I seem to recall that fda and maybe other regulatory agencies have said that approval of modified-payload mRNA vaccines will be quicker. I recall that Pfizer has tests for a delta variant shot underway and I (naively) hope they can include several mRNA sequences in the next booster. Otherwise, it’s just reloading the body’s antibody load. As the entire spike protein is presented, there’s no guarantee that each person’s antibodies are the same or even one variety. This differs from the monoclonal antibodies from Lilly and Regeneron which work as-is.