The T cells recognise the spike but the B cells recognise the nucleus amino acids so they are more effective, as spikes have different shapes in different variants. So once the Ts disappear the Bs become important.
Getting the dose right is how they increase the number of Bs in most people. But not all people have the same immune response, hence variability in results from vaccines.
But the tech is sound. Getting a bit of spike is much safer than getting the whole virus, especially if you are worried about the bogeyman of the forum, the dreaded heart inflammation.
