How do you make a COVID vaccine?

For those like me who are fascinated by the supply chain of the mRNA COVID vaccines, this article by Jonas Neubert is quite good: 

Welcome to the bottleneck of mRNA vaccine production! The number of people in the world who know how to get lipids and mRNA to combine into a lipid nanoparticle (LNP) might be in the low hundreds. And the machines to do it might not be machines at all but one-off lab bench setups like the one in this Wall Street Journal article. 

The problem at hand is this: How do you get the four lipids and the mRNA to combine in such a way that they form the protective sphere of the LNP, in a reproducible way? You can’t just combine all parts in your Vitamix and run the smoothie program. Well, you could, but it’s not going to give you a weird smoothie and not mRNA filled lipid nano-particles. What is of the essence is precise control of molecule sizes, precise control of flow rates, and probably precise control of many other parameters. Microfluidics is the technology of choice for these requirements and are most likely used for nanoparticle formation in mRNA vaccine making.

The author is not a scientist, so there are small errors in language, etc. But it's quite good, and collects a lot of information in a single place. Worth a read.