Rare C-to-G mutations occurred at an unexpectedly high frequency during emergence of SARS-CoV-2 delta variant

SARS-CoV-2 possesses a characteristic surface protein, the spike protein, which plays a central role in viral entry into host cells. The evolution of spike protein mutations has been an important factor in the emergence of viral variants with altered transmissibility and immune-evasion properties. Previous studies have shown that SARS-CoV-2 evolution is strongly biased toward certain types of…