PROS1 and COVID-19: The SFG used for comparison in this study was expressed by HEK293E cells and contained diverse glycans; this is similar to the immunogens used in many current COVID-19 vaccines that are either approved or in clinical trials, including the insect cell–expressed S protein vaccines from Sanofi and Novavax (1), the Chinese hamster ovary (CHO) cell–expressed recombinant S vaccine from Medigen (12), the adenovirus-based vaccines from AstraZeneca and Johnson & Johnson, and the mRNA vaccines from Pfizer-BioNTech and Moderna (1).