"Full of SV40": how a harmless DNA fragment became a cancer scare

The claim that COVID-19 mRNA vaccines are contaminated with a cancer-causing monkey virus rests on a confusion between a virus and a short piece of DNA copied from it. The newest paper behind the claim came from Ontario, and its publisher is investigating it.

Since 2023, a claim has moved from Substack to US Senate exhibits to oncologists' video clips: that the Pfizer-BioNTech COVID-19 vaccine contains SV40 (simian virus 40), a monkey virus that causes tumours in laboratory animals, and that this explains a wave of "turbo cancer." One part of the claim is true. The scary part is not, and the gap between the two parts carries the whole claim.

A promoter is a switch, not a virus

A promoter is a stretch of DNA that tells a cell's machinery where to start reading a gene. The SV40 promoter is a particularly strong one, so molecular biologists have used it for decades. It sits in a large share of the standard plasmids (small circles of DNA grown in bacteria) used in laboratories worldwide. Pfizer used such a plasmid as the template for producing its mRNA.

A promoter does not encode a protein. On its own it cannot make anything, infect anything or transform a cell. The part of SV40 that can push cells toward cancer is the large T antigen, a protein encoded elsewhere in the viral genome. That sequence is not in the vaccine. In December 2023, the US FDA wrote to Florida Surgeon General Joseph Ladapo that the nucleotide sequences in the mRNA vaccines encode no SV40 proteins.

Health Canada answered the same question plainly. Its spokesperson told The Canadian Press that the vaccine does not contain SV40 and that "the presence of the SV40 promoter enhancer sequence is not the same as the presence of the whole virus itself." She added that claims linking the fragment to cancer are unfounded.

Residual DNA is expected, and limited

Every vaccine or biologic made with living cells or DNA templates carries trace amounts of leftover DNA. That is why international limits exist. For mRNA vaccines, the template is digested with an enzyme (DNase) after transcription, which chops what remains into small fragments. Batches are then tested against regulatory limits. Australia's Therapeutic Goods Administration said in October 2024 that every mRNA batch released there had met those requirements.

The integration scare, that fragments will enter the nucleus and rewrite human DNA, depends on a long chain of improbable steps. Nobody has shown that chain happening in a vaccinated person.

The polio history cuts the other way

SV40 really was a vaccine contaminant once. Between 1955 and 1963, some polio vaccine lots grown in monkey kidney cells carried live SV40, and millions of people received it. Promoters of the mRNA claim cite this history as a warning. In fact it is the best evidence against them. Here was exposure to the whole, live virus, including the T antigen, on a massive scale. Long-term epidemiological studies did not find an elevated cancer risk in those recipients.

If injections of live SV40 into children did not produce a measurable cancer signal, a fragment containing no SV40 genes will not either.

That history is worth knowing in full, because the contamination was real, the contaminated lots were not recalled, and the cancer studies have one genuine weakness that honest critics do raise. All of it — the 1955–63 contamination, the epidemiology study by study, the laboratory controversy and the lawsuits — is set out in SV-40: a real contamination, a cancer that never came, which also carries a side-by-side table of the two claims.

The recent flashpoints

How the claim got an official stamp: Joseph Ladapo

Florida Surgeon General Joseph Ladapo is the person who moved the SV40 claim from Substack into an official government document. His trained field is internal medicine and health policy, and his research before his appointment was on the cost-effectiveness of cardiac diagnostic tests, not vaccines or molecular biology.

The trigger was Canadian. In late 2023, The Epoch Times reported Health Canada's confirmation that the SV40 promoter sequence was present in the plasmid used to make the Pfizer vaccine. On 6 December 2023, Ladapo wrote to the FDA and CDC arguing that the leftover DNA, carried into cells by lipid nanoparticles, posed a risk of "DNA integration." He relied on a 2007 FDA guidance document on plasmid DNA vaccines.

