paulfromcamden
Baffled
Many, capacity of the factories, chemical and process consumables, the little bottles, cold chain distribution capacity, syringes
This article also mentions the shortage of available bioprocess engineers. My guess is capacity will be scaled up but it will take a few years.
https://www.barrons.com/articles/a-plan-to-break-the-vaccine-manufacturing-bottleneck-51621952245