The currently authorized mRNA vaccines inject genetic instructions to make spike proteins, the part of the coronavirus that latches onto human cells. These proteins trigger the immune system to build up antibodies and other immune defenses against an actual infection. Crafting the shots is a multistep process that begins with mass-producing the mRNA molecules and then adding them to precisely designed lipids, or fat molecules, made at another facility.

The seemingly simple step of combining these two ingredients is seen as a major bottleneck in our current vaccine production: They have to be mixed together precisely as nanoparticles that are not much bigger than molecules. Not enough care with this step breaks up the mRNA, making it useless.

Aside from Moderna and Pfizer, few in the drug business can do this mixing, because nobody else has made an authorized mRNA vaccine before.

“I believe that you can count on one hand the number of facilities who can make the critical lipid nanoparticles,” pharmaceutical industry researcher Derek Lowe wrote in a blog post last week. He added, “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.” Notably, the recent Defense Production Act prioritization of filters for Pfizer seems aimed at aiding this step in production.