Human Cell Strains in Vaccine Development

Stanley Plotkin, M.D.

Note: The terms “cell strain” and “cell line” are sometimes used interchangeably. In this article, “cell strain” is used to describe a culture of a single type of cell; “cell line” is used to describe an immortalized culture of a single type of cell – that is, one that replicates infinitely, like the well-known HeLa cell line that was started with cancer cells taken from Henrietta Lacks.

Animals have been used in the production of human vaccines since vaccine farms were established to harvest cowpox virus from calves in the late 1800s. From that point, and through the first half of the 20th century, most vaccines would continue to be developed with the use of animals, either by growing pathogens in live animals or by using animal cells.

Although many vaccines and anti-toxin products were successfully developed this way, using animals in vaccine development – particularly live animals – is not ideal. Research animals are costly and require extensive monitoring, both to maintain their health and to ensure the continued viability of the research. They may be carrying other bacteria or viruses that could contaminate the eventual vaccine, as with polio vaccines from the mid 20th century that were made with monkey cells and eventually found to contain a monkey virus called SV40, or Simian Virus 40. (Fortunately, the virus was not found to be harmful to humans.) Moreover, some pathogens, such as the chickenpox virus, simply do not grow well in animal cells.

Even when vaccine development is done using animal products and not live animals – such as growing influenza vaccine viruses in chicken eggs – development can be hindered or even halted if the availability of the animal products drops. If an illness were to strike the egg-producing chickens, for example, they might produce too few eggs to be used in the development of seasonal flu vaccine, leading to a serious vaccine shortage. (It’s a common misconception that influenza vaccines could be produced more quickly if grown in cell cultures compared to using embryonated chicken eggs. In fact, growing the vaccine viruses in cell cultures would take about the same amount of time. However, cell cultures do not have the same potential availability issues as chicken eggs.)

For these and other reasons, using cell culture techniques to produce vaccine viruses in human cell strains is a significant advance in vaccine development.

How Cell Cultures Work

Cell cultures involve growing cells in a culture dish, often with a supportive growth medium like collagen. A primary cell culture consists of cells taken directly from living tissue, and may contain multiple types of cells such as fibroblasts, epithelial, and endothelial cells.

Cell strains, however, are designed to be a culture that contains only one type of cell. This is done by taking subcultures from the original culture until only one type remains. Primary cultures can be manipulated in many different ways in order to isolate a single type of cells; spinning the culture in a centrifuge can separate large cells from small ones, for example. Eventually, when only a single type of cell remains, researchers can try to establish a cell line. Cell lines allow for continuous observation and control that may not be possible in larger tissue cultures containing multiple types of cells.

Cell lines may be subject to the Hayflick Limit, a rule named for researcher Leonard Hayflick. Hayflick determined that a population of normal cells will reproduce only a finite number of times before they cease to reproduce. However, in contrast to those subject to Hayflick’s discovery, some cell lines can be immortalized: that is, the cells have undergone some mutation that allows them to reproduce indefinitely. One example is the so-called HeLa cell line, started from cervical cancer cells taken in the 1950s from a woman named Henrietta Lacks.

Using cell strains and cell lines, researchers can grow human pathogens like viruses in a particular type of cell to attenuate them – that is, to weaken them. One way viruses are adapted for use in vaccines is to alter them so that they are no longer able to grow well in the human body. This may be done, for example, by repeatedly growing the virus in human cells kept in culture at a much lower temperature than normal body temperature. In order to keep replicating, the virus adapts to become better at growing at the lower temperature, thus losing its original ability to grow well at normal body temperatures. Later, when it’s used in a vaccine and injected into a living human body at normal temperature, it still provokes an immune response but can’t replicate enough to cause illness.

Vaccines Developed Using Human Cell Strains

The first vaccine created with the use of human cell strains was the rubella vaccine developed by Stanley Plotkin at the Wistar Institute in Philadelphia.

