Ancestral, Delta, and Omicron (BA.1) SARS-CoV-2 strains are dependent on serine proteases for entry throughout the human respiratory tract

https://pubmed.ncbi.nlm.nih.gov/37769654/

The SARS-CoV-2 Omicron BA.1 variant emerged in late 2021 and became the globally dominant variant by January 2022. Authentic virus and pseudovirus systems have shown Omicron spike has an increased dependence on the endosomal pathway for entry.