Fetal cell lines are laboratory-grown cells that originated from tissue obtained from elective abortions performed decades ago. The original cells were placed in laboratory dishes under conditions that allowed them to keep dividing and multiplying, creating a self-sustaining supply. Today, these cell lines are many generations removed from the original tissue. They are used extensively in vaccine production, drug testing, and biomedical research.
How a Cell Line Differs From Fetal Tissue
A cell line is not the same thing as fetal tissue. When scientists first obtained a small number of cells from fetal tissue in the 1960s and 1970s, they cultured those cells so they would continue to grow and replicate in the lab. Each time a cell divides, it produces a new “generation.” The cell lines in use today have been dividing for 50 to 60 years, meaning the cells used in modern laboratories are thousands of generations removed from the original sample. No new fetal tissue is needed to maintain them.
This is the key distinction: fetal tissue is a primary biological sample, while a fetal cell line is a renewable, self-replicating population of cells maintained in a lab. A tiny number of cells harvested long ago can be expanded into an essentially limitless supply, stored in freezers, and shared between laboratories worldwide.
The Most Widely Used Fetal Cell Lines
Three fetal cell lines dominate biomedical research and vaccine manufacturing:
- WI-38: Derived from lung tissue in the 1960s. Used to produce the rubella component of the MMR vaccine. The rubella vaccine made with WI-38 is safer and more effective than versions developed using non-human cells.
- MRC-5: Also derived from lung tissue in the 1960s. Used in the production of the chickenpox (varicella) and hepatitis A vaccines. The chickenpox virus does not grow well in non-human cells, so MRC-5 or WI-38 remain necessary for that vaccine.
- HEK-293: Derived from kidney tissue in the 1970s. This is the workhorse of pharmaceutical research, used for protein production, virus cultivation, and drug testing. A search of the medical literature returns more than 30,000 published studies involving HEK-293 cells.
Why Fetal Cells Work Better Than Alternatives
Viruses need living cells to reproduce, and they don’t grow in just any cell type. Because vaccines target human viruses, the cells used to grow those viruses need to be human. Animal cells can sometimes work, but human fetal cells offer several specific advantages that other cell types cannot match.
First, fetal cells proliferate more readily and survive longer in culture than adult cells. All cells have a built-in limit on how many times they can divide, roughly 50 to 60 divisions. This cap exists because the protective caps on the ends of chromosomes, called telomeres, get slightly shorter with every division. Once telomeres become too short, the cell stops dividing and eventually dies. Fetal cells start with longer telomeres because they haven’t divided many times yet, giving scientists more usable divisions before the cells wear out.
Second, fetal cells came from the sterile environment of the womb. This meant the original cells were not already infected with other viruses, reducing the risk of contaminating vaccines during production. That sterility was a major practical advantage when these lines were first established.
Third, some viruses simply will not grow robustly in non-human cells. The chickenpox virus is a clear example: it must be produced using human fetal cell lines because it doesn’t expand well in animal-derived alternatives.
Vaccines and Drugs That Use Fetal Cell Lines
Several widely administered vaccines rely on fetal cell lines during manufacturing. The MMR vaccine (measles, mumps, rubella) uses WI-38 for its rubella component. The varicella (chickenpox) vaccine uses MRC-5 or WI-38. The hepatitis A vaccine also uses one of these two lines. During the COVID-19 pandemic, HEK-293 cells were used in the development or testing of several vaccines, though the manufacturing processes varied by product.
Beyond vaccines, fetal cell lines play a role in drug safety testing. Researchers use cells derived from fetal tissue to build miniature organ models, called organoids, that mimic human kidneys, brains, and hearts. These models help test whether drugs cause birth defects or toxicity during pregnancy. For example, brain organoid models have been used to evaluate the safety of medications like valproic acid (an anti-seizure drug) and isotretinoin (an acne medication), both known to cause birth defects. Heart organoid models have been used to study the effects of thalidomide and aspirin on developing tissue.
What Ends Up in the Final Product
A common concern is whether fetal cells or DNA are present in vaccines that reach your arm. The short answer: only trace fragments of DNA remain, in amounts so small they pose no biological risk.
The FDA requires that pharmaceutical products be free of extraneous material. For vaccines made using continuous cell lines, the maximum allowable amount of residual cellular DNA is 100 picograms per dose. A picogram is one-trillionth of a gram. To put that in perspective, a risk assessment found that exposure to 1 nanogram of cellular DNA (ten times that limit) could theoretically cause a cancer-related event in 1 out of every 1 billion recipients. The FDA also requires that any remaining DNA fragments be smaller than 200 base pairs, a size too small to encode a functional gene.
For well-established human fetal cell lines like WI-38 and MRC-5, the FDA does not even consider residual DNA a safety concern, because these cells are not tumorigenic (they don’t cause cancer). Manufacturers still follow strict purification protocols, but the regulatory posture reflects decades of safety data.
Emerging Alternatives
Several newer technologies are being developed that could eventually reduce reliance on traditional fetal cell lines. Induced pluripotent stem cells, which are adult cells reprogrammed to behave like embryonic cells, offer one potential path. Organoids grown from non-fetal sources, tissue-on-a-chip platforms, and computational biology tools are also advancing rapidly. The NIH has described these as “robust alternatives that can drive discovery while reducing ethical concerns.” For now, though, fetal cell lines remain deeply embedded in vaccine production and drug testing pipelines, and no full replacement has yet matched their reliability for all applications.