In 1941, Australian ophthalmologist Norman Gregg first realized that congenital cataracts in babies were the result of their mothers being infected with rubella during pregnancy. Along with cataracts, it was eventually determined that congenital rubella syndrome (CRS) could also cause deafness, heart disease, encephalitis, mental retardation, and pneumonia, among many other conditions. At the height of a rubella epidemic that began in Europe and spread to the United States in the mid-1960s, Plotkin calculated that 1% of all births at Philadelphia General Hospital were affected by congenital rubella syndrome. In some cases, women who were infected with rubella while pregnant terminated their pregnancies due to the serious risks from CRS.

Following one such abortion, the fetus was sent to Plotkin at the laboratory he had devoted to rubella research. Testing the kidney of the fetus, Plotkin found and isolated the rubella virus. Separately, Leonard Hayflick (also working at the Wistar Institute at that time) developed a cell strain using lung cells from an aborted fetus. Many viruses, including rubella, grew well in the resulting cell strain, and it proved to be free of contaminants. The strain was eventually called WI-38.

Plotkin grew the rubella virus he had isolated in WI-38 cells kept at 86°F (30°C), so that it eventually grew very poorly at normal body temperature. (He chose the low temperature approach following previous experiences with attenuating poliovirus.) After the virus had been grown through the cells 25 times at the lower temperature, it was no longer able to replicate enough to cause illness in a living person, but was still able to provoke a protective immune response. This rubella vaccine is still used in the United States today as part of the combined MMR (measles, mumps, and rubella) vaccine.

Ethical Issues with Human Cell Cultures

Although it has now been used in the United States for more than 30 years, Plotkin’s rubella vaccine was initially ignored in the U.S. in favor of vaccines developed using duck embryo cells and dog kidney cells. In the late 1960s, there was concern in the country that a vaccine developed using a human cell line could be contaminated with other pathogens, though this concern did not seem to have any documented evidence behind it. This is somewhat interesting in light of the discovery earlier in the decade that polio vaccines developed using animal cells were contaminated with a simian virus, which was one of the reasons researchers began using human cell lines in the first place.

Plotkin’s vaccine was first licensed in Europe in 1970 and was widely used there with a strong safety profile and high efficacy. In light of that data, and of larger side effect profiles with the other two rubella vaccines, it was licensed in the United States in 1979 and replaced the rubella vaccine component that had been previously been used for Merck’s MMR (measles, mumps, rubella) combination vaccine. Plotkin’s vaccine has been used in the country ever since. In 2005 the CDC declared rubella eliminated from the United States, though the threat from imported cases remains.

Groups that object to abortion have raised ethical questions about Plotkin’s rubella vaccine (and other vaccines developed with similar human cell strains) over the years.

Because of its position on abortion, members of the Catholic Church have asked for its moral guidance on the use of vaccines developed using cell lines started with fetal cells. This includes the vaccine against rubella as well as those against chickenpox and hepatitis A, and some of the rabies and mumps vaccines. The official position according to the National Catholic Bioethics Center is that individuals should, when possible, use vaccines not developed with the use of these cell strains. However, in the case where the only vaccine available against a particular disease was developed using this approach, the NCBC notes:

One is morally free to use the vaccine regardless of its historical association with abortion. The reason is that the risk to public health, if one chooses not to vaccinate, outweighs the legitimate concern about the origins of the vaccine. This is especially important for parents, who have a moral obligation to protect the life and health of their children and those around them.

The NCBC does note that Catholics should encourage pharmaceutical companies to develop future vaccines without the use of these cell strains. To address concerns about fetal cells remaining as actual ingredients of the vaccines, however, they specifically note that fetal cells were used only to begin the cell strains that were used in the preparation of the vaccine virus:

Descendant cells are the medium in which these vaccines are prepared. The cell lines under consideration were begun using cells taken from one or more fetuses aborted almost 40 years ago. Since that time the cell lines have grown independently. It is important to note that descendant cells are not the cells of the aborted child. They never, themselves, formed a part of the victim’s body.

In total only two fetuses, both obtained from abortions done by maternal choice, have given rise to cell strains used in vaccine development. Neither abortion was performed for the purpose of vaccine development.

