Fetal cell lines are laboratory-grown cells that descend from cells originally obtained from a small number of elective abortions performed in the 1960s and 1970s. Certain vaccines rely on these cell lines because the viruses used in those vaccines can only be grown inside living human cells, and fetal cell lines turned out to be unusually well suited for that job. The cells in today’s labs are many generations removed from the original tissue, and none of them end up in the finished vaccine you receive. That distinction between a manufacturing tool and a vaccine ingredient is central to understanding the topic, but it is also just the starting point for a set of questions worth untangling.
Where Fetal Cell Lines Came From
In the 1960s, researchers at the Wistar Institute in Philadelphia and the Medical Research Council Laboratory in London developed two cell lines that would go on to underpin decades of vaccine production: WI-38 and MRC-5. WI-38 was derived in 1962 from lung tissue of a single legally aborted fetus in Sweden. MRC-5 followed in 1966, from a single fetus in the United Kingdom. These were not ongoing collections of fetal tissue; in each case, one tissue sample was used to establish a cell line that could then be grown and replicated in the laboratory essentially without limit.1PubMed Central. Human fetal tissue is critical for biomedical research – Section: Vaccine development and production relies on cells derived from HFT
The ethical framework around these events is worth noting. The abortions that yielded WI-38 and MRC-5 predated most modern research-ethics regulations. The World Medical Association’s Declaration of Helsinki, which laid out ethical recommendations for research involving human subjects, was not published until 1964. The U.S. government did not publish basic regulations for the protection of human research subjects until 1974.2Nature. A culture of consent That historical context does not resolve the ethical questions people raise today, but it does clarify that the tissue collection happened under the norms of its era, not in violation of rules that existed at the time.
A later cell line, HEK-293, was established in 1973 from embryonic kidney cells. It became one of the most widely used human cell lines in biomedical research generally, and it played a prominent role in COVID-19 vaccine development. Another line, PER.C6, was derived from retinal cells in 1985. These four lines account for nearly all fetal-cell-line use in vaccine manufacturing.3American Journal of Health-System Pharmacy. A review of fetal cell lines used during drug development: Focus on COVID-19 vaccines, transplant medications, and biologics
Why Viruses Need Living Cells to Grow
Viruses are not free-living organisms. They cannot reproduce on their own the way bacteria can. To make copies of themselves, they need to hijack the machinery inside a living cell. This creates a basic manufacturing problem: if you want to produce a viral vaccine, whether it contains a weakened live virus or pieces of a virus, you first need enormous quantities of that virus. And to grow that virus, you need enormous quantities of the right kind of living cell.
Not just any cell will do. A virus typically infects only certain species and certain cell types. Human viruses, unsurprisingly, tend to grow best in human cells. Before the development of WI-38, vaccine researchers sometimes used primary monkey kidney cells, which brought contamination risks and batch-to-batch variability. WI-38 and MRC-5 solved both problems. They were human cells that supported robust virus growth, they were free of known contaminants, and because they were a standardized cell line rather than freshly harvested tissue, every batch was genetically identical. Researchers could expand them in culture and produce large amounts of virus reliably.1PubMed Central. Human fetal tissue is critical for biomedical research – Section: Vaccine development and production relies on cells derived from HFT
Fetal cells had specific advantages for this work. They divided many times before aging out, giving manufacturers a long production window. They were less likely to harbor latent viruses than cells taken from adults who had accumulated a lifetime of infections. And their rapid, predictable growth made them practical for industrial-scale vaccine production.
What “Derived From” Actually Means in Practice
The language around fetal cell lines trips people up. When a vaccine is described as “produced using” a fetal cell line, that does not mean fetal cells are an ingredient in the vaccine. The cell line is the factory floor, not the product. Viruses are grown inside the cells, then harvested, purified, and formulated into the vaccine. The cells themselves are waste from the perspective of the finished product.
Large-scale manufacturing typically involves multiple purification steps to remove cellular debris. These include clarification to remove large particles, chromatography to separate the virus from host cell proteins, and ultrafiltration to wash away remaining impurities. One of the key steps, anion exchange chromatography, is specifically designed to remove the majority of host cell proteins.4Biochem (Lond). Making THE vaccine – Section: Downstream process By the time the vaccine reaches a vial, the cellular material has been stripped away through these sequential rounds of processing.
Regulatory agencies set limits on how much residual host cell DNA can remain in a finished biopharmaceutical product. The WHO and U.S. FDA recommend limits of less than 10 nanograms per dose and DNA fragments below 200 base pairs in length.5Analytical Chemistry. Sensitive and Robust Capillary Electrophoretic Analysis of Host Cell Residual DNA in Biopharmaceutical Samples To put that in perspective, 10 nanograms is about one ten-millionth of a gram. The fragment size limit ensures that even the trace DNA that remains is too small to carry a functional gene.
Is Residual DNA Dangerous?
