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SciCheck’s COVID-19/Vaccination Project

Still No Evidence COVID-19 Vaccination Increases Cancer Risk, Despite Posts

This article is available in both English and Español

SciCheck Digest

It has not been shown that COVID-19 vaccines cause or accelerate cancer. Yet opponents of the vaccines say a new review article “has found that COVID-19 mRNA vaccines could aid cancer development.” The review conclusions are mainly based on the misinterpretation of a study on mRNA cancer vaccines in mice.

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Clinical trials, involving thousands of people, and multiple studies have shown that the mRNA COVID-19 vaccines from Pfizer/BioNTech and Moderna are safe. Hundreds of millions of doses have been administered under close monitoring systems that have found serious side effects are rare. Studies have also shown that the vaccines work very well in preventing severe COVID-19 disease and death, saving millions of lives across the globe. 

There is no evidence to support a link between COVID-19 vaccines and cancer, as we’ve reported. Both the National Cancer Institute and the American Cancer Society have stated there’s no information that suggests COVID-19 vaccines cause cancer, make it more aggressive or lead to recurrence of cancer. 

Yet, vaccine opponents falsely claim a review article published in April proves the contrary. 

“BREAKING: A review in the International Journal of Biological Macromolecules has found that COVID-19 mRNA vaccines could aid cancer development,” reads an April 16 Facebook post by America’s Frontline Doctors, a group that has repeatedly spread misinformation about the pandemic and whose founder was sentenced to 60 days in prison for entering the U.S. Capitol during the Jan. 6 riot.

A separate Facebook post used a Gateway Pundit headline to falsely claim, “Confirmed: Researchers Reveal COVID mRNA Vaccines Contain Component that Suppresses Immune Response and Stimulates Cancer Growth.” Similarly, a post on X said the new “study … confirms what some medical experts have been suspecting for 18 months: The COVID mRNA shots containing N1-methyl-pseudouridine SUPPRESS the immune system and STIMULATE cancer growth!” 

Messenger RNA, or mRNA, vaccines work by instructing a small number of a person’s cells to make specific proteins, which then prompt the body to mount an immune response. N1-methylpseudouridine is a modification naturally found in some RNA molecules that’s attached to mRNA in vaccines to allow it to deliver its message to the cell without being destroyed by an innate immune response, as we will explain. 

Experts told us the review paper, which is based on other published articles and does not contain original research, misleads by misinterpreting several studies and the role of N1-methylpseudouridine in vaccines. The authors also refer to an unreliable review article, written by authors known for spreading misinformation, that falsely claimed the mRNA COVID-19 vaccines impair the immune system and increase the risk of cancer, as we have explained before.

One of the most important misrepresentations, and one that the authors heavily rely on, is based on the findings of a study on mRNA cancer vaccines in mice. The study looked at the efficacy of mRNA cancer vaccines with different degrees of N1-methylpseudouridine modification in a mouse melanoma model. According to the review, the study found that “adding 100% of N1-methyl-pseudouridine (m1Ψ) to the mRNA vaccine in a melanoma model stimulated cancer growth and metastasis, while non-modified mRNA vaccines induced opposite results, thus suggesting that COVID-19 mRNA vaccines could aid cancer development.”

But that’s not what the study found. 

“[O]ur results did not show, suggest or indicate that modified mRNA promotes tumor growth/metastasis,” Tanapat Palaga, professor of microbiology at the Chulalongkorn University in Thailand and the corresponding author of that study, told us in an email.

What the study actually showed is that both unmodified mRNA and modified mRNA induced immune responses against the tumor antigens, but only the unmodified mRNA reduced cancer growth and metastasis, while the modified mRNA didn’t. The study was published in 2022 and co-authored by Drew Weissman, who won the 2023 Nobel Prize with Katalin Karikó for discovering this mRNA modification that eventually led to the mRNA COVID-19 vaccines.

Dr. James A. Hoxie, an emeritus professor of medicine at the University of Pennsylvania and co-director of the Penn Institute of RNA Innovation (directed by Weissman), told us those findings are relevant for scientists who are studying ways in which mRNA cancer vaccines can elicit immune responses needed to prevent or delay cancer progression. (See “Social Media Posts Misinterpret Biden on mRNA Cancer Vaccines” for more information about mRNA cancer vaccines.) 

“But that is a far cry from saying that the vaccine that was used to prevent COVID-19 disease causes cancer,” he said. Implying that by regulating the innate immune system, which is something scientists working in immunotherapies are trying to understand, “you’re leaving yourself open for cancer risk —  that is ludicrous.”

“I believe that the authors of this review article intentionally or [unintentionally] misinterpret our results and tried to twist the conclusion to support their agenda,” Palaga told us.

