I've been reading how there is a shortage of vaccines and countries are buying up doses for future delivery and they are all announcing their purchases.

I am not seeing the same publicity given to antigen tests. There is still a big shortage, lots of tests haven't been approved yet, and production capacity is still down. Countries need lots - billions - more tests than they do vaccines. Some say its tests that will bring the case numbers down more than the vaccines. The market is wide open.