I am pretty sure that there are four (4) different types of Covid vaccines.
See, e.g.,
https://www.gavi.org/vaccineswork/there-are-four-types-covid-19-vaccines-heres-how-they-work#:~:text=The%20four%20main%20types%20of,RNA%20 and%20DNA)
https://www.youtube.com/watch?v=lFjIVIIcCvc
There are plenty of other resources to learn about these types.
Folks should realize that in the United States, it is likely that by law, any gubmint and/or private party (e.g., your doctor's) recommendations regarding mixing and matching must likely be based on studies.
I do not know if such studies are underway in the US. If there are, they would have had to have been approved by FDA.
So hopefully we will learn more... but remember it takes time for enrollment, than the study period, then all of the information has to be organized and pass company requirements, and then, if it makes sense go to FDA.
Still not clear what will happen with the Astrazenica vaccine in the US. If I had to guess, I'd guess that it will get EUA.
The Novavax efforts are chronicled in the public information on their web site. This includes protocols.