There are no studies supporting a link between N1-methylpseudouridine and cancer in animals or mice, experts told us.

There is also no evidence mRNA COVID-19 vaccines impair, much less suppress, the immune system, as we’ve reported. In fact, the vaccines enhance immunity by teaching the immune system how to identify and fight the coronavirus.

N1-methylpseudouridine and Its Role in mRNA Vaccines

To understand the role of N1-methylpseudouridine we have to look back at the history of mRNA vaccines. 

Normally, when a cell encounters a foreign RNA, a molecule present in most living organisms and viruses, it activates a strong innate immune response against the molecule. 

Daniel Jędzura / stock.adobe.com

This was a problem for scientists trying to use mRNA as a therapeutic, since the goal was for the cell to receive the instructions carried by the mRNA and produce certain proteins. Until the mid-2000s, Karikó, Weissman and others observed that if they attached certain chemical modifications found in some kinds of natural RNA molecules, such as pseudouridine, into one of the four bases of mRNA, they could blunt that innate immune response and, at the same time, increase the mRNA’s capacity to translate its code for the cell to make the desired proteins. 

Later, scientists found N1-methylpseudouridine, another modification naturally found in some kinds of RNA molecules, worked better than pseudouridine.

The modification is not “suppressing” the immune system, Hoxie told us — it just allows for certain parts of the immune system not to activate temporarily “in order to get the desired effect.”

Jordan L. Meier, senior investigator at the National Cancer Institute who has studied the role of N1-methylpseudouridine in COVID-19 vaccines, told us the authors of the review paper misrepresent what N1-methylpseudouridine, which is abbreviated as m1Ψ, does. 

The review “incorrectly” confuses “m1Y’s ability to hide from the immune system with an ability to weaken or disable it,” he told us in an email.

To explain it, Meier compared the mRNA modification to a spy using a disguise in order to pass security guards. 

“The authors are essentially suggesting that the disguise somehow makes the guards less able to do their jobs going forward,” he wrote. “In reality, once the disguised person is through, the guards remain just as vigilant and capable as before.”

The review, he added, doesn’t provide evidence that N1-methylpseudouridine “leaves the immune system any worse off for future threats.” 

Misrepresented Studies in the Review Paper

Similarly, the review misleads by cherry-picking or misrepresenting figures and tables of this and other papers. 

For example, in the study by Palaga, Weissman and others using a mouse melanoma model (in which malignant cells from a tumor are given to a mouse), scientists found that relative to mice that received no vaccine (and instead received a saline solution) no increase in tumor growth or decrease in survival occurred when animals were vaccinated with a modified mRNA vaccine. However, when animals received a vaccine containing unmodified mRNA, the study showed a decrease in tumor growth and an increase in survival compared with the control group that received the saline solution. In other words, the study found that the unmodified mRNA generated immune responses that decreased tumor growth and improved survival, while, similar to the control group, the modified mRNA had no effect on the tumor.

Table 1 of the review, however, incorrectly says the study found that the modified mRNA vaccine “increases tumor growth” and “decreases survival.” 

“This is simply not true and is a gross misrepresentation of the data that paper actually shows. The modified RNA had no effect on the tumor, and results using that vaccine were the same as using a saline solution,” Hoxie told us.

The tumor growth in mice receiving the modified mRNA was “increased relative to the unmodified vaccine, but it was identical to when there was no intervention,” Hoxie told us. “Animals that received the modified mRNA vaccine died at the same rate and with the same amount of tumor as did animals that received the saline solution. The fact tumor progression in this model was reduced with the unmodified mRNA vaccine is the key point of this paper and indicated that in this model immune responses to unmodified mRNA may have anti-tumor activity, an important finding for the cancer immunotherapy field.”

The review also refers to a study that has been extensively misinterpreted to falsely claim that the Pfizer/BioNTech mRNA COVID-19 vaccine causes what vaccine opponents called “turbo cancer.” The study describes one mouse that died from a lymphoma after 14 mice were given a high dose of the vaccine. The review paper reproduces images from the study that show dissected mice and compares the organs of the mouse that died with one with a normal anatomy. 

As we explained, and as the authors of that paper noted in an addendum, there is no such thing as “turbo cancer,” and, more importantly, the case report does not demonstrate a causal relationship between the lymphoma and the vaccine.  

Meier told us the review also wrongly refers to a study published in 2016 to support its thesis that modified mRNA vaccines turn off an immune sensor known as RIG-I. 

“In reality, this study only showed m1Y mRNAs are unable to activate RIG-I and did not test inhibition. In other words, what was shown was that m1Y is a strong camouflage, not that it is an immune suppressor,” he wrote.  

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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