A presentation by scientists on a work group for the Centers for Disease Control and Prevention’s vaccine advisory panel highlighted various alleged “safety uncertainties” of the mRNA COVID-19 vaccines, including those about cancer and changes to the immune system. Scientists, however, told us that many of the cited studies are either misconstrued or of poor quality.
As with all medical products, the mRNA COVID-19 vaccines are not 100% safe. But the vast majority of people experience only temporary and mild side effects, which are expected. Serious side effects can occur but are rare.
In its Sept. 19 meeting, the Advisory Committee on Immunization Practices ultimately voted to recommend the COVID-19 vaccines for all ages 6 months and up as long as the decision is discussed with a health care provider. The group, however, nearly voted to require everyone to get a prescription for the shots and also endorsed unfounded or misleading claims about the risks of the vaccines. Health and Human Services Secretary Robert F. Kennedy Jr., who previously led an anti-vaccine organization, replaced the panel in an unprecedented move and installed new members, many of whom either lack expertise in vaccines or have spread inaccurate information about vaccines.
During one part of the meeting, Dr. Wafik El-Deiry, director of Brown University’s cancer center, and Charlotte Kuperwasser, a breast cancer biologist at Tufts University, gave a presentation as members of a newly reconstituted COVID-19 vaccine work group (neither are ACIP members). The presentation focused on several more technical or theoretical alleged safety concerns of the vaccines. Scientists, however, told us that the concerns are based on misinterpretations of studies or on flawed science and were presented without the proper context.
“The slide deck is an example of an anti-science ideology in which facts, evidence, and data no longer matter, but are instead abused: cherry-picked, misinterpreted, overinterpreted, or taken out of context to fit a predetermined narrative, regardless of how it conflicts with reality and the evidence obtained through the scientific method,” Marc Veldhoen, an immunologist at the Gulbenkian Institute for Molecular Medicine in Portugal who addresses scientific misinformation online, told us in an email about the presentation.
While many of the alleged “safety uncertainties” are familiar to fact-checking organizations, including those about DNA contamination and related claims about cancer that have circulated for years on social media, they have not before been given legitimacy at an ACIP meeting. Since the meeting, unfounded claims about the safety of the COVID-19 vaccines have continued to spread online.
In a commentary published in Vaccine about the meeting, former ACIP members criticized the process, content and outcome of the meeting, noting that there was an inappropriate “focus on theoretical vaccine harms using low-quality data” and that “standard methodological practices were not followed.”
According to the CDC, work groups “review relevant published and unpublished data and develop recommendation options for presentation” to ACIP. They must include at least two ACIP members and typically have involved government members, including from CDC, as well as outside experts and medical group liaisons. Under Kennedy, however, direct CDC involvement in the group has declined, and liaisons have been barred. One of the former CDC leads for the COVID-19 vaccine work group resigned in May following changes Kennedy made to the agency.
In a lawsuit that was updated on Nov. 5, the American Academy of Pediatrics, the American College of Physicians and the Infectious Diseases Society of America, among other medical and public health societies, called for the court to dissolve the current ACIP panel and void its decisions. Among many issues, the complaint cited “material inaccuracies, including an allegation that the Covid-19 vaccine resulted in ‘DNA contamination'” presented by the new COVID-19 vaccine work group.
We sent questions to El-Deiry and Kuperwasser about scientists’ concerns regarding their presentation. Kuperwasser replied with detailed answers, defending the presentation and telling us that she was aware that other scientists disagreed, but that in her view there are “gaps in knowledge that need to be addressed” and that numerous publications support her claims. On Nov. 5, Kuperwasser made some of the same statements in a piece she republished about COVID-19 vaccination and cancer for the Brownstone Institute, a think tank founded to oppose COVID-19 restrictions.
Claims of ‘Impurities’ Based on Flawed Analyses
Much of the presentation was focused on the idea that there could be safety issues, including cancer, that stem from leftover DNA in the mRNA vaccines. As we’ve explained before, a small amount of residual DNA is normal and expected in the vaccines, and there’s no reason to think it is causing cancer. Some reports have claimed to identify excessive amounts of DNA in the vaccines, but scientists have disputed those findings and the way the measurements were done.
Other vaccines made using cells also can contain small amounts of residual DNA.
The presenters cited eight references that they said reported identifying “DNA impurities” in the vaccines. Many of these, however, have not been peer-reviewed or were published in journals that are not indexed on PubMed — a minimal requirement to indicate some reliability. One is not even a paper, but rather testimony before a state legislature, which we’ve previously addressed.
