Folks should realize that the clinical trials that these companies conducted had to be approved by FDA. I don't know for sure, but I would surmise that there was a lot of discussion about trial structure with respect to number of doses, BEFORE the trials were approved.
If that is so, and the trial structures were based on two-dose endpoints, then FDA did not "shut anything down."
They likely could not authorize nor approve a change in labeling as critical as dosing because the study was likely not structured in a way that would provide sufficient safety and efficacy data.
Here is a related quote from FDA:
"Those participants who did not receive two vaccine doses at either a three-or four-week interval were generally only followed for a short period of time, such that we cannot conclude anything definitive about the depth or duration of protection after a single dose of vaccine from the single dose percentages reported by the companies. "
Did Pfizer include a robust "single dose" arm in studies? If not, why wouldn't they?
Here is a link that may provide some context:
https://clinicaltrials.gov/ct2/show/NCT04368728
This appears to be public domain info on the Pfizer study-- at least a Pfizer study.
I am sure we will learn more.
If folks are interested in learning more about trial design for vaccines and FDA submissions for changes in labeling in this type of situation, I can ask an expert--- an old friend that happens to be married to one of the OWS vaccine company CEO's. She is an expert, accomplished and experienced professional in the design and implementation of clinical studies for vaccines. I used to drink beers with her in college.