One concern that surfaces in public discussion is whether trace amounts of DNA from the host cells could cause cancer or infection in vaccine recipients. Regulatory scientists have studied this question directly. The FDA has developed assays to quantify the biological activities of residual cell-substrate DNA, producing risk estimates for both oncogenic and infectious events. These estimates were derived using the most sensitive detection methods available and are considered worst-case scenarios.6PubMed. Issues associated with residual cell-substrate DNA in viral vaccines
Probabilistic risk models have been developed to calculate the theoretical chance that residual DNA could transmit an activated oncogene or a latent viral genome to a vaccine recipient. These models factor in the amount of DNA, the size of the fragments, and the biological steps required for a fragment to actually integrate into a human cell and cause harm.7PubMed. A probabilistic model for risk assessment of residual host cell DNA in biological products The calculated risks are vanishingly small, several orders of magnitude below the threshold that regulators consider actionable. Billions of vaccine doses manufactured using these cell lines have been administered over more than half a century, and no case of cancer or infection attributable to residual cell-substrate DNA has been documented.
Which Vaccines Use Fetal Cell Lines
The vaccines most commonly associated with fetal cell lines are those against rubella (the R in the MMR vaccine), varicella (chickenpox), hepatitis A, and one type of rabies vaccine. These all use WI-38 or MRC-5 in their manufacturing process. The rubella vaccine is perhaps the most historically significant example: it was developed because rubella infection during pregnancy can cause devastating birth defects, and the cell line used to grow the vaccine virus was itself derived from fetal tissue.
During the COVID-19 pandemic, fetal cell lines entered public conversation on a much larger scale. The adenovirus-vector vaccines, such as the Johnson & Johnson (Janssen) and Oxford-AstraZeneca vaccines, used HEK-293 or PER.C6 cells to grow the modified adenovirus that serves as the delivery vehicle.3American Journal of Health-System Pharmacy. A review of fetal cell lines used during drug development: Focus on COVID-19 vaccines, transplant medications, and biologics The mRNA vaccines from Pfizer-BioNTech and Moderna, by contrast, do not use fetal cell lines in their manufacturing process, though HEK-293 cells were used in some of the early laboratory testing to confirm that the mRNA instructions worked as intended. That distinction matters if your concern is specifically about the production of the vaccine you receive.
The Rubella Story and Why It Matters
Rubella is a mild illness for most people, but when a pregnant woman contracts it, the virus can cross the placenta and cause congenital rubella syndrome in the developing fetus. The consequences include deafness, heart defects, intellectual disability, and blindness. Before widespread vaccination, rubella epidemics swept through populations regularly. A single epidemic in the United States in 1964-65 resulted in roughly 20,000 infants born with congenital rubella syndrome.
The vaccine that ended those epidemics was grown using WI-38 cells. In the Americas, combined vaccination strategies that included routine immunization for both boys and girls reduced rubella incidence by more than 98%, and some countries reported no cases of congenital rubella syndrome after 2008.8PubMed. A systematic review of rubella vaccination strategies implemented in the Americas: impact on the incidence and seroprevalence rates of rubella and congenital rubella syndrome Globally, the stakes remain high. Modeling estimates suggest that rubella vaccination in 19 countries that have not yet introduced the vaccine could prevent roughly 986,000 cases of congenital rubella syndrome between 2025 and 2055.9PubMed Central. Estimated Current and Future Congenital Rubella Syndrome Incidence with and Without Rubella Vaccine Introduction — 19 Countries, 2019–2055
The rubella vaccine story is not just a public-health statistic. It illustrates the core tension in the fetal cell line debate: the same technology that troubles some people on ethical grounds has prevented an enormous amount of suffering, including suffering to unborn children.
Ethical and Religious Perspectives
People who oppose abortion sometimes struggle with whether to accept vaccines whose production involved fetal cell lines. This is an understandable concern, and religious institutions have grappled with it carefully. The Catholic Church, which is unequivocally opposed to abortion, has concluded that the use of these vaccines is morally permissible. The Church permits temporary use of vaccines generated using fetal cell lines to protect children from preventable diseases, while urging development of alternative vaccines that do not rely on fetal tissue.10PubMed Central. Use of Aborted Fetal Tissue in Vaccines and Medical Research Obscures the Value of All Human Life
The reasoning behind this position typically centers on the distance between the original act and the current use. The abortions that gave rise to these cell lines occurred decades ago for reasons unrelated to vaccine research. No new fetal tissue is being obtained for vaccine production. Using the existing cell lines does not create demand for future abortions. And refusing vaccines carries its own moral weight, particularly when doing so puts vulnerable people, including children and pregnant women, at risk of serious disease.
Clinicians who encounter vaccine hesitancy rooted in these concerns have been encouraged to address them directly. Approaches include explaining the ethical analyses of moral complicity, discussing the altruistic dimension of protecting others from infectious disease, and noting that some vaccines (like the mRNA COVID-19 vaccines) were not produced in fetal cell lines at all.11PubMed Central. Helping patients with ethical concerns about COVID-19 vaccines in light of fetal cell lines used in some COVID-19 vaccines The goal is not to dismiss the concern but to give people enough information to make a decision that reflects their actual values rather than a misunderstanding of the science.
Alternatives to Fetal Cell Lines
Fetal cell lines are not the only way to make vaccines, and they are not even the dominant platform for newer vaccines. Several alternatives now exist and are in widespread commercial use.
