Moderna uses a higher dose of mRNA than Pfizer, and the two doses are four weeks a part instead of three weeks. They picked doses that worked and interdose times that got their trials done ASAP. Turns out that even four weeks is not optimal. Mine was five weeks because of the Texas power grid failure leading to vaccine shipment shortages. But some virologists say 5 months would be WAY better. Immunologists have known this for a long time, but WarpSpeed's aim was to make as many somewhat immune as fast as possible.

Seems the immune system does a bunch of things that take time, and the first dose just gets some processes started. If they have run for a while, the second dose has activated T-cells to turn on and get B-cell antibody production really cranking. This is why infection followed later by one vaccination gives you super-immunity.