They are considered "interchangeable" by the National Advisory Committee on Immunization of Canada.

[1]: https://www.canada.ca/en/public-health/services/immunization...

Moderna and pfizer both encode the spike protein identically.

"Follow up earlier post. Here is moderna/pfizer/wild type spike. There is only 2 amino acids different from wild to vaccine. Same for both vaccines. The change forces the protein to look like it would when stuck to the outside of the virus, without needing the rest of the virus"

From Thought Emporium on twitter (cool biohacker with particular focus on genetic engineering, but amazing breadth into physical sciences as well - I highly recommend his youtube channel)

https://twitter.com/EmporiumThought/status/14081777241920675...

CDC said it was "ok" to do so back in January. In Europe they studied mixing mRNA with non-mRNA doses (astra zeneca and pfizer) and that also appeared to work very well: https://www.pbs.org/newshour/health/a-mix-and-match-approach...