Earlier phases could have been accelerated as well. For mRNA vaccines, deleterious effects are unlikely, inefficacy is the likeliest problem. It might be possible to be challenge testing (highly informed) volunteers within a week of a novel virus being isolated.
On a similar note, national stockpiles of RT-PCR machines and reagents would allow widespread testing within the same week.
If I’m right, these indicate moral failures deeply embedded within our political and medical systems. Better luck next time.