Mick, while the structure and function of the delta variant’s spike protein has changed allowing it to be more infectious, I don’t believe that the initial immune activation characteristics has changed very much. Here’s a chart looking at immune response to the variants 5 weeks after the second Pfizer dose, with 94% of subjects producing neutralizing antibodies to Delta. While the antibody levels do drop over time, reaching the 84% level after a few months, it’s the initial robust reaction to delta that has lead to the booster strategy rather than a new vaccine variant. I also wonder how quickly a new mRNA version could be swapped out, tested, approved and rolled out. Delta’s only been the dominant variant in the US for about 5 months now.