In England, a Bishop John Sherrington, "lead" for the CBCEW on Life Issues, is responsible for a statement to the effect that Covid vaccines which use stem-cells derived from foetuses killed in the 1970s, may be used in good conscience by Catholics.
He cites a 2005 document from the Pontifical Council for Life, called Reflections on vaccines prepared from cells derived from aborted human foetus. This document was approved by the Congregation for the Doctrine of the Faith under its then Prefect Joseph Cardinal Ratzinger.
Bishop Sherrington's views are not Magisterium, neither is a document from the Pontifical Council for Life ... even if they honestly assure us that the CDF has given it the OK. The question needs to be decided against the background of Catholic moral discourse. The area of Moral Theology concerned is labelled technically "Co-operation in evil'; and it is treated in terms of formal co-operation and material co-operation. We can find discussions in reliable sources under the general heading of De Praecepto Caritatis erga proximum, answering the general question "An liceat alterius peccato materialiter co-operari?" Traditionally, the section comes early in the manuals, as part of a discussion concerning the Commanndment to love one's neighbour.
In the older books, 'Co-operation in Evil' is concerned with how far a conscientioius Catholic can go in giving a helping hand to somebody who has in mind to commit a sin. Could a servant help his master to carry a ladder to a place where the said master intends to clamber inside a bedroom in order to commit a sexual crime? Or to deliver a letter which might well be arranging an adulterous assignation? Must a young woman working in a factory producing condoms be told to give up her job? The writers of the manuals were clever men, and if they found such matters difficult to resolve, it is hardly surpising if we find the same.
In my (Marietti, 1874) copy of S Alfonso, the section on Co-operation in evil is at Liber II, Dubium 5, Articulus 3. The great Redemptorist moralist gives examples from the teaching of earlier moralists, with references, and sometimes says whether or not he agrees. Because here, as with so many other intricate questions, experts do not always agree.
Sitting beside S Alfonso, I have the immensely helpful four volumes of Henry Davis, SJ, Moral and Pastoral Theology (1935). Volume I Chapter VIII ...
And then 'Baby Pruemmer", the English summary of the large work by a Dominican; 'Big Pruemmer' was published in 1921. "Co-operation in Evil" comes towards the end of Treatise IX, "Theological Charity and Contrary Vices".
And, ex pietate, I will mention The Elements of Moral Theology (1947) by Robert Mortimer, sometime Professor of Moral and Pastoral Theology in this University and later Bishop of Exeter, drinking companion of my one-time Director, the late Prebendary John Hooper. His work is based upon Pruemmer and Merkelbach, but, he writes, "I have always gone behind them to their main source, St Thomas Aquinas".
I hope you have observed that none of my props comes from the iffy years immediately before and after "the Council". If you distrust the Moral Theology which was dreamed up during and after the 1960s, well, I share your apprehensions.
To summarise the subtle and sophisticated teaching which those earlier writers handed down to us: Formal Co-operation in the sins of others is always intrinsece malum, automatically wrong; Material Co-operation may not always be sinful, particularly where there is "grave incommodum" [grave personal inconvenience: S Alfonso's term]. In other words, the matter is up for discussion and we must consider the distance between the sin of the killing and the actions of today.
The morality of using vaccines created from cells taken out of foetuses murdered half a century ago comes under "Material Co-operation". By using such cells, how close is ones co-opertation in the evil of the killing? Near or distant?
To be concluded.