, who runs a toxicology consulting firm and has a history of sharing incorrect information about vaccines and COVID-19. She also spoke about unsubstantiated cancer risks and told the lawmakers that she believes the SV40 DNA sequences were included in the vaccines with “nefarious intent.” The idea that the presence of these sequences is nefarious is a conspiracy theory with no basis in reality.
Lindsay goes on to reference hydroxychloroquine and ivermectin, falsely concluding, “We never needed these vaccines. We had treatments that worked.” This is incorrect. The COVID-19 vaccines saved many lives, and randomized controlled trials have shown that hydroxychloroquine and ivermectin do not help people recover from COVID-19.
FactCheck.org obtained a copy of an Oct. 16 letter sent to the Senate committee by Pfizer. In the letter, Pfizer disagrees with comments made during the session, saying that statements are incorrect that “the vaccine contains plasmid DNA that could potentially impact a person’s DNA and be a theoretical cancer risk.” The letter continues, “There is no evidence to support these claims and they provide the risk of being misconstrued by either Committee members and/or the public at large.”
The letter also states that “no signs of DNA mutation or COVID-19 vaccine-induced cancer have been reported to date” related to the Pfizer/BioNTech COVID-19 vaccine.
DNA in Vaccines Is Not Inherently Dangerous
Research into residual DNA in vaccines dates back decades. Anti-vaccine fear-mongering about residual DNA or other substances in vaccines is also not a new phenomenon.
Many currently available vaccines are made using cells. Some vaccines, such as the one against chickenpox, rely on weakened virus that is grown in cells. For other vaccines, such as for hepatitis A, viruses are grown in cell culture and then inactivated. Cells also can be used to produce protein-based vaccines. One example is the COVID-19 vaccine from Novavax, which is grown in moth cells.
In all of these cases, the active ingredients for the vaccines are purified, but the vaccines can still contain small amounts of residual DNA from the cells used to make them. The FDA and other regulatory agencies have offered guidance on limiting the quantity and size of residual DNA left over from cells used to make vaccines.
The limits are based on the theoretical concern that residual DNA — specifically from mammalian cell lines — could cause cancer or a viral infection, particularly if there were a cancer-causing gene or certain viral DNA present in the cell line. But Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, told us that regulatory limits on residual DNA in vaccines are set conservatively.
Pfizer’s letter to the South Carolina Senate committee refers to a quality control process that ensures that residual DNA levels in its mRNA vaccine for COVID-19 are within regulatory limits.
“The validated method for assessment of residual DNA has shown that the Pfizer-BioNTech COVID-19 vaccine meets the requirements of the World Health Organization (WHO) and the FDA for biological products,” the letter states. “Vaccine batches are only certified and released if the criteria, during quality control testing, are met using the validated and approved method.”
The EMA spokesperson added that in the European Union, these results must be checked by an independent laboratory. “As a result, we are confident that the DNA levels in the vaccine are consistently below the approved/safe level,” the spokesperson said.
A spokesperson from the Therapeutic Goods Administration, which regulates medical products in Australia, told us that the agency has been monitoring batches of Moderna and Pfizer/BioNTech mRNA COVID-19 vaccines. “This includes independent testing performed by the TGA laboratories to confirm that residual DNA impurity levels are below the acceptable limit,” the spokesperson told us in an email. “To date all batches of COVID-19 vaccines supplied in Australia have met all quality specifications.”
Research on experimental DNA vaccines, which contain DNA as their active ingredient, also supports the idea that DNA in vaccines is unlikely to integrate into a person’s DNA. Stephen M. Kaminsky, a professor of research in genetic medicine at Weill Cornell Medical College, told us via email that “there is little concern of integration from DNA vaccines that are delivered in much greater quantities” than any residual DNA that might be found in one of the mRNA vaccines for COVID-19.
“Since amounts of DNA vaccines in the milligram range have been approved for clinical evaluation, it is difficult to imagine that the smaller quantities of residual cell-substrate DNA present in viral vaccines would pose a significant risk due to integration,” FDA scientists also concluded in one paper.
The FDA scientists went on to state that they consider the primary cancer-related concern with DNA in vaccines to be the introduction of an activated version of a cancer-causing gene to a cell — not just any DNA integrating into the genome at the wrong place.
Offit added that we are constantly exposed to DNA, including in the food we eat and from viruses that don’t cause cancer.
Cancer Theory Relies on Improbable Sequence of Events
Experts told us that theories for how residual DNA would cause cancer rely on an entire series of events, many of them unlikely.
As we’ve discussed above, changing a person’s DNA is not easy. The residual DNA would first need to get into a cell. This could happen if the DNA was inside one of the fatty bubbles called lipid nanoparticles used to package the mRNA in the vaccines, Veldhoen, the immunologist in Portugal, said. But even if this happened, the DNA would only end up in the cytoplasm, the region of a cell outside the nucleus.
Next, any residual DNA that made it into a cell would need to get access to a person’s DNA in the nucleus and insert itself. In general, a cell needs to be in the process of dividing for foreign DNA to integrate into the cell’s own DNA.
The mRNA vaccines are injected into the muscles, where the bulk of the vaccine remains. Muscle cells “do not divide rapidly and have lots of cytoplasm compared to the size of their nuclei,” Milavetz, the molecular biologist at the University of North Dakota, said. This means that it is “very unlikely” that any residual DNA from a vaccine introduced to the cytoplasm of a cell will make it into the nucleus and insert itself into the DNA there in the first place, he added.
