Have you any source for that information ?
I've read widely on this and I don't think saying they're 'not interchangeable' is necessarily correct. It's more correct to say that the combination hasn't been tested.
In the UK there's already a published protocol to mix vaccines if necessary, say if there's no record of which vaccine was first administered. Most reputable experts in the UK don't expect any problems with mixing vaccines, although the efficacy of that policy will discovered in time. It's possible that mixing vaccines may even enhance protection.
I took me less than 30 sec to google and get to this:
https://www.cdc.gov/vaccines/covid-19/info-by-product/clinical-considerations.htmlUntil a combo is tested and shown to be similarly effective, you don’t get to use it because someone thinks it’s a good idea, that’s how it works in medicine.
That should be the end of the discussion.
For humoring purposes, there can be many reasons why the combination might not work. While they technically present the same protein, the actual sequence might be slightly different between the vaccines and the immune response also depends a lot on the presentation, meaning the additional sequences “tied” to the actual mRNA. Another google search can clarify it. Make it that the doses are different and the schedules are different, 3 weeks Pfizer and 4 weeks Moderna, many things can go wrong.
In conclusion, until tested and proven, it’s not valid.