This week, the newly approved, one-shot Johnson & Johnson vaccine was cheered as a way to dramatically accelerate the COVID-19 immunization race.

But it also renewed a long-standing ethical debate because the product has a connection — albeit a remote one — with abortion.

The U.S. Conference of Catholic Bishops announced Tuesday that it is morally acceptable to get the new vaccine if neither Pfizer’s nor Moderna’s is available. (Both those products also have a limited connection with fetal tissue, but more on that later.)

The Archdiocese of Philadelphia shared that statement with educators in all Catholic schools in the five-county Philadelphia region, spokesperson Kenneth A. Gavin said in an email. The J&J vaccine will be distributed to all public and private schools in Pennsylvania beginning next week, Gov. Tom Wolf announced Wednesday.

If you are concerned, confused, or just curious about the link between the coronavirus vaccines and abortion, here is an overview.

A bit of vaccine history

After a British doctor proved that infecting humans with the relatively mild cowpox virus gave them immunity against deadly smallpox, industrialized “vaccine farms” were set up to harvest cowpox virus from calves in the late 1800s.

Animals and animal products such as eggs remained standard in vaccine research and development until the mid-20th century. Then scientists discovered a safer, cheaper, more versatile way to study, make, and test most vaccines: Use human cells grown in the lab.

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Normally, human cells reproduce only a finite number of times and then die. But some cells — from a cancer or a fetus — are inherently able to proliferate indefinitely. Scientists have also learned how to genetically manipulate cells in culture to “immortalize” them.

Immortalized fetal cells have yielded countless medical advances, particularly immunizations.

In the United States in the mid-1960s, for example, about 31,000 pregnant women infected with the rubella (German measles) virus suffered stillbirths, gave birth to severely disabled infants, or decided to end their pregnancies. One aborted fetus was sent to Philadelphia’s Wistar Institute, where vaccine pioneer Stanley Plotkin isolated the rubella virus from kidney tissue. He developed the vaccine that is given today by growing the virus in a cell line made from the lungs of an uninfected fetus aborted in Sweden.

Remdesivir, the antibody treatment approved for COVID-19, was developed using a cell line derived from kidney tissue of a fetus aborted in the 1970s.

The three COVID-19 vaccines now approved in the United States all used historically derived fetal cell lines in parts of the development process. Pfizer and Moderna used the cells only to test their vaccine, while J&J used the cells for research, production, and testing, according to the antiabortion Lozier Institute.

The antiabortion stance on vaccines

Most, but not all, antiabortion ethicists and organizations say the horrors of the pandemic outweigh their qualms about “abortion-tainted” vaccines.

However, most of them also advise that if there is a choice, get what they consider the more “morally acceptable” shot.

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The U.S. Conference of Catholic Bishops issued a statement that echoed the Vatican’s position: “When ethically irreproachable Covid-19 vaccines are not available … it is morally acceptable to receive Covid-19 vaccines that have used cell lines from aborted fetuses in their research and production process. However, if one can choose among equally safe and effective COVID-19 vaccines, the vaccine with the least connection to abortion-derived cell lines should be chosen. Therefore … Pfizer or Moderna’s vaccines should be chosen over Johnson & Johnson’s.”

A few Catholic clerics have taken a harder line. The Diocese of Bismarck, N.D. issued a statement that said the J&J vaccine is “unacceptable for any Catholic physician or health care worker to dispense and for any Catholic to receive due to its direct connection to the evil act of abortion.”

American conservative political pundit and journalist Ramesh Ponnuru called that connection “attenuated.”

“No one who takes [COVID-19 vaccines] to protect himself and his community from COVID need worry that he is either causing an abortion, encouraging abortion in the future, or conferring approval of it,” he wrote in the National Review.