What Aborted Fetuses Have to Do With Vaccines And why religious leaders still endorse vaccines.

 -- A small but growing number of parents who object to vaccinating their children on religious grounds say they do so because many common vaccines are the product of cells that once belonged to aborted fetuses.

There is a grain of truth to this statement. But even religious leaders, including a future pope, have said that shouldn't deter parents from vaccinating their children.

Vaccine and Cell Line Science

Some childhood vaccines, including the one against rubella -- which is part of the MMR vaccine given to millions of children worldwide for measles, mumps and rubella -- is cultured in "WI-38 human diploid lung fibroblasts," according to the U.S. Food and Drug Administration's fact sheet on the vaccine's ingredients.

Merck, the vaccine's manufacturer, acknowledged that those cells were originally obtained from an electively aborted fetus. They were used to start a cell line, which is a cell multiplied over and over again to produce cells that are of a consistent genetic makeup. The WI-38 cell line is used as a culture to grow live viruses that are used in vaccines.

Vaccines Developed Using Human Cell Strains

"Merck, as well as other vaccine manufacturers, uses two well-established human cell lines to grow the virus for selected vaccines," Merck said in a statement to ABC News. "The FDA has approved the use of these cell lines for the production of these Merck vaccines."

Other common vaccines, including those for chicken pox, hepatitis and rabies, are also propagated in cells originating from legally aborted human fetuses, according to the FDA.

"These abortions, which occurred decades ago, were not undertaken with the intent of producing vaccines," said a spokeswoman for the U.S. Centers Disease Control and Prevention.

The original cells were obtained more than 50 years ago and have been maintained under strict federal guidelines by the American Type Culture Collection, according to Merck.

"These cell lines are now more than three generations removed from their origin, and we have not used any new tissue to produce these vaccines," the company added in its statement.

To say that the vaccines contain a significant amount of human fetal tissue, as some objectors to the vaccines claim, is misleading, stressed Dr. Paul Offit, the director of the vaccine education center at the Children's Hospital of Philadelphia.

"There are perhaps nanograms of DNA fragments still found in the vaccine, perhaps billionths of a gram," he said. "You would find as much if you analyzed the fruits and vegetables you eat."

And to remove human fibroblast cells entirely from vaccines is out of the question, Offit explained, noting they are necessary because human viruses don't grow well in animal cells.

"They have also been tested for safety and the fetal cells can go through many more divisions than most other cells before dying," he said.

Ethical Considerations

Religious organizations have sided in favor of vaccines as well, even those generally opposed to abortion.

"We should always ask our physician whether the product he proposes for our use has an historical association with abortion," the National Catholic Bioethics Center states on its website, but then goes on to say "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," the center's position statement continued. "This is especially important for parents, who have a moral obligation to protect the life and health of their children and those around them."

Offit said he was glad the Catholic Church supports vaccination.

He noted it is particularly ironic to object to the rubella vaccine using fetal cells because Cardinal Joseph Ratzinger, who later became Pope Benedict XVI, commented on the subject in 2003, saying: "Universal vaccination has resulted in a considerable fall in the incidence of congenital rubella, with a general incidence reduced to less than 5 cases per 100,000 livebirths."

In other words, Offit explained, the rubella virus increases the risk of spontaneous abortion.

In the U.S., vaccination prevents up to 5,000 miscarriages each year in the U.S. alone, he said.