More than 4,000 Americans have contracted measles since January 2025. Two children and one adult have died. Sixteen states have fallen below the vaccination threshold required to prevent sustained transmission. The United States is at risk of losing its measles elimination status, which it achieved in 2000.
Secretary of Health and Human Services Robert F. Kennedy Jr. has a message for the communities at the center of these outbreaks: the measles, mumps, and rubella (MMR) vaccine contains “millions of particles that were created from aborted fetal tissue, millions of DNA fragments.” It does not.
He is the nation’s chief public health officer. He is making this claim during the worst measles outbreak in more than three decades. He is making it about the one vaccine that could stop that outbreak. And he is directing it at the religious communities—Mennonite, Orthodox, conservative Catholic—in which vaccination rates are lowest and where the current cases are concentrated.
Let’s explore what the manufacturing, regulatory, and scientific records actually show.
Where the vaccine comes from
Viruses can replicate only inside living cells. To make the rubella component of the MMR vaccine, manufacturers grow the live weakened virus in a human cell strain called WI-38.
In 1962, a researcher named Leonard Hayflick isolated cells from lung tissue obtained from a single elective abortion performed in Sweden. Those cells were placed in laboratory cell cultures—controlled environments where cells are grown outside the body—where they have been propagating ever since. The cell strain used in Merck’s MMR-II traces its lineage to that original culture.
The chickenpox and hepatitis A vaccines use a similar strain, MRC-5, established in 1966 by J.P. Jacobs from lung cells obtained from an elective abortion performed in the United Kingdom.
Both of these facts come directly from the product pages of the American Type Culture Collection, the national repository that catalogs and sells biological materials to researchers—the same database cited by those making these claims.
These cell strains are not immortal. They replicate for roughly 50 passages, then reach the end of their replicative lifespan. Manufacturers maintain frozen seed stocks that can be thawed to start fresh cultures as needed. This finite nature is a safety feature: It distinguishes these strains from continuous cell lines, which replicate indefinitely and carry a theoretical cancer risk.
These cell strains are not immortal. They replicate for roughly 50 passages, then reach the end of their replicative lifespan.
The cells used today are the laboratory descendants of those original strains, not material from any abortion. The original cells have not existed for sixty years. No new abortions are performed to manufacture these vaccines.
Once the virus finishes growing in these cell cultures, it is extracted and purified. What ends up in the final vaccine is the weakened virus, stabilizing ingredients, and trace amounts of residual protein and DNA left over from the production process. There are no intact human cells in the vaccine.
The residual DNA question
Trace amounts of DNA from the WI-38 strain do remain in the final product. This is a known manufacturing reality, disclosed in the package insert, and studied extensively by regulators. Their conclusion: Residual DNA from normal human cell strains like WI-38 and MRC-5 does not pose a safety risk.
That determination requires some context. The World Health Organization (WHO) sets a strict limit of 10 nanograms of residual DNA per vaccine dose. That limit was established specifically for a different category of production cells called continuous cell lines, which can be derived from tumors and carry theoretical cancer risk. WI-38 and MRC-5 are not in that category. They are normal, finite human cell strains with no capacity to form tumors, and WHO guidance explicitly states that the 10-nanogram limit does not apply to products made from normal human cell strains like these.
The DNA fragments that remain are intentionally reduced to below 200 base pairs in length. A base pair is the basic structural unit of DNA. The average human gene is thousands to tens of thousands of base pairs long. A 200 base pair fragment cannot encode a functional protein or integrate meaningfully into a human genome.
The Food and Drug Administration’s advisory committee evaluated the cellular DNA in the chickenpox vaccine, which contains the highest DNA content of any approved childhood vaccine, and concluded specifically that it was unlikely to integrate into host cells and cause harm during vaccination.
The “billions of fragments” figure that circulates widely online comes from real arithmetic applied to a negligible mass. A nanogram is a billionth of a gram. When you divide nanograms of DNA by the length of each fragment, you get an enormous fragment count. You do not get a biologically meaningful quantity of material.
The paper behind the autism claim
The claim that vaccine DNA causes autism traces almost entirely to a 2015 paper by Theresa Deisher and colleagues, published in Issues in Law and Medicine. That journal is published by the Alliance for Hippocratic Medicine, a right-to-life advocacy organization. It is not a molecular biology or epidemiology journal.
