[hbrav]

Some Canadian provinces treated mRNA vaccines as interchangeable. In Ontario a lot of people got a first dose of Pfizer and a second of Moderna.

There's currently a study underway in the UK (I forget the name, but it run by the University of Oxford) to assess the efficacy of mixed-dose vaccination.

[gpm]

More than mRNA vaccines as interchangeable, we (Ontario) mostly treated the oxford vaccine as interchangeable with the mRNA vaccines too, until we started giving it to less people.