One credible reference, a paper published in Vaccine in March 2025, specifically debunks one of the cited published papers that alleged DNA contamination. It shows in detail why the methods used would overestimate the amount of DNA. Journal editors added an expression of concern in June to the paper that alleged contamination, which was not noted by the presenters.
The reference the presenters relied on the most — including citing its figures claiming DNA levels in the vaccines 36 to 627 times the regulatory limit — is a paper published in the journal Autoimmunity on Sept. 6. Co-written by an individual with a history of spreading incorrect information about vaccines, the paper is by its own description “an expanded version of a previous preprint” that we and another fact-checking organization previously wrote about. Like its predecessor, the published paper has been criticized by other scientists and is being investigated, according to a pop-up message on the journal’s website and Retraction Watch.
As before, the Autoimmunity paper’s claimed high levels of DNA come from measurements using a particular method that uses fluorescent dyes, which is not the standard method regulatory agencies use to evaluate residual DNA. As the Vaccine paper shows, the method is not reliable if the sample is not prepared properly, as the high levels of mRNA and lipids in the vaccine can lead to erroneously high DNA measurements. In addition, many of the tested vaccine vials were expired, and some had been previously opened (see table 1).
Rolf Marschalek, a molecular biologist who studies cancer mechanisms at Goethe University Frankfurt and the senior author of the Vaccine paper, said the Autoimmunity paper should not have been published.
“To be very frank: all these studies have used methods that do not allow [for] … correct measurement” of DNA, he told us in an email. He also said that another paper, currently unpublished but under review at the journal npj Vaccines, did a similar analysis as his group and did not find any vaccine samples with too much residual DNA.
Those authors, from the Slovak Academy of Sciences, used four different methods to measure the amount of DNA in the mRNA vaccines. According to their paper, their results “clearly demonstrate that the quantity of residual DNA in all analysed vaccine batches is below approved limits, in very low quantities relative to mRNA, and degraded into small fragments.”
Kuperwasser said that the DNA in the mRNA vaccines remains a concern, noting that the manufacturers, the Food and Drug Administration and others “all acknowledge that residual DNA impurities are present in mRNA vaccines.”
“We simply do not know the fate of the DNA impurities in the mRNA vaccine products when they come into contact with cells,” she told us, emphasizing that the DNA is present within lipid nanoparticles. “Therefore, no claims can be made that this DNA does not pose a health risk. It is complete speculation to assume that this cannot and is not the case. Said differently, no studies have yet shown that these impurities are too minimal to enter cells, integrate into DNA, or cause any adverse reaction” (emphasis is hers).
Regulatory agencies from around the world have repeatedly said that concerns about DNA contamination of the mRNA vaccines are misplaced, and that testing with established methods has not revealed concerning levels of DNA.
The notion that residual DNA would be a problem is theoretical, as there’s no evidence of harm. It’s also highly unlikely that it poses a risk, since there are multiple layers of protection that prevent outside DNA from altering DNA in a cell or otherwise causing harm, as we explained in a 2023 article.
The Annenberg Public Policy Center of the University of Pennsylvania, our parent organization, and the Vaccine Education Center at the Children’s Hospital of Philadelphia created a short video this year explaining how cells defend against foreign DNA fragments. (We provided some input, based on our reporting, in the early stages of that project.)
As Veldhoen told us, “these vaccines have undergone extensive testing, and the results clearly demonstrate their safety and efficacy, with minimal risk of side effects. This evidence remains robust and will not change retrospectively.”
“While it is important to discuss potential risks of new products or technologies,” researchers with the Paul Ehrlich Institute in Germany noted in a commentary associated with the Vaccine paper, “it must be ensured that such discussions are based on solid data generated on the basis of the accepted rules of good scientific practice.” They, too, found in their own testing that vaccine vials did not contain more residual DNA than expected.
IgG4 Antibodies
The presenters also pointed to a 2023 Science Immunology paper by a team in Germany to suggest that the COVID-19 vaccines could lead to concerning changes to the immune system.
The paper is one of several that have found that of the spike-specific antibodies people produce after vaccination, people who have been repeatedly vaccinated with an mRNA COVID-19 vaccine tend to have more of a particular type of antibody called IgG4.
Because IgG4 is a type of antibody that in some respects might fight an infection less well than other antibody types, this has been misinterpreted by those with a history of sharing incorrect or misleading information to mean that COVID-19 vaccination makes people more susceptible to infections or even to cancer. Scientists say there isn’t evidence that the increase in spike-specific IgG4 makes any difference clinically, as repeated COVID-19 vaccination continues to protect against severe disease.
