According to this article, they break down the manufacture and distribution of mRNA vaccines into six steps. They label step 4 as the bottleneck.
https://blogs.sciencemag.org/pipeline/archives/2021/02/02/myths-of-vaccine-manufacturing
Step One: Produce the appropriate stretch of DNA, containing the sequence that you need to have transcribed into mRNA. This is generally done in bacterial culture.
Step Two: Produce that mRNA from your DNA template using enzymes in a bioreactor.
Step Three: Produce the lipids that you need for the formulation. Some of these are pretty common (such as cholesterol), but the key ones are very much not (more on this below).
Step Four: take your mRNA and your lipids and combine these into lipid nanoparticles (LNPs). I have just breezed past the single biggest technological hurdle in the whole process, and below you will learn why it’s such a beast.
Step Five: combine the LNPs with the other components of the formulation (phosphate buffers, saline, sucrose and such) and fill those into vials.
Step Six: get those vials into trays, into packages, into boxes, into crates, and out the door into trucks and airplanes
Ah, but now we get back to Step Four. As Neubert says, “Welcome to the bottleneck!” Turning a mixture of mRNA and a set of lipids into a well-defined mix of solid nanoparticles with consistent mRNA encapsulation, well, that’s the hard part. Moderna appears to be doing this step in-house, although details are scarce, and Pfizer/BioNTech seems to be doing this in Kalamazoo, MI and probably in Europe as well. Everyone is almost certainly having to use some sort of specially-built microfluidics device to get this to happen – I would be extremely surprised to find that it would be feasible without such technology. Microfluidics (a hot area of research for some years now) involves liquid flow through very small channels, allowing for precise mixing and timing on a very small scale. Liquids behave quite differently on that scale than they do when you pour them out of drums or pump them into reactors (which is what we’re used to in more traditional drug manufacturing). That’s the whole idea. My own guess as to what such a Vaccine Machine involves is a large number of very small reaction chambers, running in parallel, that have equally small and very precisely controlled flows of the mRNA and the various lipid components heading into them. You will have to control the flow rates, the concentrations, the temperature, and who knows what else, and you can be sure that the channel sizes and the size and shape of the mixing chambers are critical as well.
These will be special-purpose bespoke machines, and if you ask other drug companies if they have one sitting around, the answer will be “Of course not”. This is not anything close to a traditional drug manufacturing process. And this is the single biggest reason why you cannot simply call up those “dozens” of other companies and ask them to shift their existing production over to making the mRNA vaccines. There are not dozens of companies who make DNA templates on the needed scale. There are definitely not dozens of companies who can make enough RNA. But most importantly, I believe that you can count on one hand the number of facilities who can make the critical lipid nanoparticles. That doesn’t mean that you can’t build more of the machines, but I would assume that Pfizer, BioNTech, Moderna (and CureVac as well) have largely taken up the production capacity for that sort of expansion as well.