Non-fetal animal cell lines have been used successfully for multiple approved vaccines. MDCK cells (from dog kidney tissue) are used for some influenza vaccines. Vero cells (from African green monkey kidney) are used for polio, rabies, and some other viral vaccines. CHO cells (from Chinese hamster ovary) are a workhorse of the biopharmaceutical industry, used to produce a wide range of protein-based therapeutics and some vaccines.12ACS Omega. Animal Cell Lines as Expression Platforms in Viral Vaccine Production: A Post Covid-19 Perspective – Section: 5. Animal Cell Culture-Based Expression Platforms Used in Vaccine Production
Insect cell technology has also matured considerably. The baculovirus expression system, which uses insect cells to produce viral proteins, achieved a regulatory milestone in 2007 with the approval of Cervarix, a human papillomavirus vaccine. That approval opened the door for broader use of insect-cell-derived vaccines, and the platform is now dominant in veterinary vaccine manufacturing.13PubMed. Insect cell technology is a versatile and robust vaccine manufacturing platform
The most significant recent shift is mRNA technology. The Pfizer-BioNTech and Moderna COVID-19 vaccines demonstrated that you can produce a vaccine without growing a virus at all. Instead, synthetic mRNA instructions are manufactured chemically and packaged in lipid nanoparticles. No cell line of any kind is needed for the production process itself. If mRNA platforms continue to expand to other diseases, the practical reliance on fetal cell lines for new vaccine development will shrink further.
Fetal Cell Lines Beyond Vaccines
Vaccines get most of the public attention, but fetal cell lines play a much broader role in biomedical research and drug development. HEK-293 cells, in particular, have become one of the most commonly used cell lines in biology. They are used as a research tool to study gene function, to screen drug candidates, and to produce therapeutic proteins.
Many biologics and cellular therapies for genetic diseases and cancers have been directly developed from HEK-293 cells or contain proteins produced using fetal cell lines.14PubMed. A review of fetal cell lines used during drug development: Focus on COVID-19 vaccines, transplant medications, and biologics Gene therapies, for instance, often rely on adeno-associated virus (AAV) vectors that are grown in HEK-293 cells. Some transplant medications have been tested or manufactured using these lines. The reach of fetal cell lines in modern medicine extends well beyond the handful of vaccines that dominate public conversation.
This broader use means that a person who objects to fetal cell lines on ethical grounds faces a more complex landscape than they might initially realize. Avoiding all medical products that were developed with or tested in fetal cell lines would exclude not just certain vaccines but also some treatments for cancer, genetic disorders, and organ transplant rejection. How people weigh that reality against their values is a personal decision, but it helps to know the full scope of what is involved.
Cell Banking and Why New Fetal Tissue Is Not Needed
A common misconception is that ongoing fetal tissue collection is necessary to keep producing vaccines. In reality, the original cell lines have been banked in a system designed for long-term sustainability. A master cell bank consists of carefully frozen vials of cells, all genetically identical, stored under controlled conditions. From a single tissue donation, researchers can develop a master bank of dozens of homogeneous vials, each containing millions of cells.15PubMed. Consistency and safety of cell banks for research and clinical use: preliminary analysis of fetal skin banks When production cells age out after many divisions, a fresh vial from the master bank is thawed to start a new working stock. This tiered banking system means the original cell lines can supply manufacturing needs for decades, potentially centuries, without any new tissue collection.
WI-38, for example, is now more than 60 years old as a cell line. The cells being used today are descendants of the original cells, separated by many generations of division and passaging. They are as much “fetal cells” as a great-great-grandchild is their ancestor. The biological connection to the original tissue is real but increasingly distant with every passage.
How Manufacturing May Change
The vaccine field is moving in directions that will likely reduce, though not eliminate, reliance on fetal cell lines. Human cell lines remain widely used in biopharmaceutical research and production, but there is still relatively limited clinical experience with newer human cell lines compared to well-established non-human platforms like CHO cells. Additional research investment could optimize human cell lines for routine commercial production of a broader range of therapeutics, potentially including lines that do not carry the same ethical concerns.16Taylor & Francis Online / PubMed Central. Human cell lines for biopharmaceutical manufacturing: history, status, and future perspectives
Meanwhile, the expansion of mRNA and protein-subunit technologies is changing the equation for new vaccines. These platforms can sidestep cell-line-based virus propagation entirely, producing vaccines through chemical synthesis or expression in non-controversial cell systems. For existing vaccines like rubella, chickenpox, and hepatitis A, however, reformulating with a different cell line would require extensive new clinical trials to prove the new version is safe and effective. That is an expensive, years-long process with no guarantee of regulatory success, which is why these older vaccines continue to be made the way they have been for decades.
The practical result is a split. New vaccines increasingly have options that avoid fetal cell lines. Legacy vaccines, particularly live-virus vaccines for which WI-38 and MRC-5 are the established manufacturing substrate, will likely continue using them for the foreseeable future. For people navigating this landscape, the specific vaccine and the specific cell line involved are worth checking rather than assuming all vaccines are the same.