PPIB and COVID-19: A previously obtained serum sample from COVID-19 convalescent and sera from mice immunized three times with recombinant N(B.1) protein, as well as anti-N monoclonal antibodies [20], were added to Vero CCL81 cells infected with HcoV-19/Russia/StPetersburg-3524/2020 (B.1 Lineage, Wuhan) virus, followed by incubation with guinea pig naïve sera (the complement source); the cell control wells contained no antibodies or sera.