There have been a number of smaller studies mixing and matching vaccines but I haven’t seen any directly on point (2 Pfizer followed by Moderna booster). The CDC authorized a 3rd mRNA shot in those with compromised immune systems and the 3rd does not have to be the same as the first 2, but only if the same vaccine is not available for the third.
If you find any publications talking about that, please link it to us. I saw the recent news story that the Moderna vaccine looses less imunity over time than the Pfizer vaccine. What I read didn't make much sense, and left me scratching my head. Basically, I was surprised that there was such a difference between the two.
I’ve had 2 Pfizer jabs and I suspect that I’ll probably be exposed to the Delta variant at some point. I don’t have a compromised immune system (as far as I know). On the other hand, if a 3rd Moderna jab is at some point proven to be both safe and more effective against the Delta variant I’d probably get it.
We all will be exposed to the Delta variant, if we haven't already.
Last March-April, I got two jabs of the Moderna vaccine, and my wife got the Pfizer. I am also moderately immune-compromised because of medication I take to suppress an auto-immune disease. I don't yet know what the details are about which immune-compromised people get first priority for a 3rd dose. And, I also don't yet know which flavor vaccine will be used. I aim to find out asap.
In Maryland, no effort was made to prioritize people for vaccination, other than by age. It is my
guess that this will be repeated now – people will only need to claim (but not prove) that they need a 3rd dose due to a fully or partially impaired immune response.