For those of you far more knowledgable than I, perhaps you can tell me why the emphasis (at least here in the USA) for vaccines seems to be antibody levels? I mean, I know a little about immunology (granted was like 20 years ago from college), but the human immune system is more than just antibodies, yet the CDC, etc. seems to be focused solely on antibody detection/levels as being the sole determining factor for vaccine durability/etc.

Do we (meaning the USA, or possibly others) normally *only* rely on antibody levels for all vaccines?
Or is this something "new" for Coronvaviruses?