This might answer it.
Moderna did some testing of one shot effectiveness and reportedly realized 80-90% which is pretty damn good.
The Sinovac (chinese covid vaccine) recently approved for use in brazil, delivers just over 50% effectiveness... (50.38%).
If we are operating in an apparent shortage of vaccine doses, it seems prudent to give twice as many people 80% effective protection. This seems like a no brainer, but moderna has not been formally tested as a one jab vaccine, so not authorized for use that way. The 80-90% result was from a single 2,000 person trial.
It has not been tested any other way than the two jab regimen 3-4 weeks apart so we have no idea if delaying the second jab until supply catches up would have an adverse impact. It is easy to imagine the benefit from vaccinating twice as many people with first jabs.
JR
PS: Moderna is reportedly working on new vaccine variants better matched to new COVID strains.