Comparative Perspective of Evolution of SARS-CoV-2 and 1918 Influenza Virus

Omicron variants of SARS-CoV-2 virus have higher infectivity, more immune escape but lower risk of severe clinical outcomes than the ancestral variants. However, people are still wondering whether SARS-CoV-2 may become lethal again or whether some highly-pathogenic strain may emerge in the future. Compared with ancestral variants, Omicron has more selective proliferation in the upper respiratory tract (URT). The temperature of human URT is 33–34°C, and the virus strains other than Omicron cannot replicate effectively at this temperature. While Omicron adapted to the low temperature environment, so its distribution mainly depends on the expression levels of ACE2 in cells. Similar distribution switch from lungs to URT has been observed for the 1918 H1N1 influenza virus, which vanished after 1921 and has been turned to become the seasonal human influenza A viruses. However, some relatively high-pathogenic SARS-CoV-2 strains and influenza virus strains may still emerge in the future. Future evolutionary directions of SARS-CoV-2 and influenza viruses are discussed.