The paper makes two main arguments.
The first examines MMR vaccination rates and autism diagnoses in the United Kingdom, Norway, and Sweden during the years following Andrew Wakefield’s 1998 Lancet paper—the fraudulent study that falsely linked the MMR vaccine to autism, was formally retracted, and cost Wakefield his medical license. After this publication, MMR vaccination coverage dropped across Britain and Scandinavia. Deisher argues that autism diagnoses also fell during this period and that this constitutes a “natural experiment” proving the vaccine causes autism through its fetal DNA content.
Two problems undercut this entirely. The paper’s own authors acknowledge that publicly available autism prevalence data for this period is “disappointingly scarce.” And the apparent drop in diagnoses during the height of the Wakefield panic almost certainly reflects disrupted pediatric care, not a genuine fall in autism rates. When parents stop bringing children to doctors out of fear, fewer children get diagnosed. The fact that two trends moved in the same direction during a period of social disruption is not evidence that one caused the other.
The apparent drop in diagnoses during the height of the Wakefield panic almost certainly reflects disrupted pediatric care, not a genuine fall in autism rates.
The second part of the paper describes laboratory experiments in which DNA fragments were added to cancer cells in lab dishes and observed to enter those cells. Cells in a dish behave very differently from cells in a living person. For vaccine-derived DNA to cause autism would require a chain of events that has never been demonstrated: DNA surviving the body’s constant degradation of foreign material, traveling from the injection site, crossing the blood-brain barrier, entering neurons, and integrating at precisely the right genomic location to disrupt autism-associated genes.
Also, Deisher’s measurements were taken from Meruvax II, Merck’s discontinued standalone rubella vaccine, not from MMR-II. The data being used as evidence of danger in the MMR vaccine were not even collected from the MMR vaccine.
The Deisher paper is not a new scientific finding. It’s an attempt to revive the Wakefield hypothesis with a different proposed mechanism, using the same kind of comparison that gave Wakefield’s argument its superficial plausibility.
The religious argument
Secretary Kennedy cited Mennonite communities’ objections as the context for his claim. The Catholic Church, which holds the most theologically developed position on this question among major Christian denominations, has examined it carefully.
In 2005, the Vatican’s Pontifical Academy for Life formally evaluated the ethics of vaccines made using fetal cell strains. Its 2017 follow-up statement concluded that receiving these vaccines involves no morally relevant cooperation with the original abortions. A 2020 statement from the Congregation for the Doctrine of the Faith regarding COVID vaccination, approved by Pope Francis, confirmed that receiving vaccines made using these cell strains is morally acceptable given the gravity of preventable disease.
The institution most publicly committed to opposing abortion has concluded that these vaccines may be used in good conscience.
The toll on kids of misleading claims
The original Wakefield fraud drove down MMR coverage across the United Kingdom and caused measles outbreaks that infected thousands of children. The same argument—now amplified by the top US public health official, targeted at the communities most at risk, during an active outbreak of the disease it discourages people from preventing—is contributing to the outbreaks under way now.
MMR vaccination rates among kindergartners have fallen from 95.2% to 92.5% nationally. Two children died from measles in 2025, the first pediatric measles deaths in this country in more than a decade, from a disease for which we have had a safe and effective vaccine since 1963.
The institution most publicly committed to opposing abortion has concluded that these vaccines may be used in good conscience.
The claim that the MMR vaccine contains aborted fetal tissue is false. The trace residual DNA it does contain has been evaluated by regulators, found not to pose a safety risk, and administered safely to hundreds of millions of people across six decades.
Fear about vaccine ingredients, contradicted by 60 years of safety data, has driven vaccination rates to the point where children are dying from a disease we eliminated a generation ago.
Dr. Scott is a Clinical Associate Professor of Infectious Diseases at Stanford University School of Medicine and a co-author of “Updated Evidence for Covid-19, RSV, and Influenza Vaccines for 2025-2026“ in the New England Journal of Medicine.
The opinions voiced in CIDRAP Op-Ed pieces are the authors’ own and do not necessarily represent the official position of CIDRAP.
