From the Catholic Standard:
I think that it is important to understand that none of the COVID-19 vaccines being developed are actually derived from cell lines, whether fetally derived or not, but rather, the cell lines are needed to maintain the culture (the propagation) of the virus that forms the basis of the vaccine. Many of the cell lines used for this purpose have been around for decades, and I would hazard a guess to say that most researchers don’t even know the origin of most of these cell lines. Definitely, if some of the cell lines used in maintaining the vaccine virus cultures of some of the COVID-19 vaccines under development were originally derived from aborted fetuses, this could present ethical concerns for Catholics. However, I think that it is important to understand that there is not just one COVID-19 vaccine being developed, and that there will likely be several different ones that are eventually approved and licensed, each using a different means of production. I am confident that there will be options for concerned Catholics who wish to avoid receiving a vaccine that may have utilized a cell line in its production that was originally derived from an aborted fetus.
Regarding mRNA vaccines in particular, such as the ones being developed by Moderna and Pfizer, these are not manufactured by using cell lines, so there should be no ethical or religious concern regarding their use, if licensed.