I do not have experience or expertise in this topic, but people I know in Canada will be going from AZ first dose to Pfizer second dose (with perhaps another booster needed?). This has been approved by the Canadian authorities and I have not heard anyone discuss a downside (unless they are administered too closely together, say within one month). Here’s what person wrote to me:
- one [vaccine] is good to make T cells, one is good for antibodies. It makes perfect sense to mix and match.
- if you look at the history, we have always mixed and matched. Until COVID nobody ever paid attention to brands. We have 10 or more manufacturers of flu shots every flu season. Nobody ever knows what they are getting, even through they are not all the same. Not even close.
- there is nothing from a theoretical perspective other makes this risky. Vaccine orthodoxy says don’t get more than one vaccine per month.