On 14 December, Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, replied that the 2007 guidance was written for DNA vaccines, in which DNA is the active ingredient. He said it does not apply to trace DNA left over in other vaccines. He also wrote that integration of residual fragments into chromosomal DNA was implausible, and that no SV40 proteins are encoded in the vaccines. On 3 January 2024, Ladapo called for a halt to mRNA COVID-19 vaccination anyway, repeating the same reading of the same guidance.

For the rest of Ladapo's record as Florida Surgeon General, see the Ladapo running file.

The Ontario paper

The paper now cited most often has a Canadian address. "Quantification of residual plasmid DNA and SV40 promoter-enhancer sequences in Pfizer/BioNTech and Moderna modRNA COVID-19 vaccines from Ontario, Canada" was published in the Taylor & Francis journal Autoimmunity on 6 September 2025. Its authors are David J. Speicher of the University of Guelph, independent researcher Jessica Rose, and Kevin McKernan. Senator Ron Johnson entered it into the record of a US Senate subcommittee hearing. McKernan said he had been contacted by the CDC's vaccine advisory committee (ACIP) to discuss it.

The problems were raised quickly:

• The publisher opened an investigation. Concerns posted on PubPeer were forwarded to Taylor & Francis, which began investigating within about two weeks of publication. Retraction Watch reported it on 18 September 2025.
• The cited standard doesn't apply. Retraction Watch found that the FDA guidance the authors measured against was written for gene therapies and states explicitly that it does not apply to vaccines for infectious diseases.
• The measuring method inflates results. A letter to the editor argued that fluorometric DNA measurement is unsuitable in RNA-rich samples. RNA interferes with the reading and pushes DNA numbers up. A vaccine is almost entirely RNA.
• The dose-response claim runs backwards. The same letter noted that the lot with the most reported adverse events in the authors' own data had among the lowest DNA levels. That undercuts the link the paper proposed between DNA content and harm.
• Peer reviewers recommended against publication. According to Retraction Watch, reviews of both the original and revised manuscripts advised against publishing because of problems with the fluorometry data.

As of early October 2026, co-author Jessica Rose was still describing the paper as under investigation. QuackeryWatch has found no retraction notice. If one is issued, this page will be updated. Rose and McKernan describe the investigation as an organised attack. Readers can compare the paper and the criticism directly through the PubPeer thread linked below.

The tell

When you see "SV40" attached to a vaccine claim, ask one question: does the source distinguish the virus from the promoter? If it says the vaccine "contains SV40" or is "full of SV40" without making that distinction, it is either confused or counting on you to be. If it does make the distinction, ask what harm a promoter that makes no protein is supposed to cause. Then look for evidence in people, not hypotheticals.

Sources

1. Canadian Press fact check: Health Canada statement on DNA in COVID-19 vaccines misrepresented online
2. US FDA, letter from Peter Marks to Joseph Ladapo, 14 December 2023: fda.gov/media/174875
3. Florida Department of Health, Ladapo letter to FDA, 6 December 2023: floridahealth.gov
4. Retraction Watch, 18 September 2025: Publisher investigating DNA contamination paper
5. Letter to the Editor, Autoimmunity (via Retraction Watch): PDF
6. PubPeer discussion of Speicher, Rose & McKernan (2025), doi 10.1080/08916934.2025.2551517: pubpeer.com
7. Full Fact, November 2025: COVID-19 vaccines do not contain "cancer-causing" SV40 monkey virus
8. AAP FactCheck, July 2025: Video wrong on link between mRNA vaccines and monkey virus
9. Logically Facts: No cancer-causing DNA from green monkeys in mRNA vaccines
10. Science Feedback: DNA contaminant claims based on vials of unknown provenance
11. Institute of Medicine, Immunization Safety Review: SV40 Contamination of Polio Vaccine and Cancer, released 22 October 2002 (print volume 2003). National Academies Press.

Related on QuackeryWatch

• The 82% miscarriage claim: a denominator error
• The Allison Inquiry and its American speakers
• Fenbendazole, Joe Tippens and William Makis ("turbo cancer")
• SV-40: a real contamination, a cancer that never came, and a modern claim that borrows the name — the 1955–63 polio contamination in full, the epidemiology, and the two claims side by side
• Joseph Ladapo: the running file