On Wednesday Florida’s state surgeon general Joseph Ladapo called for stopping the use of messenger RNA–based COVID vaccines, citing—without convincing evidence—concerns about DNA fragments from the vaccines entering the human genome. Experts, including those at the U.S. Food and Drug Administration, say the claims are unfounded and that this warning could cause great harm by preventing people from getting a potentially lifesaving vaccine.
Last December Ladapo sent a letter to the commissioner of the FDA and the director of the Centers for Disease Control and Prevention in which he questioned the safety of DNA fragments in Pfizer’s and Moderna’s COVID mRNA vaccines. The letter laid out unfounded concerns about these fragments entering human cell nuclei in the presence of the lipid nanoparticles that are used to deliver the mRNA to cells. He also stated concerns about DNA contamination from a virus called simian virus 40 (SV40). Ladapo suggested that if such DNA were to be integrated into cells, it could activate cancer-causing genes or cause chromosomal instability.
That month the FDA said in a response letter that, “based on a thorough assessment of the entire manufacturing process, FDA is confident in the quality, safety, and effectiveness of the COVID-19 vaccines.”
Many scientists have dismissed the risks Ladapo asserted. They include Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, who serves on an FDA advisory committee for the COVID vaccines. The way mRNA vaccines are made does result in small amounts of DNA in the final product, Offit says—but that’s true of any vaccine grown in cells, including the measles and chickenpox vaccines. “There are trace quantities” of DNA (billionths to trillionths of a gram) per vaccine dose, “which is utterly and completely harmless for several reasons,” he says.
To make an mRNA vaccine against COVID, scientists start with circular pieces of DNA called plasmids that contain a gene for the spike protein of SARS-CoV-2, the virus that causes the disease. The plasmids are amplified into billions of copies inside of bacteria, and chemicals are then added to release them from the bacteria. Enzymes are used to cut the plasmids into linear pieces of DNA that encode the spike protein, and a different enzyme converts that DNA into mRNA. Another enzyme is added to chop any remaining DNA into tiny harmless fragments.
In order to enter human cell nuclei, any such residual viral DNA would first have to enter the cell’s main compartment, or cytoplasm, which normally keeps foreign DNA out. Next it would have to cross the nuclear membrane; this would be impossible without an access signal, which these fragments don’t have, Offit notes. The residual DNA would also have to integrate into the nuclear DNA, which would require DNA-cutting enzymes that aren’t present in the mRNA vaccine. The chances that mRNA vaccination would in any way affect your DNA “are zero,” Offit says.
This is not the first time Ladapo has questioned the safety of COVID mRNA vaccines. In 2022 he recommended against children age 17 or younger getting the vaccines, falsely claiming that the shots didn’t help and might even cause harm.
Scientist and physician Robert Malone also made unfounded claims about the supposed dangers of DNA fragments in mRNA vaccines altering human DNA when he testified in a November 2023 committee hearing held by Republican Representative Marjorie Taylor Greene of Georgia.
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Offit points out that we encounter much greater quantities of foreign DNA all the time from the bacteria we’re exposed to and the plants and animals we eat. In his statement on Wednesday, Ladapo claimed that people can get other COVID vaccines that don’t use mRNA. Yet the only alternative in the U.S., made by the company Novavax, is grown in moth cells—which also contain DNA. “The minute you say the word DNA, people think, ‘Oh, my God, there’s DNA in this? Is that going to affect my DNA?’” Offit says. “But you have better chance of becoming Spider-Man” than being harmed by DNA from the COVID vaccines.
As for the concerns about simian virus 40, the COVID vaccines do not contain SV40 proteins or any genetic material encoding them. Although SV40 was a contaminant in early polio vaccines, it has not been shown to cause cancer in humans.
Offit acknowledges that any vaccine or medicine comes with potential risks as well as benefits. The Johnson & Johnson COVID vaccine was found to cause rare and sometimes fatal blood clots in some people and was removed from the market. The mRNA vaccines, while largely safe, come with a small but nonzero risk of myocarditis, primarily in teenage boys and young men. The risk of myocarditis from COVID itself is higher, however, and COVID-related myocarditis tends to be more severe. For those concerned about mRNA vaccines, there are other vaccines available, such as the one made by Novavax.
Offit says the benefits of vaccination still clearly outweigh any risks. He recommends the primary series for everyone aged six months and older. He says getting the booster is less important for healthy young people, who are not the highest-risk group for hospitalization. People older than age 65 and people of any age with underlying health conditions that put them at risk of severe disease—including pregnant people—should definitely be up to date on their vaccine, he says. Additionally, recent studies have shown that receiving a vaccination followed by one or more boosters greatly reduces the risk of developing long COVID.
“There’s no avoiding risk,” Offit says. “People say, ‘I’m not going to risk that vaccine.’ Okay, so you’re going to risk the disease. Realize that that’s the risk you’re taking.”
TANYA LEWIS is a senior editor covering health and medicine at Scientific American. She writes and edits stories for the website and print magazine on topics ranging from COVID to organ transplants. She also co-hosts Your Health, Quickly on Scientific American's podcast Science, Quickly and writes Scientific American's weekly Health & Biology newsletter. She has held a number of positions over her seven years at Scientific American, including health editor, assistant news editor and associate editor at Scientific American Mind. Previously, she has written for outlets that include Insider, Wired, Science News, and others. She has a degree in biomedical engineering from Brown University and one in science communication from the University of California, Santa Cruz.
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