Dr. Kilian Schober, an immunologist at Friedrich-Alexander University and one of the co-senior authors of the Science Immunology paper, told us in an email that his results “do not raise any safety concerns” and are “primarily an ‘interesting immunological observation.’”
During the presentation, however, the work group presenters cast the findings as a potential problem.
In citing the Science Immunology paper, Kuperwasser called the IgG4 findings “one of the best documented” and said the “changes were linked to reduced antibody effector function, consistent with IgG4’s role in promoting tumor immune tolerance.”
Later, El-Deiry attempted to link the IgG4 phenomenon to cancer, citing a study of 96 pancreatic cancer patients — something he later emphasized on X.
“The IgG4 mechanism suppresses the immune system after multiple doses of mRNA vaccines,” he also said on X in late September, while defending a study that had just been published purporting to find an association between the vaccines and cancer. (That study, from South Korea, has methodological flaws and is inconsistent with data showing no uptick in cancer cases in the country, Science Feedback and Full Fact have explained. There is a notice on the paper saying it is under investigation by journal editors after readers raised concerns.)
Dr. Robert Malone, one of Kennedy’s new vaccine panelists and a known spreader of inaccurate information about vaccines, also characterized the IgG4 findings as “[i]mmune suppression” on X.
Experts told us that these interpretations are flawed.
“IgG4 is innocuous. In general, it is a ‘good thing’ and reduces inflammation and dampens allergic responses. It does NOT suppress the immune system,” Dr. Shiv Pillai, an immunologist at Harvard Medical School and the Ragon Institute who specializes in B cells, the immune cells that produce antibodies, told us in an email.
He added that while it’s true that viruses “may be theoretically cleared less well” with IgG4, there’s “no evidence” that this makes any difference to an infection. Even with elevated IgG4, he said, there’s still far more of another antibody subtype with strong pathogen-clearing abilities. There is “no scientific basis to consider that IgG4 has adverse effects on immunity,” Pillai said.
IgG4 “interacts less aggressively with immune cells, leading to a more regulated, balanced response rather than immune suppression,” Veldhoen said. “Importantly, IgG4 antibodies remain highly specific to the spike protein and retain neutralising capacity. So, this is not a weakening of immunity; it’s simply a refinement of it.”
Schober noted that even if the increase in spike-specific IgG4 did lower the antiviral response to the coronavirus to some degree, it might not even be a bad thing, since immune system overactivation is one reason why COVID-19 can be so dangerous. It’s the clinical results that matter — and “clearly prove the positive value” of vaccination, he said.
Schober also said that claims about general immune suppression are incorrect since his results are specific to antibodies that recognize the coronavirus spike protein. Overall IgG4 is not elevated in his study, he said. For the same reason, he added, there is no reason to think that this phenomenon would increase cancer risk, either.
Veldhoen said claims linking IgG4 to cancer “are unfounded,” as elevated IgG4 in cancer “is typically a consequence of the disease’s chronic inflammation, not a cause.”
Indeed, Pillai said there is confusion “even among some physicians” about IgG4, likely because they misunderstand a particular autoimmune condition known as IgG4-related disease, which Pillai studies. In the disorder, IgG4 accumulates, but it’s not what causes the disease, he said.
Pillai was also skeptical of the pancreatic cancer study El-Deiry highlighted, which claims to have identified a correlation between repeated vaccination and IgG4 levels with poorer survival among patients with pancreatic cancer.
Veldhoen said there were numerous problems and limitations with that paper. The reported correlations are more plausibly explained by “confounding, selection and timing biases, and small sample sizes in the key subgroup analyses,” he said. Confounding is when one or more other factors is influencing an exposure and outcome, making it look like there is an association between the studied factors, even when there is no causal relationship.
Thomas Winkler, a geneticist at FAU and another co-senior author of the Science Immunology paper, told us that very high overall IgG4 is associated with certain forms of pancreatic cancer when patients have IgG4-related disease. “This is a rare disorder that does not develop in the billions of individuals who have received mRNA vaccines,” he said in an email (emphasis is his).
“To date, only two single case reports from Japan have described a possible temporal association between mRNA vaccination and the onset of IgG4-related disease,” he added. “These isolated observations are entirely consistent with a rare incidental coincidence rather than a causal relationship.”
When asked about her claims about IgG4, Kuperwasser said that the “persistence, clinical significance, and potential long-term consequences of IgG4 production is uncertain,” but that “its excessive production can be associated with autoimmunity, chronic inflammation, immune tolerance, impaired immune surveillance, as well as immune fatigue or immune suppression. Its effects depend on the immunological context and mechanism of induction.”
