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I think this has been inferred in some quarters from the fact that the mix n match trial showed better protection if the second dose was an mRNA vaccine. But that was just looking at immune responses rather than real life infection, and 2 doses rather than a 3rd dose booster.
Either way I wouldn’t be surprised if they do Pfizer boosters to use up stock and switch tactics if/when variant specific boosters become available.
AZ is probably less effective against Delta than Pfizer but neither provide 100% protection.
I think all boosters in the UK are planned to be Pfizer.