As Catholics, we ought to be concerned about how treatments for Covid-19 are developed because of our strong moral stance against the use of embryonic stem cells in research. Because these cells are harvested by killing living human beings, we would be benefiting from the use of elected abortion which is an intrinsic evil. We have a moral obligation to stand up against medicine like this because as Catholics we believe that the ends can never justify the means. That means that we can never do something evil even if it has a beneficial effect.
Many faithful Catholics question whether or not using the Covid-19 vaccines that are being produced by Pfizer and Moderna are morally licit. I am happy to say that neither of these vaccines are based on embryonic stem cells. However, unfortunately, they were tested and developed with the use of embryonic stem cells. While this is not ideal, there is still no moral issue with using either one of these vaccines for grave reason and the United States Conference of Catholic Bishops have issued statements saying so. However, Catholics are morally obliged not to use the vaccines developed by companies that did base them on embryonic stem cells such as Johnson and Johnson and AstraZeneca. If a vaccine is developed that is not even tested on embryonic stem cells at all, then Catholics would be obligated to use that vaccine rather than the ones that were based on illicit research.
Of course, it would be ideal that we could live in a world where we could have no cooperation with evil whatsoever. But, realistically, we never know the full consequence of all of our actions and what people will do with what we give them. Medical moral issues are incredibly complex and it is really tough to know if what one is doing is correct. With the information that we have now, the Church has decided that the Pfizer and Moderna vaccines are morally licit because they do not directly cooperate with evil and there is grave reason to use them. Of course, this does not require any Catholic to take these vaccines.
I hope this clears up the Church’s stance on this issue. If you would like to hear a more complete treatment on this subject, then I suggest you look up one of my favorite podcasts: Trent Horn’s “Council of Trent” episode “Covid-19 Vaccines and Catholic Morality” first posted on November 17th.