There are several issues getting deeply confused. I'll break it down this way:
1. Is it immoral to develop a cell line from an aborted fetus? Yes, always, and gravely immoral.
2. Is it immoral to conduct research using an aborted fetal cell line? Yes, but with varying culpability depending on the degree of cooperation.
3. Is it immoral to use a product developed from the use of an aborted fetal cell line? Yes or no, depending on the principle of double effect. In this particular case, very obviously yes.
Based on how some people here are arguing, I wonder if they don't have any problem with embryonic stem cell research either. The moral objections to HESC are the same for developing aborted fetal cell lines.
The use of these cell lines is not morally neutral in this particular case because we know they were produced in a gravely immoral manner; it is irrelevant to this conversation to speak of using cell lines in a totally generic way and as being morally neutral. We're not talking about any cell line but aborted fetal cell lines, and their use is immoral.
Further, it is irrelevant that the original fetal cells of the aborted child are or are not currently present in the cell line. Those cells were the initial cause of the cell line (in all four senses of causality); what happens subsequently is irrelevant insofar as we are talking about the causal chain as a whole. Many people here have been confusing per se vs. per accidens causality. In the case of a cell line, we have a per accidens chain, but we must remember that every per accidens chain presupposes a per se causal chain. This is how I read Fr. Scott's original article arguing that using vaccines derived in any way from aborted fetal cells is gravely immoral because there is a direct line of causality, that is, the entire chronological development of the cell line ultimately depends on the initial fetal cells. I think arguing this way is a bit ambiguous, but I think it is trying to overcome the objection that the original cells are no longer present. The ambiguity of arguing like this also shows itself in the somewhat convoluted thought experiments given earlier to show how cooperation in some evil act would be formal vs. material.
As I understand it, Church teaching clearly says that the development of aborted fetal cell lines is always gravely immoral. The culpability that follows from using these cell lines depends on the degrees of cooperation in any particular line by any given person; not everyone will have equal degree of culpability. Because the culpability is determined by applying the principles of moral cooperation and hence will always be a contingent, circuмstantial matter, it is unnecessary for the Church to make any definite statement about that; it is up to the moral theologians to assess the degrees of culpability in any particular instance.
The morality of using a product developed from such a fetal cell line is guided by the principle of double effect. In this case, we ask the standard questions: is there a proportionate reason, no reasonable alternative, obvious case for pursuing this solution, and do we not will the evil? If there is a ~99.76% survival rate for Covid-19, then it is clear a vaccine (putting aside for the moment how it was produced) is completely unnecessary for the vast majority of humans. Since the vast majority don't even need a vaccine for this disease, certainly one cannot argue that proportionate reason exists or reasonable alternatives don't exist. Many effective alternatives exist.
So if a particular Covid vaccine was developed using an aborted fetal cell line, it very obviously and thoroughly fails the standard of double effect and hence would be gravely immoral to use.
There are so many mindboggling aspects to the SSPX statement, one doesn't know where to begin. I'm bewildered that the SSPX statement goes further than the 2005 PAL docuмent does and strongly suggests that a Covid vaccine would be OBLIGATORY for parents to provide to their children. What is also shocking is that when the SSPX claims to be quoting Church teaching, they're actually quoting their own online Q&A on vaccines, unless they see themselves as the Church? The 2005 docuмent is tame, mild, and uneventful compared to the overzealous, pro-vaccine mentality of the SSPX statement. In fact, it clearly contradicts the 2005 docuмent by going further than it and by not mentioning the necessary caveats that the 2005 docuмent makes.