Had another thought Mark, and is basically the same point to consider as I posted on last week's discussion. Couldn't the difficulty in protection of the respiratory tract by vaccine induced antibodies(and probably to a lesser extent previous infection - IgA vs IgG) together with the increase of infectiousness associated with Delta explain what we're seeing? Difficulty in protecting respiratory tract discussed here:
https://www.abc.net.au/news/health/2020-04-17/coronavirus-vaccine-ian-frazer/12146616