wherever possible.
Really?
That little caveat says it all.
Measles and mumps both have FDA approved alternatives which were developed using ethical cell lines. Just Rubella that doesn’t have one approved for use in the US. Japan has one available, but it isn’t approved here.
For Catholics, the governing document around vaccines and fetal stem cells is here: https://www.immunize.org/talking-about-vaccines/vaticandocument.htm
The gist is you have a moral and ethical responsibility to see out vaccines which have not used fetal stem cells in their development. If such a vaccine is not yet available, you must weigh the greater good (is this a vaccine for something that has a high prevalence in your region and is the particular medical risk to you and your loved ones such that not getting it would present a grave danger?) If taking the problematic vaccine prevents a greater harm to you or your family, you are permitted to do so, but then your responsibility becomes advocating for an ethical alternative to be developed, produced, and approved for use.