From what I understand, covid is a relatively small rna strand vs e.g. the flu which makes significant mutation way more rare

SARS-CoV-2 genome size is ~30 kb [1], influenza A genome size is ~14 kb [2]. A protein encoded in the SARS-CoV-2 genome is predicted to have exonuclease proofreading activity, which would result in a lower mutation rate [3].

[1] https://mra.asm.org/content/9/11/e00169-20

[2] https://en.m.wikipedia.org/wiki/Influenza_A_virus#Structure_...

[3] https://translational-medicine.biomedcentral.com/articles/10...