No Credible Link to Cancer
In addition to the pancreatic cancer study, the work group presenters also pointed to a list of case reports to argue that the COVID-19 vaccines might cause cancer.
“There are over 50+ papers on this subject, and the list keeps growing,” Kuperwasser told us, when asked about her cancer risk claims. “We have reached a critical mass of publications where the temporal association between the vaccine and cancer cannot simply be ignored or dismissed.”
The notion that the vaccines cause or exacerbate cancer has been a persistent claim from those that spread incorrect information about vaccines over the last several years. As we and others have explained, there isn’t credible evidence linking COVID-19 vaccination to cancer.
“There is no evidence that COVID-19 vaccines cause cancer, lead to recurrence, or lead to disease progression,” a National Cancer Institute website states.
“[M]any of the alarming claims rely on anecdotal case reports, the weakest form of scientific evidence, often involving exceptional outlier patients,” Veldhoen said. “Such correlations should never be interpreted as proof of causation.”
Large-scale epidemiology studies have not found increases in cancer after vaccination, he explained, and there isn’t a demonstrated mechanism for how the vaccines would cause cancer.
“What’s plausible, and already shown in animal models (correlation in humans), is that infections (which cause much stronger, longer inflammation) can awaken already-present cancer cells,” Veldhoen said. “Vaccination produces a short, localised inflammatory burst and therefore is far less likely to have that effect than an actual infection.”
“Taken together, the totality of epidemiology and biology makes a causal link between COVID-19 vaccination and new cancers very unlikely; isolated case reports and correlations need careful context and controlled follow-up studies before they can be interpreted as evidence,” he said.
A study published in Nature in October found that, far from causing cancer, COVID-19 vaccination in certain cancer patients may help them live longer, as the vaccine appears to make a type of cancer immunotherapy more effective.
Frameshifting
The work group presenters also raised concerns about the findings of a 2023 Nature paper that found that due to modified mRNA in the COVID-19 vaccines, the protein-making factories in cells tend to “slip” a bit more when reading the mRNA letters that code for the coronavirus’ spike protein. This occasionally leads to a so-called frameshift and the production of a slightly different protein, which people can then mount an immune response against.
In the study, about a third of vaccinated participants produced unintended proteins. Experiments, however, suggest that more than 90% of the time the correct protein is made, which is consistent with all the evidence showing that the vaccines trigger protective immune responses and are effective in preventing severe disease.
The authors of the Nature paper have emphasized that there’s no evidence this frameshifting with the COVID-19 vaccines is harmful. In their paper and in a press release and other comments, they have repeatedly said that their findings are useful to know when designing other mRNA vaccines or treatments in the future — particularly because there is an easy fix to avoid the issue — but that they don’t call into question the safety of the COVID-19 vaccines.
“The data is clear: there is no evidence linking unintended proteins and immune responses with harm,” the two senior authors of the Nature study, from the University of Cambridge, explained in a Conversation piece shortly after the publication of their paper. Noting the strong safety data supporting the vaccines, they added: “[O]ur latest study should not affect the safety assessment of existing mRNA COVID vaccines.”
Other experts have agreed with this interpretation of the findings.
But the work group presenters instead highlighted the uncertainty, saying the “[i]mmunogenic or toxic properties” of the off-target proteins are “unknown” and the “health consequences … have not been studied.”
Dr. Retsef Levi, the chair of the COVID-19 vaccine work group and ACIP member, also later cited the frameshifting finding in a slide titled, “mRNA Vaccines Don’t Work as Intended.”
In an email, Kuperwasser defended her interpretation of the data. “We neither know the quantities of frameshifted products produced by cells expressing the mRNA nor what their protein products do within cells,” she said. “There are so many unknowns with frameshifting and it is pure speculation at this time to assume they are safe.”
As Veldhoen has noted before, because the mRNA sequence in the COVID-19 vaccines is known, the unintended proteins can be predicted. It’s not the case, as some opposed to COVID-19 vaccination have argued, that the proteins produced are completely “random” or produce self proteins that could trigger autoimmunity.
“A frameshift simply produces a slightly shorter or minimally altered version of the Spike protein,” Veldhoen told us. “Some variants may include a few extra amino acids, but these are too short to make any meaningful difference.”
The Nature paper authors also note in their Conversation piece that “humans regularly encounter unintended proteins and generate harmless immune responses, as seen with proteins produced from our food or by harmless gut bacteria. These immune responses, which occur in all of us constantly, are controlled by our immune system to prevent them from causing damage to our bodies.”
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