Pfizer now approved as a booster for all aged over 18 starting at 6 months.
SV and PZ have been compared, and it appears SV is coming out ahead.
For me AZ and SV is top of the table and PZ third for longevity of effect.
Don't forget that even though SV is also an mRNA vax, it has 3x the mRNA content than PZ.
PZ doses were reduced early on in development because the reduction in mRNA content still provided excellent (initial) seroconversion and allowed the company to sell more doses.
It is impossible to know whether that difference is significant.
There are no studies that I am aware of that looks at comparing equal dosing of PZ and SV
Mix and Match has been tried in Spain, Canada with some studies suggesting M&M is better than x2 of the same in an M&M of 2 doses.
it was not officially called M&M rather P&B: "Prime and Boost"
Immunologically makes sense and entirely predictable.
Currently as a 3rd jab, I'm leaning toward SV rather than AZ because there is potential for an immune response against the adenovirus carrier in AZ after repeated doses which might destroy the virus carrier before it has a chance to get into the cells
Ultimately we should be looking at CovidVax specifically designed for the Delta variant for a booster.