The U.S. Centers for Disease Control and Prevention said the COVID-19 vaccines caused the largest carcinogenic exposure in history.
In the summer of 2026, a rumor spread that the U.S. Centers for Disease Control and Prevention had “admitted” that COVID-19 vaccines caused cancer.
Several social media posts made the claim on Facebook, including one that said “CDC admits COVID-19 vaccines are now the largest carcinogenic exposure in history. With the vaccinated at risk of most aggressive cancers & tumors” (archived):
To back the same claim, another Facebook post cited two separate papers. One, published in September 2025, analyzed data from 8.4 million people in South Korea. The other, published in March 2025, was a study of nearly 300,000 people in Italy.
Some social media users may have been referring to a slide presentation uploaded to the CDC website, which outlined some uncertainties surrounding the COVID-19 vaccines. The presentation listed several cases of aggressive cancers that occurred after individuals received the vaccines.
But as we outline below, neither that presentation nor the studies established a causal link between the vaccine and cancer. For this reason, we rated the rumor false.
In addition, the South Korea paper, which analyzed the largest population, presented enough problems that the editors of the journal added a warning at the top of the page on which it appeared, saying they launched an editorial investigation. We have reached out to the journal asking for details on the inquiry and its findings. We will update this report should we receive a response.
The slide presentation
The presentation, by Wafik El-Deiry of Brown University and Charlotte Kuperwasser of Tufts University listed on Page 14 several cases of aggressive cancers that occurred after people received immunization against COVID-19. The slide’s title read, in part, “Cancers have been reported in mRNA vaccinated individuals in temporal association to immunization.”
While some may have taken this to mean that the cases of cancer were a result of the COVID-19 vaccines, the only thing the slide evoked was the “temporal association” — that is, the fact that the cancers were diagnosed after the immunization. This alone did not represent a confirmation or “admission” that there is a proven causal link between the two.
The South Korean data
A September 2025 letter to the editor of the peer-reviewed journal Biomarker Research cited a cohort study in South Korea. The authors concluded that COVID-19 vaccination correlated with a higher incidence of certain types of cancer, including lung, breast, colorectal, prostate and thyroid cancer.
While this study said it had examined data from 8.4 million people, the journal added a warning in October 2025 at the top of the letter that read:
Readers are alerted that concerns with this article have been raised with the Editors. Editorial action will be taken as appropriate once the concerns have been fully investigated.
We have reached out to the journal’s publisher, Springer Nature, to ask whether the inquiry had concluded and, if so, what it revealed. We will update this report should we receive a response.
Meanwhile, the same journal published a commentary on this letter by a team of Belgian researchers in March 2026. The note outlined concerns about the methodology the South Korean team followed to reach their conclusion, casting doubt on their findings.
For example, the Belgian researchers pointed out that the vaccinated group and unvaccinated group were assigned different dates for the start of the analysis, which they said may have inflated the numbers.
They also said a better way to analyze the data would have been to compare a cohort of vaccinated and unvaccinated people who have not been infected by the COVID-19 virus. The reason for this is that there has been some indication that the virus itself is “oncogenic” (cancer-causing), as one 2023 paper showed. They also said that the South Korean authors did not adjust their analysis for oncogenic factors in the population they studied, such as whether the individuals smoked, drank alcohol, or had a family history or genes that may suggest a higher risk of cancer.
A third point the Belgian researchers made was that the South Korean study did not account for “surveillance bias.” They suggested that individuals who seek COVID-19 vaccines may be more likely to seek regular cancer screenings as they emphasize prevention. More regular screenings increase the likelihood of a cancer diagnosis.
In other words, the Belgian authors said there were several reasons to believe that the quality of the South Korean study was low, and they recommended the data be reviewed with new parameters:
We therefore recommend a re-analysis emulating a target trial framework, incorporating time-varying exposure, appropriate comparison groups, longer follow-up, exclusion of early diagnoses, healthcare utilization matching, statistical multiple testing corrections, and rigorous sensitivity analyses.
The Italian study
The March 2025 paper in EXCLI Journal, a German journal focused on experimental and clinical sciences, said that a study of 296,015 individuals older than 11 years of age in the Pescara province indicated a slight uptick in cancer diagnoses after vaccination, especially in colorectal, breast and bladder cancer.
The Italian study was of higher quality than the South Korean one as it accounted for prior COVID-19 virus infection, sex, age and other comorbitities.
However, the authors themselves said the quality of their data may have met certain limiting factors, because it did not account for some important elements, such as whether the subjects smoked, or whether they sought preventative care such as vaccination and regular cancer screenings.
Meanwhile, a 2026 paper by the above presentation’s two authors, which reviewed both the South Korean and the Italian studies, noted these limitations. Kuperwasser and El-Deiry said that neither study showed a causal relationship between the COVID-19 vaccine on cancer.
“Both studies provide early, population-level associations rather than causal estimates and underscore the importance of long-term follow-up and molecular correlation to distinguish true biological effects from health-system or behavioral confounders,” the authors wrote.
For further reading, Snopes has previously covered the story that the COVID-19 vaccine has been shown to extend the lives of some cancer patients.
