At least until we have enough vaccine supply to offer everyone 2 doses, the best way to use our limited supply of vaccine is to give as many first shots to as many at-risk people as we can.
Because the data from nearly everywhere is indicating that a 2nd shot doesn’t increase immunity by all that much over one shot.
Or put another way, most people seem to get nearly as much immunity (nearly, but not totally) from one shot as they get from a full regimen of two shots.
Take this preliminary study from Quebec, where some just-published data indicates that the COVID-19 vaccines have offered about 80 % effectiveness after just a first dose, although that protection kicked in after 2 weeks in health care workers, and only after 3 weeks in people over the age of 80.
With the studies indicating that the overall protection rate from the RNA vaccines goes up to 92 % or so after 2 doses – roughly 10-12 % better than with this real-world data from one-dose – this really argues forcefully for postponing 2nd shots until way more people get protected by a first shot (it may also be, BTW, that protection after one-dose continues to go up somewhat after those 3 weeks, but that data is still no in).