My impression is that critical points in the process (combining mRNA with lipids) is a rate limiting step and can only be performed by a handful of specialized manufacturing facilities.

Derek Lowes “In the Pipeline” column has a number of articles relating to mRNA vaccine manufacturing:

https://www.science.org/content/blog-post/more-mrna-vaccine-... https://www.science.org/content/blog-post/myths-vaccine-manu...