Current Vaccines Developed Using Human Cell Strains

Two main human cell strains have been used to develop currently available vaccines, in each case with the original fetal cells in question obtained in the 1960s. The WI-38 cell strain was developed in 1961 in the United States, and the MRC-5 cell strain (also started with fetal lung cells) was developed in 1965 in the United Kingdom. No new or additional fetal cells are required in order to sustain the two cell strains.

The vaccines below were developed using either the WI-38 or the MRC-5 cell strains.

Hepatitis A vaccines [VAQTA/Merck, Havrix/GlaxoSmithKline, and part of Twinrix/GlaxoSmithKline]
Rubella vaccine [MERUVAX II/Merck, part of MMR II/Merck, and ProQuad/Merck]
Varicella (chickenpox) vaccine [Varivax/Merck, and part of ProQuad/Merck]
Zoster (shingles) vaccine [Zostavax/Merck]
Adenovirus Type 4 and Type 7 oral vaccine [Barr Labs] *
Rabies vaccine [IMOVAX/Sanofi Pasteur] *
* Vaccine not routinely given

Several vaccines currently available in the United States were developed using animal cell lines, primarily using cells from African green monkeys. These include vaccines against Japanese encephalitis, rotavirus, polio, and smallpox. Of these, only rotavirus and polio vaccines are routinely given.

Sources and Additional Reading

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.

Barr Labs. Package Insert – Adenovirus Type 4 and Type 7 Vaccine, Live, Oral. 2011. Available at (179 KB). Accessed 7/31/14.

Centers for Disease Control and Prevention. Elimination of rubella and congenital rubella syndrome–United States, 1969-2004. MMWR Morb. Mortal. Wkly. Rep. 2005. 54 (11): 279–82. Available at: Accessed 7/31/14.

GlaxoSmithKline. Package Insert – Havrix. 2011. Available at (123 KB). Accessed 7/31/2014.

GlaxoSmithKline. Package Insert – Hepatitis A Inactivated & Hepatitis B (Recombinant) Vaccine. 2011. Available at (332 KB). Accessed 7/31/2014.

Lindquist JM, Plotkin SA, Shaw L, Gilden RV, Williams ML. Congenital rubella syndrome as a systemic infection: studies of affected infants born in Philadelphia, USA. Br Med J 1965;2:1401-6.

Merck & Co, Inc. Package Insert – Measles, Mumps, and Rubella Virus Vaccine Live. 2009. Available at (196 KB). Accessed 7/31/2014.

Merck & Co, Inc. Package Insert – MERUVAX II. 2006. Available at (88.6 KB). Accessed 7/31/2014.

Merck & Co, Inc. Package Insert – Refrigerator-Stable Formulation – ProQuad. 2010. Available at (448 KB). Accessed 7/31/2014.

Merck & Co, Inc. Package Insert – VAQTA – Hepatitis A Vaccine, Inactivated. 2011. Available at (332 KB). Accessed 7/31/2014.

Merck & Co, Inc. Package Insert – Varivax (Frozen). 2010. Available at (220 KB). Accessed 7/31/2014.

Merck & Co, Inc. Package Insert – Zostavax. 2011. Available at (159 KB). 7/31/2014.

National Catholic Bioethics Center. FAQ on Vaccines. 2006. Available at: Accessed 7/31/2014.

National Network for Immunization Information. Human Fetal Links with Some Vaccines. 2010. Available at Accessed 7/31/2014.

Plotkin SA. The History of Rubella and Rubella Vaccination Leading to Elimination. Clin Infect Dis. 2006 43 (Supplement 3): S164-S168.

Plotkin SA, Orenstein WA, Offit PA, eds. Vaccines. Philadelphia: Saunders; 2008.

Sanofi Pasteur. Package Insert – ACAM2000. 2009. Available at (285 KB). Accessed 7/31/2014.

Sanofi Pasteur. Package Insert – IMOVAX. 2013. Available at (213 KB). Accessed 7/31/2014.

Sgreccia E. Statement from the Pontifical Academy for Life, including English translation of “Moral Reflections on Vaccines Prepared from Cells Derived from Aborted Human Foetuses.” 2005. Available at: Accessed 7/31/2014.