“Even if it enters the nucleus, which it probably can’t, it would still have to be integrated into DNA, which requires an integrase, which it also doesn’t have,” Offit said. An integrase is an enzyme some viruses use to insert themselves into cellular DNA.
In the event that some residual DNA did manage to insert into a person’s DNA, it would need to be exactly the wrong kind of DNA, land in exactly the wrong place or a combination of the two.
And then, if this entire sequence of events occurred in one of a person’s trillions of cells, the cell would need to avoid destruction by the immune system, divide and give rise to other cells, which would need to continue along the path toward becoming cancerous.
In reality, the immune system can detect when cells take up foreign DNA or mRNA, Veldhoen said. In the end, cells that had taken up residual DNA would not survive, he said, and the DNA bits would be “broken down, its individual parts recycled.”
SV40 Sequence Unlikely to Cause Cancer
As we’ve said, social media posts misleadingly refer to the presence of “SV40, a cancer causing sequence.” This brings to mind past concerns, which were not borne out, that contamination of polio vaccines with the entire SV40 virus could cause cancer. Researchers discovered in 1960 that monkey kidney cells that had been used to produce some polio vaccines were contaminated with SV40, which was found to cause cancer in rodents. But the virus has not been shown to cause cancer in humans, and the contamination did not ultimately lead to more cancer in children who received the contaminated vaccines compared with those who didn’t.
The small amount of SV40 DNA in the DNA template for the Pfizer/BioNTech vaccine does not encode the entire virus. SV40 “is a naturally occurring virus and the virus itself is not included in either starting materials, plasmid DNA, or in the final product of the Pfizer-BioNTech COVID-19 vaccine,” the Pfizer spokesperson said.
McKernan’s original preprint did not indicate the presence of the whole virus or any DNA encoding viral proteins, but rather highlighted regulatory DNA. Regulatory DNA, including a type of sequence called a promoter, helps control which genes in a cell are turned on.
Milavetz said that the portion of SV40 shown to have the potential for causing cancer in the lab — encoding a protein called T-antigen — is not among the sequences McKernan identified in the vaccine.
It is unclear why the Pfizer/BioNTech DNA template would include SV40 regulatory DNA. The EMA told us that “the sequence is not directly relevant” for producing copies of the DNA template or for producing mRNA for the vaccine, “so it is considered to be a non-functional part of the structure of the source plasmid.”
McKernan has suggested that a piece of SV40 regulatory DNA could cause cancer by integrating into a person’s DNA and turning on a cancer-causing gene. In response to criticisms that it’s difficult for DNA to get into the nucleus, McKernan points to research showing a role for part of that sequence in helping to bring DNA into the cell nucleus.
But it’s hardly clear that any nuclear entry mechanism would be at play in human cells exposed to residual DNA fragments. And as we have previously explained, there are multiple reasons why residual DNA is unlikely to integrate into a person’s DNA.
“Fragments of the SV40 sequence may only be present as residual impurities at very low levels that are routinely controlled,” the EMA spokesperson said. “There is no scientific evidence that any of these SV40 fragments can act as insertional mutagens,” the spokesperson said, meaning there is no evidence the fragments would integrate into a person’s DNA.
Buckhaults, who also found SV40 regulatory DNA in Pfizer/BioNTech vaccine vials, told us the bits of SV40 DNA aren’t “any more dangerous than all the other bits” of DNA he found in the vaccine vials.
Milavetz pointed out the improbability of the SV40 regulatory sequence causing cancer, even if it did somehow integrate into a person’s DNA.
He said that any residual DNA present would be unlikely to contain only the SV40 sequence needed to turn on a gene. There would likely be extra chunks of DNA that would prevent it from functioning.
“For this to be a viable problem only critical portions of the promoter would have to be introduced into the regulatory region of only a very small subset of genes in a human in a very specific way,” he said. “In my opinion there are too many things that would have to occur perfectly for the promoter to be integrated into one of these critical human genes.”
‘No Meaningful Difference’ from Manufacturing Process Change
Various posts also reference a change in the DNA template used to produce the Pfizer/BioNTech vaccine between the clinical trials and the rollout of the vaccine to the general public. To make the vaccine supply that was primarily used in the clinical trials, manufacturers produced copies of the DNA template using a process called PCR, in which DNA is amplified in a lab without the help of biological organisms. To help scale up production, manufacturers enlisted bacteria to make many copies of a plasmid, a circular piece of DNA. The bacteria divide rapidly and can make large quantities of DNA.
Based on this process change, social media posts have said that the “Pfizer covid vaccine approved the for emergency use was not the same one used on the public!” or posted the “BREAKING” news that “Pfizer’s COVID vaccine that was approved for emergency use was not the same one they injected into billions of arms.”
To be clear, the fact there was a process change has long been publicly available information. It is mentioned in the Pfizer clinical trial protocol, the emergency use authorization from the FDA and an EMA public assessment report first published in December 2020. The EMA spokesperson confirmed that vaccine batches produced by both processes were tested in clinical studies, adding that the manufacturer provided test results and other information to show the comparability of the product resulting from both processes. “This assessment of comparability confirmed there was no meaningful difference in the quality of material from process 1 and process 2 that could impact the safety and/or efficacy of the vaccine,” the EMA spokesperson said.
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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