The fetal cell line that van der Eb extracted became known as HEK 293 and is still used frequently in medical research. The line has been used to study an enormous range of pharmaceuticals, from common over-the-counter drugs such as ibuprofen and aspirin to vaccines against lethal diseases such as tuberculosis, Ebola, and COVID-19.

For Catholics, the conundrum posed by the fruits of van der Eb’s research has to do with the roots of his process: Is willingly using products created from human remains procured from abortion ever morally permissible? The Congregation for the Doctrine of the Faith, a Vatican office devoted to defining and promulgating Catholic doctrine, issued a 2005 letter detailing an uneasy conclusion: Catholics have a duty, it said, to seek out alternative vaccines that were not developed using fetal cells, when those options exist. “As regards the diseases against which there are no alternative vaccines which are available and ethically acceptable, it is right to abstain from using these vaccines if it can be done without causing children, and indirectly the population as a whole, to undergo significant risks to their health,” the office wrote. “However, if the latter are exposed to considerable dangers to their health, vaccines with moral problems pertaining to them may also be used.”

Or, in common parlance: Receiving vaccines developed using fetal cell lines such as HEK 293 can be morally permissible, as long as there aren’t suitable alternatives and the risks posed by the illness itself are sufficiently serious. The Church’s position, complex and contingent as it was, left plenty of Catholics uncertain about how to consider the COVID-19 vaccines. Was the coronavirus dangerous enough to justify the vaccines? Some of them approached John Di Camillo, a staff ethicist at the National Catholic Bioethics Center, in Philadelphia. I asked him how he had been responding to the many queries he had received regarding how to weigh moral concerns about the origins of the vaccines against the urgent threat of the pandemic itself.