It’s possible you could have Memory B-cells in place today for SARS-CoV-2, if your body fought off an infection already. You could have a serology test done to see if you’re currently producing antibodies to SARS-CoV-2. If so, you have no need of a vaccine.
The basics of how this works is that when a pathogen invades your body, general defenses already in the area respond first. This is primarily going to be macrophages and neutrophils. If the macrophages can’t beat it, they’ll signal for dendritic cells, which are kind of the brains of the immune system. The dendritic cells will take samples of the pathogen to lymph nodes, where a complicated process activates the right kind of T-cell that fits with the samples taken. Those T-cells then activate virgin B-cells, which produce the specific antibody necessary to kill that pathogen. After infection is over, 90% of those B-cells die (they reproduced in mass numbers to begin antibody production and most of them work themselves to death producing antibodies). The 10% that remain become Memory B-cells, ready to resume production of antibodies if that pathogen is ever encountered again.
I don’t want to keep picking your brain, but after this virus has been around as long as it has...what percentage of the population would you estimate is producing antibodies?