There were many, many English canonized during this time period as martyrs, most notably St Thomas More and Bishop St John Fisher, who died EXACTLY because they would not take the Oath.

Of course, if abortions were "constantly" needed for vaccines, then their use would be 100% prohibited.  As it is, it appears that the process is using CLONES/COPIES of the cells, which originally came from abortion long ago.  To me, a copy is a new thing, even if it's cellular makeup is identical.  Since it's a new thing, then philosophically speaking, it's essence is not the same as the original fetal cells.

Yeah, I making same assumptions and for the sake of discussion presume a good effect is 100% valid (while in the reality we obviously know it is a lie - vaccines bring in a lot of bad, in fact way more then good, if at all).
Now if the process of developing/testing/producing/packaging is the same and the only differentiator here is that for a vaccine A we use 50 years (since the kill) old cell line and for exactly same vaccine B we have 1 month old (since the kill) cell line, it would be an absurd to think that taking A makes one less culpable (due to a "very" "remote" cooperation) than B. As both are equivalent, it is of the same value. (or you could not tell them apart by taking into consideration mere function).

I am not a biochemist nor a theologian of course, but do have a very quite advanced physics background - and I hope I can still logically think - and all looks for me that the main difficulty is understanding what that "CLONES/COPIES of the cells" truly means. It is probably worth mentioning that human body replaces all the cells every 7-10 years (source: a stupid google search). So the existence of the same set of cell is not an essence of our being here.

Personally, I side with an opinion, that there is a direct continuation of an original corpse/cavader of a killed baby, as they share the same DNA, regardless how long the process lasts. The analogy would be, we artificially maintain a body of a dead person or part of it (by e.g. freezing it). Probably a better example would be a frozen embryo. Having said that, it does not matter if the abortion was procured 50 years ago or 5 months ago. Simple as that.

Now, that dramatically shortens number of steps and completely removes time from "the direct line of causality from the abortion to the available fetal cells to the development of the vaccine, to the immunization".
I have also an example where a principle of double effect could be safely used. For instance, a scientists kills a person and using this dead body and his wicked genius comes up with a medical procedure to ... let's say a cure for a leprosy. Now the method itself, which developing took some evil measures, in itself does not, in any form or fashion, rely on the evil which took place. In other words, it is possible that another genius discovered it without a convenient murder which made it easier. Then using that method is morally neutral in any case.

There were many many more Catholics who who were coerced and took the Oath out of great fear .  In other words ...without full consent, a necessary component for mortal sin. So no one here should be asserting that these Catholics committed mortal sin.

Even if the Church determined that the COVID vaccine could result in mortal sin it would have to be taken with full consent.  I suspect that anyone here who would end up doing so against such a Church determination would be doing so unwillingly.

"The three most reliable sources to date of abundant fetal stem cells are the placenta, amniotic fluid, and umbilical cord blood." Source: https://www.sciencedirect.com/topics/medicine-and-dentistry/fetal-stem-cell

Realize that ((science)) must blur distinctions between fetal and supportive gestational tissues that are NOT fetal to muddy the waters and redefine what is objectively true so most will accept this generic unthreatening false definition of what fetal cells lines are. Babies (fetuses) are NOT placentas or umbilical cords. Like I said before, they should use these cells (placental and umbilical) for vaccines as they are just as effective and leave the poor baby alone, but they do not.

For example ((science)) has redefined  implantation as the new "conception". Why ? So they can  call abortifacients which kill embryos before implantation just "contraception. Today, because of the purposeful language distortion, you must use the term "fertilization" for clarity about conception. Women who take the abortifacients thinking they are contraceptives' never realize they are committing abortion. if you control the language you control it all . The deception is very thick

Do you accept that using human cells to make vaccines is morally indifferent?

It depends on the method that has been used to acquire the human cells. 
If the method is moral, say through placental cells, it could be morally indifferent. 
If the cells are acquired at the cost of the death of the subject, say an aborted infant, it is not morally indifferent but morally repugnant - a grave sin.

https://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/indifferent-acts
Determinants of Morality. The morality of a human act is determined by three aspects of the action. First, there is the object about which the choice is concerned. This object, even when the choice remains purely internal, can be considered the substance of the act. Second, the circuмstances of time, place, status of the person, means, and manner qualify the object of the act and are concomitant determinants of the morality of the act. Third, the circuмstance of purpose or reason for the act deserves special attention because the end colors the entire choice as qualified by the other circuмstances. For a human act to be morally good, all three of these moral determinants must be good; that is, they must conform to objective norms of morality. Subjectively, the individual must follow his certain conscience dictating that all three elements are moral. If any of the three moral determinants is evil, then the entire act is morally evil. To choose something good, but for an evil purpose, vitiates the entire act. To choose something good for a good purpose but at a wrong time can make the whole act evil.

What a bunch of fools they were, to die for that when they could have just taken the Oath if it wasn't a mortal sin not to.  Just like those silly Catholics in the Roman Empire, who let themselves be fed to lions.  

Except Catholic morality teaches that a mortal sin requires full consent.  These men were certainly courageous in dying.  That's why they were canonized.

But I see you're posting sarcastic, condescending posts...again.

I was researching to refute 2Vermont, and stumbled across this from St. Thomas Aquinas (Q. 125, Art. 4, Secunda Secundae):

"Now the evils of the soul are more to be feared than the evils of the body. and evils of the body more than evils of external things. Wherefore if one were to incur evils of the soul, namely sins, in order to avoid evils of the body, such as blows or death, or evils of external things, such as loss of money; or if one were to endure evils of the body in order to avoid loss of money, one would not be wholly excused from sin. Yet one's sin would be extenuated somewhat, for what is done through fear is less voluntary, because when fear lays hold of a man he is under a certain necessity of doing a certain thing. Hence the Philosopher (Ethic. iii, 1) says that these things that are done through fear are not simply voluntary, but a mixture of voluntary and involuntary."
https://www.newadvent.org/summa/3125.htm

Consequently, I modify my own opinion quoted above:

It would certainly be grave matter, but may not be mortal sin.

Concedo.

Well, thanks for that.  This is very important to this discussion and I am surprised that none of the clerical guidance posted thus far has included it (unless I missed it).