Interesting study on mix and match and the antibody response for 9 different combinations of Moderna, Pfizer, and J&J (which would probably be a pretty good proxy for AZ in Canada). Basic conclusion is, mix and match looks to be every bit as good as using two of the same brand in generating an antibody response. And for viral vector vaccines, boosting with an mRNA vaccine dramatically increases the antibody response and brings it more or less on par with 2 doses of mRNA. And interestingly, the reverse works very well too, where a 1st dose of mRNA with a 2nd dose of J&J produces a somewhat similar antibody response to 2 doses of mRNA.
Here's how the combos rank (first 8 are fairly close together with #9 being significantly lower):
1) Moderna -> Moderna
2) Moderna -> Pfizer
3) Pfizer -> Moderna
4/5) Pfizer -> Pfizer & J&J -> Moderna (basically the same)
6) J&J -> Pfizer
7) Moderna -> J&J
8) Pfizer -> J&J
9) J&J -> J&J (this was significantly lower than the rest)
https://twitter.com/user/status/1448353685965131779